diff --git a/Makefile b/Makefile
index 4a2fbcc..a719205 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 PROTO_FILES := $(wildcard proto/sisyphus/v1/*.proto) $(wildcard proto/sisyphus/node/v1/*.proto)
-COVERED := ./apps/...,./packages/ai/...,./packages/geo/...,./packages/hardware/...,./packages/identity/...,./packages/job-model/...,./packages/kubo/...,./packages/nodedb/...,./packages/runtime/...,./packages/sealed/...,./packages/storage/...
+COVERED := ./apps/...,./packages/ai/...,./packages/geo/...,./packages/hardware/...,./packages/identity/...,./packages/job-model/...,./packages/kubo/...,./packages/nodedb/...,./packages/runtime/...,./packages/s3/...,./packages/sealed/...,./packages/storage/...
 
 .PHONY: build test cover demo proto tools fmt
 
diff --git a/apps/sisyphusd/README.md b/apps/sisyphusd/README.md
index 8f95da1..6ae00ad 100644
--- a/apps/sisyphusd/README.md
+++ b/apps/sisyphusd/README.md
@@ -236,6 +236,22 @@ A coordinator also collects every `--gc-interval` (default one hour), and with `
 
 A job's result must be stored by the workload's aggregation step to be kept; a blob a task stored is treated as intermediate unless the aggregation stores it again.
 
+### In a bucket
+
+A coordinator can keep the pool's stored data in a bucket of an object store that speaks S3, instead of on its own disk: Ceph, MinIO, SeaweedFS, Storj, Wasabi, AWS.
+
+```sh
+printf '%s\n%s\n' "$ACCESS_KEY" "$SECRET_KEY" > s3.keys && chmod 600 s3.keys
+bin/sisyphusd run --s3-endpoint https://s3.eu-central-1.wasabisys.com --s3-bucket my-pool --s3-credentials s3.keys
+```
+
+- **The bucket must exist**; the node checks that it can reach it and stops if not. `--s3-prefix` puts the pool's objects under a name of their own, to share a bucket; `--s3-region` is for stores that have regions.
+- **Nothing else changes.** A file has the same CID wherever its bytes are, so jobs, workers and clients need know nothing of it. Workers still fetch from the coordinator and each other; only the coordinator talks to the bucket, and the credentials never leave it.
+- **Each block is an object**, of up to 256 KiB, under `blocks/`. What is kept and for how long is still the node's to decide: pins are in its data directory, and `blob gc` deletes from the bucket what nothing is keeping.
+- **It is slower than a disk** when the bucket is far away: reading a gigabyte is four thousand requests, one after another. Nothing is cached on the coordinator. Use it for durability and room, and a bucket near the coordinator.
+- It cannot be combined with `--kubo`, which is another answer to where the data is kept.
+- Tried against SeaweedFS's S3 gateway. The others named speak the same dialect and have not been tried.
+
 ## What each machine has
 
 A worker looks at the machine it runs on and tells its coordinator what it finds: the processor, how much memory, and any NVIDIA graphics cards. `sisyphusd nodes` shows it.
diff --git a/apps/sisyphusd/cli_test.go b/apps/sisyphusd/cli_test.go
index ac8ce40..4c4d445 100644
--- a/apps/sisyphusd/cli_test.go
+++ b/apps/sisyphusd/cli_test.go
@@ -29,10 +29,17 @@ func TestMain(m *testing.M) {
 	// Nothing a test does may land in the real user's data directory.
 	os.Unsetenv("XDG_DATA_HOME")
 	code := m.Run()
+	for _, stop := range stopAtExit {
+		stop()
+	}
 	os.RemoveAll(home)
 	os.Exit(code)
 }
 
+// stopAtExit is what to stop when the tests are over: whatever a test
+// started that outlives it, such as a container shared between tests.
+var stopAtExit []func()
+
 // dataDirs records, for each address a test node listens on, the data
 // directory holding that node's key. Commands sent to the address are run
 // from that directory, which makes them the node's owner.
diff --git a/apps/sisyphusd/daemon.go b/apps/sisyphusd/daemon.go
index 503477b..06981ab 100644
--- a/apps/sisyphusd/daemon.go
+++ b/apps/sisyphusd/daemon.go
@@ -46,6 +46,7 @@ import (
 	nodepb "github.com/sisyphus-network/Sisyphus/packages/protocol/sisyphus/node/v1"
 	pb "github.com/sisyphus-network/Sisyphus/packages/protocol/sisyphus/v1"
 	"github.com/sisyphus-network/Sisyphus/packages/runtime"
+	"github.com/sisyphus-network/Sisyphus/packages/s3"
 	"github.com/sisyphus-network/Sisyphus/packages/sealed"
 	"github.com/sisyphus-network/Sisyphus/packages/storage"
 )
@@ -79,6 +80,11 @@ func runDaemon(ctx context.Context, args []string) error {
 	gcInterval := fs.Duration("gc-interval", time.Hour, "coordinator role: how often to delete stored data nothing is keeping; 0 never does")
 	maxStore := fs.Uint64("max-store-bytes", 0, "coordinator role: refuse uploads once stored data uses this much disk; 0 means no limit")
 	maxCache := fs.Uint64("max-cache-bytes", 0, "worker-only node: evict the least recently used cached blobs once the cache uses this much disk; 0 means no limit")
+	s3Endpoint := fs.String("s3-endpoint", "", "coordinator: keep stored data in a bucket of the S3-style object store at this address, such as https://s3.wasabisys.com or http://127.0.0.1:9000, instead of on this machine's disk")
+	s3Bucket := fs.String("s3-bucket", "", "the bucket, which must exist")
+	s3Prefix := fs.String("s3-prefix", "", "put this before the name of every object, to share the bucket with other things")
+	s3Region := fs.String("s3-region", "", "the bucket's region, for stores that have them")
+	s3Credentials := fs.String("s3-credentials", "", "a file with the access key on its first line and the secret key on its second; they stay on this node")
 	useKubo := fs.Bool("kubo", false, "keep stored data in a Kubo (IPFS) daemon that this node starts and runs alongside itself, on a private network with the rest of its pool; needs the ipfs program installed, and for a worker, a coordinator that uses it too")
 	swarmPort := fs.Int("swarm-port", 0, "coordinator role with --kubo: TCP port to open so that members' Kubo daemons can connect to this node's directly, which is faster; 0 opens none, and they reach it through --listen")
 	syncCache := fs.Bool("sync-cache", false, "worker-only node: wait for the disk when caching a blob; slower, but the cache then survives a power cut without downloading again")
@@ -128,6 +134,16 @@ func runDaemon(ctx context.Context, args []string) error {
 	if host, _, err := net.SplitHostPort(*webListen); *webListen != "" && (err != nil || !net.ParseIP(host).IsLoopback()) {
 		return errors.New("--web-listen must be a loopback address such as 127.0.0.1:50052: the local API is for this machine only, its web pages included")
 	}
+	if *s3Endpoint != "" {
+		switch {
+		case !isCoordinator:
+			return errors.New("--s3-endpoint is for a node that coordinates a pool: it is the pool's stored data that is kept there")
+		case *useKubo:
+			return errors.New("--s3-endpoint and --kubo each say where stored data is kept: choose one")
+		case *s3Bucket == "":
+			return errors.New("--s3-endpoint needs --s3-bucket, the bucket to keep stored data in")
+		}
+	}
 	if isCoordinator && *invitation != "" {
 		return errors.New("--join is for worker-only nodes")
 	}
@@ -243,6 +259,15 @@ func runDaemon(ctx context.Context, args []string) error {
 	// stopped.
 	var store *storage.Store
 	switch {
+	case *s3Endpoint != "":
+		bucket, err := openBucket(ctx, *s3Endpoint, *s3Bucket, *s3Prefix, *s3Region, *s3Credentials)
+		if err != nil {
+			return err
+		}
+		log.Info("stored data is kept in a bucket", "endpoint", *s3Endpoint, "bucket", *s3Bucket)
+		if store, err = storage.OpenBucket(bucket, filepath.Join(*dataDir, "s3-pins")); err != nil {
+			return fmt.Errorf("%w (nodes sharing a machine each need their own --data-dir)", err)
+		}
 	case isCoordinator && *useKubo:
 		store, err = storage.OpenKubo(sidecar, filepath.Join(*dataDir, "kubo-pins"))
 	case isCoordinator:
@@ -756,6 +781,25 @@ func withTrust(peers []*nodepb.Peer, takes []string, worksFor func(id string) bo
 	return peers
 }
 
+// openBucket opens the bucket a node keeps its stored data in, with the
+// credentials in a file: the access key on the first line and the secret
+// key on the second.
+func openBucket(ctx context.Context, endpoint, bucket, prefix, region, credentialsFile string) (*s3.Client, error) {
+	cfg := s3.Config{Endpoint: endpoint, Bucket: bucket, Prefix: prefix, Region: region}
+	if credentialsFile != "" {
+		keys, err := os.ReadFile(credentialsFile)
+		if err != nil {
+			return nil, fmt.Errorf("read the bucket's credentials: %w", err)
+		}
+		lines := strings.Fields(string(keys))
+		if len(lines) != 2 {
+			return nil, fmt.Errorf("%s should hold the access key on its first line and the secret key on its second", credentialsFile)
+		}
+		cfg.AccessKey, cfg.SecretKey = lines[0], lines[1]
+	}
+	return s3.Open(ctx, cfg)
+}
+
 // countryOf says which country an address is registered in. It is the
 // table the daemon carries; tests put another here.
 var countryOf = geo.Country
diff --git a/apps/sisyphusd/s3_test.go b/apps/sisyphusd/s3_test.go
new file mode 100644
index 0000000..43f6aa3
--- /dev/null
+++ b/apps/sisyphusd/s3_test.go
@@ -0,0 +1,156 @@
+package main
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"net/http"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/minio/minio-go/v7"
+	"github.com/minio/minio-go/v7/pkg/credentials"
+
+	"github.com/sisyphus-network/Sisyphus/packages/s3"
+)
+
+// objectStore is a real object store, SeaweedFS with its S3 gateway in a
+// container, started the first time a test wants it.
+var objectStore struct {
+	once     sync.Once
+	endpoint string
+	err      error
+}
+
+const (
+	objectStoreAccess = "sisyphus-test"
+	objectStoreSecret = "sisyphus-test-secret"
+)
+
+// bucketFor starts the object store if need be, makes a bucket in it for a
+// test, and returns where it is and the bucket's name. A machine with no
+// Docker skips the test, unless told that it must have it.
+func bucketFor(t *testing.T) (endpoint, bucket string) {
+	t.Helper()
+	objectStore.once.Do(func() {
+		lis, err := net.Listen("tcp", "127.0.0.1:0")
+		if err != nil {
+			objectStore.err = err
+			return
+		}
+		port := lis.Addr().(*net.TCPAddr).Port
+		lis.Close()
+		name := fmt.Sprintf("sisyphus-daemon-s3-test-%d", os.Getpid())
+		out, err := exec.Command("docker", "run", "-d", "--rm", "--name", name, "-p", fmt.Sprintf("127.0.0.1:%d:8333", port),
+			"-e", "AWS_ACCESS_KEY_ID="+objectStoreAccess, "-e", "AWS_SECRET_ACCESS_KEY="+objectStoreSecret,
+			"chrislusf/seaweedfs:latest", "server", "-s3", "-dir=/data").CombinedOutput()
+		if err != nil {
+			objectStore.err = fmt.Errorf("start the object store: %v: %s", err, bytes.TrimSpace(out))
+			return
+		}
+		stopAtExit = append(stopAtExit, func() { exec.Command("docker", "rm", "-f", name).Run() })
+		at := fmt.Sprintf("http://127.0.0.1:%d", port)
+		for deadline := time.Now().Add(90 * time.Second); time.Now().Before(deadline); time.Sleep(250 * time.Millisecond) {
+			if res, err := http.Get(at); err == nil {
+				res.Body.Close()
+				objectStore.endpoint = at
+				return
+			}
+		}
+		objectStore.err = errors.New("the object store did not start in time")
+	})
+	if objectStore.err != nil {
+		if os.Getenv("SISYPHUS_REQUIRE_DOCKER") != "" {
+			t.Fatalf("no object store to test against, and SISYPHUS_REQUIRE_DOCKER is set: %v", objectStore.err)
+		}
+		t.Skipf("no object store to test against: %v", objectStore.err)
+	}
+	api, err := minio.New(strings.TrimPrefix(objectStore.endpoint, "http://"), &minio.Options{Creds: credentials.NewStaticV4(objectStoreAccess, objectStoreSecret, "")})
+	if err != nil {
+		t.Fatal(err)
+	}
+	bucket = fmt.Sprintf("test-%d", time.Now().UnixNano())
+	// The gateway answers before it is ready to make buckets.
+	for deadline := time.Now().Add(60 * time.Second); ; time.Sleep(500 * time.Millisecond) {
+		if err = api.MakeBucket(context.Background(), bucket, minio.MakeBucketOptions{}); err == nil {
+			return objectStore.endpoint, bucket
+		}
+		if time.Now().After(deadline) {
+			t.Fatalf("make bucket: %v", err)
+		}
+	}
+}
+
+func TestACoordinatorKeepsItsStoredDataInABucket(t *testing.T) {
+	endpoint, bucket := bucketFor(t)
+	dataDir := t.TempDir()
+	keys := filepath.Join(t.TempDir(), "keys")
+	os.WriteFile(keys, []byte(objectStoreAccess+"\n"+objectStoreSecret+"\n"), 0o600)
+	addr := freeAddr(t)
+	args := []string{"--data-dir", dataDir, "--listen", addr, "--name", "rig", "--slots", "2",
+		"--s3-endpoint", endpoint, "--s3-bucket", bucket, "--s3-prefix", "pool-one", "--s3-credentials", keys}
+	stop := startDaemon(t, args...)
+	waitForOutput(t, "rig", "nodes", "--addr", addr)
+
+	input := filepath.Join(t.TempDir(), "words.txt")
+	os.WriteFile(input, []byte(strings.Repeat("the boulder rolls and the boulder rolls again\n", 2000)), 0o600)
+	cid := strings.TrimSpace(mustCLI(t, "blob", "put", "--addr", addr, input))
+	out := mustCLI(t, "job", "submit", "--addr", addr, "--workload", "wordcount", "--params", `{"input":"`+cid+`"}`, "--tasks", "2")
+	if !strings.Contains(out, `"words":16000`) {
+		t.Fatalf("a job over data kept in a bucket:\n%s", out)
+	}
+	// The data is in the bucket, under the prefix, and nowhere on this
+	// machine but for the record of what is kept.
+	client, err := s3.Open(context.Background(), s3.Config{Endpoint: endpoint, Bucket: bucket, Prefix: "pool-one", AccessKey: objectStoreAccess, SecretKey: objectStoreSecret})
+	if err != nil {
+		t.Fatal(err)
+	}
+	blocks := 0
+	client.List(context.Background(), func(name string, _ int64) bool {
+		if strings.HasPrefix(name, "blocks/") {
+			blocks++
+		}
+		return true
+	})
+	if blocks < 2 {
+		t.Errorf("the bucket holds %d blocks", blocks)
+	}
+	if _, err := os.Stat(filepath.Join(dataDir, "blobs")); !os.IsNotExist(err) {
+		t.Errorf("a store on this machine's disk beside the bucket: %v", err)
+	}
+	// It is all still there for a coordinator started again.
+	stop()
+	startDaemon(t, args...)
+	waitForOutput(t, "rig", "nodes", "--addr", addr)
+	if got := mustCLI(t, "blob", "stat", "--addr", addr, cid); !strings.Contains(got, "92000") {
+		t.Errorf("the file after a restart: %s", got)
+	}
+
+	oneLine := filepath.Join(t.TempDir(), "keys")
+	os.WriteFile(oneLine, []byte(objectStoreAccess+"\n"), 0o600)
+	inUse := []string{"--data-dir", dataDir, "--s3-endpoint", endpoint, "--s3-bucket", bucket, "--s3-prefix", "pool-one", "--s3-credentials", keys}
+	for _, tt := range []struct {
+		name string
+		args []string
+		want string
+	}{
+		{"no bucket", []string{"--data-dir", t.TempDir(), "--s3-endpoint", endpoint}, "needs --s3-bucket"},
+		{"with Kubo too", []string{"--data-dir", t.TempDir(), "--s3-endpoint", endpoint, "--s3-bucket", bucket, "--kubo"}, "choose one"},
+		{"on a node with no pool", []string{"--data-dir", t.TempDir(), "--role", "worker", "--coordinator", "127.0.0.1:1", "--s3-endpoint", endpoint, "--s3-bucket", bucket}, "is for a node that coordinates a pool"},
+		{"credentials that are not there", []string{"--data-dir", t.TempDir(), "--s3-endpoint", endpoint, "--s3-bucket", bucket, "--s3-credentials", filepath.Join(t.TempDir(), "absent")}, "read the bucket's credentials"},
+		{"credentials of one line", []string{"--data-dir", t.TempDir(), "--s3-endpoint", endpoint, "--s3-bucket", bucket, "--s3-credentials", oneLine}, "the access key on its first line and the secret key on its second"},
+		{"a bucket that is not there", []string{"--data-dir", t.TempDir(), "--s3-endpoint", endpoint, "--s3-bucket", "no-such-bucket-here", "--s3-credentials", keys}, "there is no bucket"},
+		{"a data directory in use", inUse, "in use by another process"},
+	} {
+		if _, err := cli(t, append([]string{"run", "--listen", freeAddr(t)}, tt.args...)...); err == nil || !strings.Contains(err.Error(), tt.want) {
+			t.Errorf("--s3-endpoint %s: %v, want %q", tt.name, err, tt.want)
+		}
+	}
+}
diff --git a/docs/development.md b/docs/development.md
index ebab44a..fb7a176 100644
--- a/docs/development.md
+++ b/docs/development.md
@@ -32,6 +32,7 @@ CI runs formatting, `go vet`, the tests under the race detector, `make cover` an
 | `packages/job-model` | Job and task state machines, and which blobs a job consumed and produced. No I/O, no locks. |
 | `packages/nodedb` | The node's SQLite database, where a coordinator keeps its jobs, its members and the invitations it has issued. Schema changes are new numbered files in `migrations/`, never edits to old ones. |
 | `packages/runtime` | The `Workload` interface, the built-in workloads, and the recorder that notes what a workload reads and writes. |
+| `packages/s3` | A bucket in an S3-style object store, for a store that keeps its blocks there. Its tests run against a real one, SeaweedFS in a container. |
 | `packages/sealed` | Encrypting a private job's blobs: the format, sealing and seekable unsealing. |
 | `packages/storage` | The blob store: IPFS-compatible import, pins, garbage collection, verification. |
 | `apps/sisyphusd/coordinator` | Scheduling, retries, aggregation, worker connections, pinning a job's data. |
diff --git a/go.mod b/go.mod
index 3fb7cd2..3810c92 100644
--- a/go.mod
+++ b/go.mod
@@ -14,6 +14,7 @@ require (
 	github.com/ipfs/go-ipld-format v0.6.4
 	github.com/libp2p/go-libp2p v0.50.0
 	github.com/libp2p/go-libp2p-kad-dht v0.42.2
+	github.com/minio/minio-go/v7 v7.3.0
 	github.com/modelcontextprotocol/go-sdk v1.8.0
 	github.com/multiformats/go-multiaddr v0.16.1
 	github.com/multiformats/go-multihash v0.2.3
@@ -63,7 +64,9 @@ require (
 	github.com/ipld/go-ipld-prime v0.24.0 // indirect
 	github.com/jackpal/go-nat-pmp v1.0.2 // indirect
 	github.com/jbenet/go-temp-err-catcher v0.1.0 // indirect
+	github.com/klauspost/compress v1.19.2 // indirect
 	github.com/klauspost/cpuid/v2 v2.4.0 // indirect
+	github.com/klauspost/crc32 v1.3.0 // indirect
 	github.com/koron/go-ssdp v0.9.1 // indirect
 	github.com/libp2p/go-buffer-pool v0.1.0 // indirect
 	github.com/libp2p/go-cidranger v1.1.0 // indirect
@@ -82,6 +85,8 @@ require (
 	github.com/miekg/dns v1.1.73 // indirect
 	github.com/mikioh/tcpinfo v0.0.0-20190314235526-30a79bb1804b // indirect
 	github.com/mikioh/tcpopt v0.0.0-20190314235656-172688c1accc // indirect
+	github.com/minio/crc64nvme v1.1.1 // indirect
+	github.com/minio/md5-simd v1.1.2 // indirect
 	github.com/minio/sha256-simd v1.0.1 // indirect
 	github.com/mr-tron/base58 v1.3.0 // indirect
 	github.com/multiformats/go-base32 v0.1.0 // indirect
@@ -96,6 +101,7 @@ require (
 	github.com/ncruces/go-strftime v1.0.0 // indirect
 	github.com/pb33f/ordered-map/v2 v2.3.1 // indirect
 	github.com/pbnjay/memory v0.0.0-20210728143218-7b4eea64cf58 // indirect
+	github.com/philhofer/fwd v1.2.0 // indirect
 	github.com/pion/datachannel v1.5.10 // indirect
 	github.com/pion/dtls/v3 v3.1.4 // indirect
 	github.com/pion/ice/v4 v4.0.10 // indirect
@@ -122,6 +128,7 @@ require (
 	github.com/quic-go/quic-go v0.62.0 // indirect
 	github.com/quic-go/webtransport-go v0.13.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
+	github.com/rs/xid v1.6.0 // indirect
 	github.com/segmentio/asm v1.1.3 // indirect
 	github.com/segmentio/encoding v0.5.4 // indirect
 	github.com/spaolacci/murmur3 v1.1.0 // indirect
@@ -130,10 +137,12 @@ require (
 	github.com/tidwall/match v1.1.1 // indirect
 	github.com/tidwall/pretty v1.2.1 // indirect
 	github.com/tidwall/sjson v1.2.5 // indirect
+	github.com/tinylib/msgp v1.6.4 // indirect
 	github.com/whyrusleeping/chunker v0.0.0-20181014151217-fe64bd25879f // indirect
 	github.com/whyrusleeping/go-keyspace v0.0.0-20160322163242-5b898ac5add1 // indirect
 	github.com/wlynxg/anet v0.0.5 // indirect
 	github.com/yosida95/uritemplate/v3 v3.0.2 // indirect
+	github.com/zeebo/xxh3 v1.1.0 // indirect
 	go.opentelemetry.io/auto/sdk v1.2.1 // indirect
 	go.opentelemetry.io/otel v1.46.0 // indirect
 	go.opentelemetry.io/otel/metric v1.46.0 // indirect
@@ -142,6 +151,7 @@ require (
 	go.uber.org/mock v0.6.0 // indirect
 	go.uber.org/multierr v1.11.0 // indirect
 	go.uber.org/zap v1.28.0 // indirect
+	go.yaml.in/yaml/v3 v3.0.5 // indirect
 	go.yaml.in/yaml/v4 v4.0.0-rc.2 // indirect
 	golang.org/x/crypto v0.57.0 // indirect
 	golang.org/x/exp v0.0.0-20260908205506-85c1c2202aba // indirect
@@ -157,6 +167,7 @@ require (
 	gonum.org/v1/gonum v0.17.0 // indirect
 	google.golang.org/genproto/googleapis/api v0.0.0-20260819154853-08b0e4226688 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20260819154853-08b0e4226688 // indirect
+	gopkg.in/ini.v1 v1.67.3 // indirect
 	lukechampine.com/blake3 v1.4.1 // indirect
 	modernc.org/libc v1.77.1 // indirect
 	modernc.org/mathutil v1.7.1 // indirect
diff --git a/go.sum b/go.sum
index 6e9ee80..39fd575 100644
--- a/go.sum
+++ b/go.sum
@@ -31,6 +31,7 @@ github.com/crackcomm/go-gitignore v0.0.0-20241020182519-7843d2ba8fdf/go.mod h1:p
 github.com/cskr/pubsub v1.0.2 h1:vlOzMhl6PFn60gRlTQQsIfVwaPB/B/8MziK8FhEPt/0=
 github.com/cskr/pubsub v1.0.2/go.mod h1:/8MzYXk/NJAz782G8RPkFzXTZVu63VotefPnR9TIRis=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davidlazar/go-crypto v0.0.0-20200604182044-b73af7476f6c h1:pFUpOrbxDR6AkioZ1ySsx5yxlDQZ8stG2b88gTPxgJU=
 github.com/davidlazar/go-crypto v0.0.0-20200604182044-b73af7476f6c/go.mod h1:6UhI8N9EjYm1c2odKpFpAYeR8dsBeM7PtzQhRgxRr9U=
 github.com/decred/dcrd/crypto/blake256 v1.1.0 h1:zPMNGQCm0g4QTY27fOCorQW7EryeQ/U0x++OzVrdms8=
@@ -138,8 +139,13 @@ github.com/jbenet/go-temp-err-catcher v0.1.0 h1:zpb3ZH6wIE8Shj2sKS+khgRvf7T7RABo
 github.com/jbenet/go-temp-err-catcher v0.1.0/go.mod h1:0kJRvmDZXNMIiJirNPEYfhpPwbGVtZVWC34vc5WLsDk=
 github.com/jtolds/gls v4.20.0+incompatible h1:xdiiI2gbIgH/gLH7ADydsJ1uDOEzR8yvV7C0MuV77Wo=
 github.com/jtolds/gls v4.20.0+incompatible/go.mod h1:QJZ7F/aHp+rZTRtaJ1ow/lLfFfVYBRgL+9YlvaHOwJU=
+github.com/klauspost/compress v1.19.2 h1:hMRETovs/pu/dVWN7zIT1PGG8t509MwT6bO7XSi26R8=
+github.com/klauspost/compress v1.19.2/go.mod h1:cwPg85FWrGar70rWktvGQj8/hthj3wpl0PGDogxkrSQ=
+github.com/klauspost/cpuid/v2 v2.0.1/go.mod h1:FInQzS24/EEf25PyTYn52gqo7WaD8xa0213Md/qVLRg=
 github.com/klauspost/cpuid/v2 v2.4.0 h1:S6Hrbc7+ywsr0r+RLapfGBHfyefhCTwEh3A0tV913Dw=
 github.com/klauspost/cpuid/v2 v2.4.0/go.mod h1:19jmZ9mjzoF//ddRSUsv0zfBTJWh3QJh9FNxZTMrGxU=
+github.com/klauspost/crc32 v1.3.0 h1:sSmTt3gUt81RP655XGZPElI0PelVTZ6YwCRnPSupoFM=
+github.com/klauspost/crc32 v1.3.0/go.mod h1:D7kQaZhnkX/Y0tstFGf8VUzv2UofNGqCjnC3zdHB0Hw=
 github.com/koron/go-ssdp v0.9.1 h1:zvxbAAuJftJIZ8Jh8mda+LI7V92hYZf/sKprmOxpxwA=
 github.com/koron/go-ssdp v0.9.1/go.mod h1:C43c047jWkDaeg9YuZlSh/QGqOieuWV6dbhWi/jcaLk=
 github.com/kr/pretty v0.2.1/go.mod h1:ipq/a2n7PKx3OHsz4KJII5eveXtPO4qwEXGdVfWzfnI=
@@ -195,6 +201,12 @@ github.com/mikioh/tcpinfo v0.0.0-20190314235526-30a79bb1804b/go.mod h1:lxPUiZwKo
 github.com/mikioh/tcpopt v0.0.0-20190314235656-172688c1accc h1:PTfri+PuQmWDqERdnNMiD9ZejrlswWrCpBEZgWOiTrc=
 github.com/mikioh/tcpopt v0.0.0-20190314235656-172688c1accc/go.mod h1:cGKTAVKx4SxOuR/czcZ/E2RSJ3sfHs8FpHhQ5CWMf9s=
 github.com/minio/blake2b-simd v0.0.0-20160723061019-3f5f724cb5b1/go.mod h1:pD8RvIylQ358TN4wwqatJ8rNavkEINozVn9DtGI3dfQ=
+github.com/minio/crc64nvme v1.1.1 h1:8dwx/Pz49suywbO+auHCBpCtlW1OfpcLN7wYgVR6wAI=
+github.com/minio/crc64nvme v1.1.1/go.mod h1:eVfm2fAzLlxMdUGc0EEBGSMmPwmXD5XiNRpnu9J3bvg=
+github.com/minio/md5-simd v1.1.2 h1:Gdi1DZK69+ZVMoNHRXJyNcxrMA4dSxoYHZSQbirFg34=
+github.com/minio/md5-simd v1.1.2/go.mod h1:MzdKDxYpY2BT9XQFocsiZf/NKVtR7nkE4RoEpN+20RM=
+github.com/minio/minio-go/v7 v7.3.0 h1:HM4pFCSQq/TK+j0/zmorSh5ddh81iDgRgU0BG0Vz/YU=
+github.com/minio/minio-go/v7 v7.3.0/go.mod h1:KUPWdecEO1LWyUz+sTGXAuf2jZHrPh5fCsRH86QbPfk=
 github.com/minio/sha256-simd v0.1.1-0.20190913151208-6de447530771/go.mod h1:B5e1o+1/KgNmWrSQK08Y6Z1Vb5pwIktudl0J58iy0KM=
 github.com/minio/sha256-simd v1.0.1 h1:6kaan5IFmwTNynnKKpDHe6FWHohJOHhCPchzK49dzMM=
 github.com/minio/sha256-simd v1.0.1/go.mod h1:Pz6AKMiUdngCLpeTL/RJY1M9rUuPMYujV5xJjtbRSN8=
@@ -233,6 +245,8 @@ github.com/pb33f/ordered-map/v2 v2.3.1 h1:5319HDO0aw4DA4gzi+zv4FXU9UlSs3xGZ40wcP
 github.com/pb33f/ordered-map/v2 v2.3.1/go.mod h1:qxFQgd0PkVUtOMCkTapqotNgzRhMPL7VvaHKbd1HnmQ=
 github.com/pbnjay/memory v0.0.0-20210728143218-7b4eea64cf58 h1:onHthvaw9LFnH4t2DcNVpwGmV9E1BkGknEliJkfwQj0=
 github.com/pbnjay/memory v0.0.0-20210728143218-7b4eea64cf58/go.mod h1:DXv8WO4yhMYhSNPKjeNKa5WY9YCIEBRbNzFFPJbWO6Y=
+github.com/philhofer/fwd v1.2.0 h1:e6DnBTl7vGY+Gz322/ASL4Gyp1FspeMvx1RNDoToZuM=
+github.com/philhofer/fwd v1.2.0/go.mod h1:RqIHx9QI14HlwKwm98g9Re5prTQ6LdeRQn+gXJFxsJM=
 github.com/pion/datachannel v1.5.10 h1:ly0Q26K1i6ZkGf42W7D4hQYR90pZwzFOjTq5AuCKk4o=
 github.com/pion/datachannel v1.5.10/go.mod h1:p/jJfC9arb29W7WrxyKbepTU20CFgyx5oLo8Rs4Py/M=
 github.com/pion/dtls/v3 v3.1.4 h1:QhvtMflMfu9Kf0RcDC5BJBle4caPskByrKQR6uuYqpY=
@@ -290,6 +304,8 @@ github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec h1:W09IVJc94
 github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec/go.mod h1:qqbHyh8v60DhA7CoWK5oRCqLrMHRGoxYCSS9EjAz6Eo=
 github.com/rogpeppe/go-internal v1.14.1 h1:UQB4HGPB6osV0SQTLymcB4TgvyWu6ZyliaW0tI/otEQ=
 github.com/rogpeppe/go-internal v1.14.1/go.mod h1:MaRKkUm5W0goXpeCfT7UZI6fk/L7L7so1lCWt35ZSgc=
+github.com/rs/xid v1.6.0 h1:fV591PaemRlL6JfRxGDEPl69wICngIQ3shQtzfy2gxU=
+github.com/rs/xid v1.6.0/go.mod h1:7XoLgs4eV+QndskICGsho+ADou8ySMSjJKDIan90Nz0=
 github.com/segmentio/asm v1.1.3 h1:WM03sfUOENvvKexOLp+pCqgb/WDjsi7EK8gIsICtzhc=
 github.com/segmentio/asm v1.1.3/go.mod h1:Ld3L4ZXGNcSLRg4JBsZ3//1+f/TjYl0Mzen/DQy1EJg=
 github.com/segmentio/encoding v0.5.4 h1:OW1VRern8Nw6ITAtwSZ7Idrl3MXCFwXHPgqESYfvNt0=
@@ -303,7 +319,14 @@ github.com/spaolacci/murmur3 v1.1.0/go.mod h1:JwIasOWyU6f++ZhiEuf87xNszmSA2myDM2
 github.com/standard-webhooks/standard-webhooks/libraries v0.0.1 h1:uOfcYT+3QungH6tIGSVCR/Y3KJmgJiHcojJbMTPDZAI=
 github.com/standard-webhooks/standard-webhooks/libraries v0.0.1/go.mod h1:L1MQhA6x4dn9r007T033lsaZMv9EmBAdXyU/+EF40fo=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/objx v0.4.0/go.mod h1:YvHI0jy2hoMjB+UWwv71VJQ9isScKT/TqJzVSSt89Yw=
+github.com/stretchr/objx v0.5.0/go.mod h1:Yh+to48EsGEfYuaHDzXPcE3xhTkx73EhmCGUpEOglKo=
+github.com/stretchr/objx v0.5.2/go.mod h1:FRsXN1f5AsAjCGJKqEizvkpNtU+EGNCLh3NxZ/8L+MA=
 github.com/stretchr/testify v1.4.0/go.mod h1:j7eGeouHqKxXV5pUuKE4zz7dFj8WfuZ+81PSLYec5m4=
+github.com/stretchr/testify v1.7.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+github.com/stretchr/testify v1.8.0/go.mod h1:yNjHg4UonilssWZ8iaSj1OCr/vHnekPRkoO+kdMU+MU=
+github.com/stretchr/testify v1.8.4/go.mod h1:sz/lmYIOXD/1dqDmKjjqLyZ2RngseejIcXlSw2iwfAo=
+github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
 github.com/stretchr/testify v1.12.1 h1:EuwCh5fleGS7H32xRwO3wRGT7DxrDhLAT6FF8MpWDWE=
 github.com/stretchr/testify v1.12.1/go.mod h1:MDEgiDPPsNp5cuIrHPPCyornHKgEVbtFUmoNlxoYthg=
 github.com/syndtr/goleveldb v1.0.0 h1:fBdIW9lB4Iz0n9khmH8w27SJ3QEJ7+IgjPEwGSZiFdE=
@@ -318,6 +341,8 @@ github.com/tidwall/pretty v1.2.1 h1:qjsOFOWWQl+N3RsoF5/ssm1pHmJJwhjlSbZ51I6wMl4=
 github.com/tidwall/pretty v1.2.1/go.mod h1:ITEVvHYasfjBbM0u2Pg8T2nJnzm8xPwvNhhsoaGGjNU=
 github.com/tidwall/sjson v1.2.5 h1:kLy8mja+1c9jlljvWTlSazM7cKDRfJuR/bOJhcY5NcY=
 github.com/tidwall/sjson v1.2.5/go.mod h1:Fvgq9kS/6ociJEDnK0Fk1cpYF4FIW6ZF7LAe+6jwd28=
+github.com/tinylib/msgp v1.6.4 h1:mOwYbyYDLPj35mkA2BjjYejgJk9BuHxDdvRnb6v2ZcQ=
+github.com/tinylib/msgp v1.6.4/go.mod h1:RSp0LW9oSxFut3KzESt5Voq4GVWyS+PSulT77roAqEA=
 github.com/warpfork/go-testmark v0.12.1 h1:rMgCpJfwy1sJ50x0M0NgyphxYYPMOODIJHhsXyEHU0s=
 github.com/warpfork/go-testmark v0.12.1/go.mod h1:kHwy7wfvGSPh1rQJYKayD4AbtNaeyZdcGi9tNJTaa5Y=
 github.com/whyrusleeping/chunker v0.0.0-20181014151217-fe64bd25879f h1:jQa4QT2UP9WYv2nzyawpKMOCl+Z/jW7djv2/J50lj9E=
@@ -328,6 +353,10 @@ github.com/wlynxg/anet v0.0.5 h1:J3VJGi1gvo0JwZ/P1/Yc/8p63SoW98B5dHkYDmpgvvU=
 github.com/wlynxg/anet v0.0.5/go.mod h1:eay5PRQr7fIVAMbTbchTnO9gG65Hg/uYGdc7mguHxoA=
 github.com/yosida95/uritemplate/v3 v3.0.2 h1:Ed3Oyj9yrmi9087+NczuL5BwkIc4wvTb5zIM+UJPGz4=
 github.com/yosida95/uritemplate/v3 v3.0.2/go.mod h1:ILOh0sOhIJR3+L/8afwt/kE++YT040gmv5BQTMR2HP4=
+github.com/zeebo/assert v1.3.0 h1:g7C04CbJuIDKNPFHmsk4hwZDO5O+kntRxzaUoNXj+IQ=
+github.com/zeebo/assert v1.3.0/go.mod h1:Pq9JiuJQpG8JLJdtkwrJESF0Foym2/D9XMU5ciN/wJ0=
+github.com/zeebo/xxh3 v1.1.0 h1:s7DLGDK45Dyfg7++yxI0khrfwq9661w9EN78eP/UZVs=
+github.com/zeebo/xxh3 v1.1.0/go.mod h1:IisAie1LELR4xhVinxWS5+zf1lA4p0MW4T+w+W07F5s=
 go.opentelemetry.io/auto/sdk v1.2.1 h1:jXsnJ4Lmnqd11kwkBV2LgLoFMZKizbCi5fNZ/ipaZ64=
 go.opentelemetry.io/auto/sdk v1.2.1/go.mod h1:KRTj+aOaElaLi+wW1kO/DZRXwkF4C5xPbEe3ZiIhN7Y=
 go.opentelemetry.io/otel v1.46.0 h1:FHt5/CDyVxi/8IM1CH7VE/rRgq3kLHa2mSTVMO8AWyc=
@@ -419,9 +448,13 @@ google.golang.org/protobuf v1.36.12/go.mod h1:HTf+CrKn2C3g5S8VImy6tdcUvCska2kB7j
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c h1:Hei/4ADfdWqJk1ZMxUNpqntNwaWcugrBjAiHlqqRiVk=
 gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c/go.mod h1:JHkPIbrfpd72SG/EVd6muEfDQjcINNoR0C8j2r3qZ4Q=
+gopkg.in/ini.v1 v1.67.3 h1:iM9Lhz5MRSGhHVGGwCuzG9KO8PoirCXj/m/qTmOJJQw=
+gopkg.in/ini.v1 v1.67.3/go.mod h1:x/cyOwCgZqOkJoDIJ3c1KNHMo10+nLGAhh+kn3Zizss=
 gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.4.0 h1:D8xgwECY7CYvx+Y2n4sBz93Jn9JRvxdiyyo8CTfuKaY=
 gopkg.in/yaml.v2 v2.4.0/go.mod h1:RDklbk79AGWmwhnvt/jBztapEOGDOx6ZbXqjP6csGnQ=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 lukechampine.com/blake3 v1.4.1 h1:I3Smz7gso8w4/TunLKec6K2fn+kyKtDxr/xcQEN84Wg=
 lukechampine.com/blake3 v1.4.1/go.mod h1:QFosUxmjB8mnrWFSNwKmvxHpfY72bmD2tQ0kBMM3kwo=
 modernc.org/cc/v4 v4.29.7 h1:q+NXGJ0bK3b4TXFYQQVr9pYETGnmwFWkrUzJnMya/Tg=
diff --git a/packages/s3/s3.go b/packages/s3/s3.go
new file mode 100644
index 0000000..5690e2d
--- /dev/null
+++ b/packages/s3/s3.go
@@ -0,0 +1,125 @@
+// Package s3 reaches a bucket in an object store that speaks Amazon's S3
+// dialect, which most do: Ceph's gateway, MinIO, SeaweedFS, Storj's
+// gateway, Wasabi, and AWS itself.
+package s3
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+
+	"github.com/minio/minio-go/v7"
+	"github.com/minio/minio-go/v7/pkg/credentials"
+
+	"github.com/sisyphus-network/Sisyphus/packages/storage"
+)
+
+// Config says which bucket, where, and as whom.
+type Config struct {
+	// Endpoint is where the object store is, such as
+	// https://s3.eu-central-1.wasabisys.com or http://127.0.0.1:9000. One
+	// with no scheme is taken to be https.
+	Endpoint string
+	Bucket   string
+	// Prefix, if set, goes before the name of every object, so that a
+	// bucket can be shared with other things.
+	Prefix string
+	// Region is the region the bucket is in, for stores that have them.
+	Region string
+	// AccessKey and SecretKey are the credentials. They stay on this node.
+	AccessKey, SecretKey string
+}
+
+// Client is a bucket. It is a storage.Bucket.
+type Client struct {
+	api    *minio.Client
+	bucket string
+	prefix string
+}
+
+// Open returns the bucket cfg describes, having checked that it is there
+// and can be reached with the credentials given.
+func Open(ctx context.Context, cfg Config) (*Client, error) {
+	endpoint := cfg.Endpoint
+	if !strings.Contains(endpoint, "://") {
+		endpoint = "https://" + endpoint
+	}
+	at, err := url.Parse(endpoint)
+	if err != nil || at.Host == "" {
+		return nil, fmt.Errorf("object store address %q is not one", cfg.Endpoint)
+	}
+	if cfg.Bucket == "" {
+		return nil, fmt.Errorf("no bucket was named")
+	}
+	api, err := minio.New(at.Host, &minio.Options{
+		Creds:  credentials.NewStaticV4(cfg.AccessKey, cfg.SecretKey, ""),
+		Secure: at.Scheme == "https",
+		Region: cfg.Region,
+	})
+	if err != nil {
+		return nil, fmt.Errorf("object store at %s: %w", cfg.Endpoint, err)
+	}
+	found, err := api.BucketExists(ctx, cfg.Bucket)
+	if err != nil {
+		return nil, fmt.Errorf("reach bucket %s at %s: %w", cfg.Bucket, cfg.Endpoint, err)
+	}
+	if !found {
+		return nil, fmt.Errorf("there is no bucket %s at %s: make it first, this does not", cfg.Bucket, cfg.Endpoint)
+	}
+	prefix := strings.Trim(cfg.Prefix, "/")
+	if prefix != "" {
+		prefix += "/"
+	}
+	return &Client{api: api, bucket: cfg.Bucket, prefix: prefix}, nil
+}
+
+func (c *Client) Put(ctx context.Context, name string, data []byte) error {
+	_, err := c.api.PutObject(ctx, c.bucket, c.prefix+name, bytes.NewReader(data), int64(len(data)), minio.PutObjectOptions{ContentType: "application/octet-stream"})
+	return err
+}
+
+func (c *Client) Get(ctx context.Context, name string) ([]byte, error) {
+	// Opening an object asks for nothing: what is wrong with it is found
+	// on reading it.
+	object, _ := c.api.GetObject(ctx, c.bucket, c.prefix+name, minio.GetObjectOptions{})
+	defer object.Close()
+	data, err := io.ReadAll(object)
+	return data, absent(err)
+}
+
+func (c *Client) Size(ctx context.Context, name string) (int, error) {
+	info, err := c.api.StatObject(ctx, c.bucket, c.prefix+name, minio.StatObjectOptions{})
+	return int(info.Size), absent(err)
+}
+
+func (c *Client) Remove(ctx context.Context, name string) error {
+	return c.api.RemoveObject(ctx, c.bucket, c.prefix+name, minio.RemoveObjectOptions{})
+}
+
+func (c *Client) List(ctx context.Context, visit func(name string, size int64) bool) error {
+	// Stopping the listing early needs its context ended, or it goes on
+	// listing to nobody.
+	ctx, stop := context.WithCancel(ctx)
+	defer stop()
+	for object := range c.api.ListObjects(ctx, c.bucket, minio.ListObjectsOptions{Prefix: c.prefix, Recursive: true}) {
+		if object.Err != nil {
+			return object.Err
+		}
+		if !visit(strings.TrimPrefix(object.Key, c.prefix), object.Size) {
+			return nil
+		}
+	}
+	return nil
+}
+
+// absent turns the object store's "not there" into the one a store knows.
+func absent(err error) error {
+	if minio.ToErrorResponse(err).StatusCode == http.StatusNotFound {
+		return storage.ErrNoObject
+	}
+	return err
+}
diff --git a/packages/s3/s3_test.go b/packages/s3/s3_test.go
new file mode 100644
index 0000000..85ff7c5
--- /dev/null
+++ b/packages/s3/s3_test.go
@@ -0,0 +1,209 @@
+package s3
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"net/http"
+	"os"
+	"os/exec"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/minio/minio-go/v7"
+	"github.com/minio/minio-go/v7/pkg/credentials"
+
+	"github.com/sisyphus-network/Sisyphus/packages/storage"
+)
+
+// objectStore is a real object store for the tests to use: SeaweedFS with
+// its S3 gateway, in a container, started once and stopped when the tests
+// are over.
+var objectStore struct {
+	once     sync.Once
+	endpoint string
+	err      error
+}
+
+const (
+	storeImage = "chrislusf/seaweedfs:latest"
+	accessKey  = "sisyphus-test"
+	secretKey  = "sisyphus-test-secret"
+)
+
+func TestMain(m *testing.M) {
+	code := m.Run()
+	if objectStore.endpoint != "" {
+		exec.Command("docker", "rm", "-f", containerName()).Run()
+	}
+	os.Exit(code)
+}
+
+func containerName() string { return fmt.Sprintf("sisyphus-s3-test-%d", os.Getpid()) }
+
+// endpoint returns where the object store is, starting it if need be. A
+// machine with no Docker skips the test, unless told that it must have it.
+func endpoint(t *testing.T) string {
+	t.Helper()
+	objectStore.once.Do(func() {
+		lis, err := net.Listen("tcp", "127.0.0.1:0")
+		if err != nil {
+			objectStore.err = err
+			return
+		}
+		port := lis.Addr().(*net.TCPAddr).Port
+		lis.Close()
+		out, err := exec.Command("docker", "run", "-d", "--rm", "--name", containerName(), "-p", fmt.Sprintf("127.0.0.1:%d:8333", port),
+			"-e", "AWS_ACCESS_KEY_ID="+accessKey, "-e", "AWS_SECRET_ACCESS_KEY="+secretKey,
+			storeImage, "server", "-s3", "-dir=/data").CombinedOutput()
+		if err != nil {
+			objectStore.err = fmt.Errorf("start %s: %v: %s", storeImage, err, bytes.TrimSpace(out))
+			return
+		}
+		at := fmt.Sprintf("http://127.0.0.1:%d", port)
+		for deadline := time.Now().Add(90 * time.Second); time.Now().Before(deadline); time.Sleep(250 * time.Millisecond) {
+			if res, err := http.Get(at); err == nil {
+				res.Body.Close()
+				objectStore.endpoint = at
+				return
+			}
+		}
+		objectStore.err = errors.New("the object store did not start in time")
+	})
+	if objectStore.err != nil {
+		if os.Getenv("SISYPHUS_REQUIRE_DOCKER") != "" {
+			t.Fatalf("no object store to test against, and SISYPHUS_REQUIRE_DOCKER is set: %v", objectStore.err)
+		}
+		t.Skipf("no object store to test against: %v", objectStore.err)
+	}
+	return objectStore.endpoint
+}
+
+// newBucket makes a bucket of its own for a test and returns its name.
+func newBucket(t *testing.T) string {
+	t.Helper()
+	at := endpoint(t)
+	api, err := minio.New(strings.TrimPrefix(at, "http://"), &minio.Options{Creds: credentials.NewStaticV4(accessKey, secretKey, "")})
+	if err != nil {
+		t.Fatal(err)
+	}
+	name := fmt.Sprintf("test-%d", time.Now().UnixNano())
+	// The gateway answers before it is ready to make buckets.
+	for deadline := time.Now().Add(60 * time.Second); ; time.Sleep(500 * time.Millisecond) {
+		if err = api.MakeBucket(context.Background(), name, minio.MakeBucketOptions{}); err == nil {
+			return name
+		}
+		if time.Now().After(deadline) {
+			t.Fatalf("make bucket: %v", err)
+		}
+	}
+}
+
+func TestBlocksAreKeptInARealObjectStore(t *testing.T) {
+	ctx := context.Background()
+	bucket := newBucket(t)
+	client, err := Open(ctx, Config{Endpoint: endpoint(t), Bucket: bucket, Prefix: "/pool-one/", AccessKey: accessKey, SecretKey: secretKey})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := client.Put(ctx, "blocks/a", []byte("the boulder")); err != nil {
+		t.Fatal(err)
+	}
+	if err := client.Put(ctx, "blocks/b", []byte("rolls")); err != nil {
+		t.Fatal(err)
+	}
+	if got, err := client.Get(ctx, "blocks/a"); err != nil || string(got) != "the boulder" {
+		t.Errorf("Get = %q, %v", got, err)
+	}
+	if size, err := client.Size(ctx, "blocks/b"); err != nil || size != 5 {
+		t.Errorf("Size = %d, %v", size, err)
+	}
+	// What is not there is said to be not there, in the store's own words.
+	if _, err := client.Get(ctx, "blocks/absent"); !errors.Is(err, storage.ErrNoObject) {
+		t.Errorf("Get of nothing: %v", err)
+	}
+	if _, err := client.Size(ctx, "blocks/absent"); !errors.Is(err, storage.ErrNoObject) {
+		t.Errorf("Size of nothing: %v", err)
+	}
+	// Another prefix in the same bucket is another store's, and unseen.
+	other, err := Open(ctx, Config{Endpoint: endpoint(t), Bucket: bucket, AccessKey: accessKey, SecretKey: secretKey})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := other.Put(ctx, "elsewhere", []byte("x")); err != nil {
+		t.Fatal(err)
+	}
+	listed := map[string]int64{}
+	if err := client.List(ctx, func(name string, size int64) bool { listed[name] = size; return true }); err != nil || len(listed) != 2 || listed["blocks/a"] != 11 || listed["blocks/b"] != 5 {
+		t.Errorf("List = %v, %v", listed, err)
+	}
+	// A listing can be stopped part way.
+	seen := 0
+	if err := client.List(ctx, func(string, int64) bool { seen++; return false }); err != nil || seen != 1 {
+		t.Errorf("a listing stopped at once saw %d, %v", seen, err)
+	}
+	if err := client.Remove(ctx, "blocks/a"); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := client.Size(ctx, "blocks/a"); !errors.Is(err, storage.ErrNoObject) {
+		t.Errorf("Size of what was removed: %v", err)
+	}
+
+	// And a whole store over it: a blob put, read back and verified.
+	store, err := storage.OpenBucket(client, t.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer store.Close()
+	blob := bytes.Repeat([]byte("one must imagine Sisyphus happy. "), 20_000)
+	c, err := store.Put(ctx, bytes.NewReader(blob))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := store.Verify(ctx, c); err != nil {
+		t.Errorf("Verify: %v", err)
+	}
+	if size, err := store.Size(ctx); err != nil || size < uint64(len(blob)) {
+		t.Errorf("Size = %d, %v", size, err)
+	}
+}
+
+func TestABucketThatCannotBeUsedIsSaidSo(t *testing.T) {
+	ctx := context.Background()
+	at := endpoint(t)
+	bucket := newBucket(t)
+	soon, done := context.WithTimeout(ctx, 3*time.Second)
+	defer done()
+	for name, tt := range map[string]struct {
+		cfg  Config
+		want string
+	}{
+		"an address that is none":        {Config{Endpoint: "http://", Bucket: bucket}, "is not one"},
+		"an address that cannot be read": {Config{Endpoint: "http://a b", Bucket: bucket}, "is not one"},
+		"no bucket named":                {Config{Endpoint: at}, "no bucket was named"},
+		"a host that is no host":         {Config{Endpoint: "http://-not_a_host-:9", Bucket: bucket}, "object store at"},
+		"a bucket that is not there":     {Config{Endpoint: at, Bucket: "no-such-bucket-here", AccessKey: accessKey, SecretKey: secretKey}, "there is no bucket no-such-bucket-here"},
+		"nothing at the address":         {Config{Endpoint: "127.0.0.1:1", Bucket: bucket}, "reach bucket"},
+	} {
+		if _, err := Open(soon, tt.cfg); err == nil || !strings.Contains(err.Error(), tt.want) {
+			t.Errorf("%s: %v, want %q", name, err, tt.want)
+		}
+	}
+	// A listing that fails part way is a failure, not a short list.
+	client, err := Open(ctx, Config{Endpoint: at, Bucket: bucket, AccessKey: accessKey, SecretKey: secretKey})
+	if err != nil {
+		t.Fatal(err)
+	}
+	gone, cancel := context.WithCancel(ctx)
+	cancel()
+	if err := client.List(gone, func(string, int64) bool { return true }); err == nil {
+		t.Error("a listing that could not be made was taken for an empty one")
+	}
+	if err := client.Put(gone, "x", []byte("x")); err == nil {
+		t.Error("a put that could not be made was taken for done")
+	}
+}
diff --git a/packages/storage/s3.go b/packages/storage/s3.go
new file mode 100644
index 0000000..874fbfd
--- /dev/null
+++ b/packages/storage/s3.go
@@ -0,0 +1,162 @@
+package storage
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/gofrs/flock"
+	blocks "github.com/ipfs/go-block-format"
+	"github.com/ipfs/go-cid"
+	ipld "github.com/ipfs/go-ipld-format"
+)
+
+// Bucket is the part of an S3-style object store that a store keeps its
+// blocks in: one object for each block, named by the block's CID. An
+// s3.Client is one.
+type Bucket interface {
+	Put(ctx context.Context, name string, data []byte) error
+	// Get returns an object, and Size how long it is; both return
+	// ErrNoObject for one that is not there.
+	Get(ctx context.Context, name string) ([]byte, error)
+	Size(ctx context.Context, name string) (int, error)
+	Remove(ctx context.Context, name string) error
+	// List calls visit with the name and size of every object, until it
+	// returns false.
+	List(ctx context.Context, visit func(name string, size int64) bool) error
+}
+
+// ErrNoObject is what a Bucket returns for an object it does not hold.
+var ErrNoObject = errors.New("no such object")
+
+// OpenBucket opens a durable store whose blocks are kept as objects in a
+// bucket, for a pool that runs or rents object storage: Ceph, MinIO,
+// SeaweedFS, Storj, Wasabi, AWS. A blob has the same CID wherever its bytes
+// are, so nothing else need know. Pins are this store's own, kept in dir as
+// OpenLocal keeps them.
+//
+// A block is an object and reading one is a request, so a store in a bucket
+// far away is slower than a disk: a blob of a gigabyte is four thousand
+// blocks. Nothing is kept on this machine but the pins.
+func OpenBucket(bucket Bucket, dir string) (*Store, error) {
+	if err := os.MkdirAll(dir, 0o700); err != nil {
+		return nil, fmt.Errorf("open blob store %s: %w", dir, err)
+	}
+	lock := flock.New(filepath.Join(dir, "LOCK"))
+	locked, err := lock.TryLock()
+	if err != nil {
+		return nil, fmt.Errorf("open blob store %s: %w", dir, err)
+	}
+	if !locked {
+		return nil, fmt.Errorf("open blob store %s: in use by another process", dir)
+	}
+	held := bucketBlocks{bucket}
+	store := newStoreOver(held, false)
+	store.size = held.size
+	store.close = func() error { return nil } // the bucket is its owner's to close
+	store.lock = lock
+	store.pinFile = filepath.Join(dir, "pins.json")
+	if store.pins, err = loadPins(store.pinFile); err != nil {
+		store.Close()
+		return nil, fmt.Errorf("open blob store %s: %w", dir, err)
+	}
+	return store, nil
+}
+
+// bucketBlocks keeps blocks as objects in a bucket.
+type bucketBlocks struct {
+	bucket Bucket
+}
+
+// blockPrefix begins the name of every object that is a block, so that a
+// bucket may hold other things beside them and they are left alone.
+const blockPrefix = "blocks/"
+
+func objectName(c cid.Cid) string { return blockPrefix + c.String() }
+
+func (b bucketBlocks) Put(ctx context.Context, block blocks.Block) error {
+	return b.bucket.Put(ctx, objectName(block.Cid()), block.RawData())
+}
+
+func (b bucketBlocks) PutMany(ctx context.Context, many []blocks.Block) error {
+	for _, block := range many {
+		if err := b.Put(ctx, block); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (b bucketBlocks) Get(ctx context.Context, c cid.Cid) (blocks.Block, error) {
+	data, err := b.bucket.Get(ctx, objectName(c))
+	if err != nil {
+		return nil, b.translate(err, c)
+	}
+	return blocks.NewBlockWithCid(data, c)
+}
+
+func (b bucketBlocks) GetSize(ctx context.Context, c cid.Cid) (int, error) {
+	size, err := b.bucket.Size(ctx, objectName(c))
+	return size, b.translate(err, c)
+}
+
+func (b bucketBlocks) Has(ctx context.Context, c cid.Cid) (bool, error) {
+	_, err := b.bucket.Size(ctx, objectName(c))
+	if errors.Is(err, ErrNoObject) {
+		return false, nil
+	}
+	return err == nil, err
+}
+
+func (b bucketBlocks) DeleteBlock(ctx context.Context, c cid.Cid) error {
+	return b.bucket.Remove(ctx, objectName(c))
+}
+
+// AllKeysChan lists the blocks in the bucket. The listing is read to its
+// end before anything is handed on, so that a failure part-way is reported
+// and does not pass for a short list, which a garbage collection would act
+// on.
+func (b bucketBlocks) AllKeysChan(ctx context.Context) (<-chan cid.Cid, error) {
+	var all []cid.Cid
+	err := b.bucket.List(ctx, func(name string, _ int64) bool {
+		if id, is := strings.CutPrefix(name, blockPrefix); is {
+			if c, err := cid.Decode(id); err == nil {
+				all = append(all, c)
+			}
+		}
+		return true
+	})
+	if err != nil {
+		return nil, err
+	}
+	keys := make(chan cid.Cid, len(all))
+	for _, c := range all {
+		keys <- c
+	}
+	close(keys)
+	return keys, nil
+}
+
+// size adds up the blocks in the bucket.
+func (b bucketBlocks) size(ctx context.Context) (uint64, error) {
+	var total uint64
+	err := b.bucket.List(ctx, func(name string, size int64) bool {
+		if strings.HasPrefix(name, blockPrefix) {
+			total += uint64(size)
+		}
+		return true
+	})
+	return total, err
+}
+
+// translate turns the bucket's "not there" into the one the rest of the
+// store recognises.
+func (bucketBlocks) translate(err error, c cid.Cid) error {
+	if errors.Is(err, ErrNoObject) {
+		return ipld.ErrNotFound{Cid: c}
+	}
+	return err
+}
diff --git a/packages/storage/s3_test.go b/packages/storage/s3_test.go
new file mode 100644
index 0000000..3f37858
--- /dev/null
+++ b/packages/storage/s3_test.go
@@ -0,0 +1,209 @@
+package storage
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"io"
+	"os"
+	"path/filepath"
+	"sort"
+	"testing"
+	"time"
+
+	blocks "github.com/ipfs/go-block-format"
+)
+
+// memoryBucket is a bucket kept in memory, which can be made to fail.
+type memoryBucket struct {
+	objects map[string][]byte
+	fail    error
+}
+
+var errBucket = errors.New("the object store is down")
+
+func newBucket() *memoryBucket { return &memoryBucket{objects: map[string][]byte{}} }
+
+func (b *memoryBucket) Put(_ context.Context, name string, data []byte) error {
+	if b.fail != nil {
+		return b.fail
+	}
+	b.objects[name] = bytes.Clone(data)
+	return nil
+}
+
+func (b *memoryBucket) Get(_ context.Context, name string) ([]byte, error) {
+	data, held := b.objects[name]
+	if b.fail != nil {
+		return nil, b.fail
+	}
+	if !held {
+		return nil, ErrNoObject
+	}
+	return data, nil
+}
+
+func (b *memoryBucket) Size(ctx context.Context, name string) (int, error) {
+	data, err := b.Get(ctx, name)
+	return len(data), err
+}
+
+func (b *memoryBucket) Remove(_ context.Context, name string) error {
+	delete(b.objects, name)
+	return b.fail
+}
+
+func (b *memoryBucket) List(_ context.Context, visit func(string, int64) bool) error {
+	if b.fail != nil {
+		return b.fail
+	}
+	names := make([]string, 0, len(b.objects))
+	for name := range b.objects {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	for _, name := range names {
+		if !visit(name, int64(len(b.objects[name]))) {
+			break
+		}
+	}
+	return nil
+}
+
+func openBucket(t *testing.T, bucket Bucket, dir string) *Store {
+	t.Helper()
+	s, err := OpenBucket(bucket, dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { s.Close() })
+	return s
+}
+
+func TestAStoreInABucketKeepsEachBlockAsAnObject(t *testing.T) {
+	bucket := newBucket()
+	// The bucket may hold other things, which are not the store's.
+	bucket.objects["notes.txt"] = []byte("someone else's")
+	bucket.objects["blocks/not-a-cid"] = []byte("nor is this a block")
+	dir := t.TempDir()
+	s := openBucket(t, bucket, dir)
+
+	a := put(t, s, blobA)
+	blob, err := s.Open(ctx, a)
+	if err != nil {
+		t.Fatal(err)
+	}
+	read, _ := io.ReadAll(blob)
+	blob.Close()
+	if !bytes.Equal(read, blobA) || blob.Size() != uint64(len(blobA)) {
+		t.Fatalf("read back %d bytes of %d", len(read), len(blobA))
+	}
+	// 700,000 bytes is three blocks and a node that lists them.
+	if len(bucket.objects) != 6 {
+		t.Errorf("the bucket holds %d objects", len(bucket.objects))
+	}
+	if held, err := s.Has(ctx, a); err != nil || !held {
+		t.Errorf("Has = %v, %v", held, err)
+	}
+	if err := s.Verify(ctx, a); err != nil {
+		t.Errorf("Verify: %v", err)
+	}
+	// Its size is that of its blocks, not of what else is in the bucket.
+	if size, err := s.Size(ctx); err != nil || size < uint64(len(blobA)) || size > uint64(len(blobA))+1000 {
+		t.Errorf("Size = %d, %v", size, err)
+	}
+
+	// Pins and collection work as on any store, and are kept beside it:
+	// what is pinned stays, what is not goes, and what is not the store's
+	// is left alone.
+	b := put(t, s, blobB)
+	if err := s.Pin(ctx, "someone", time.Time{}, a); err != nil {
+		t.Fatal(err)
+	}
+	collected, err := s.GC(ctx, afterGrace())
+	if err != nil || collected.Blocks == 0 {
+		t.Fatalf("GC = %+v, %v", collected, err)
+	}
+	if held, _ := s.Has(ctx, b); held {
+		t.Error("an unpinned blob outlived the collection")
+	}
+	if _, err := s.Open(ctx, b); !errors.Is(err, ErrNotFound) {
+		t.Errorf("opening what was collected: %v", err)
+	}
+	if held, _ := s.Has(ctx, a); !held || string(bucket.objects["notes.txt"]) != "someone else's" {
+		t.Error("the collection took what it should have left")
+	}
+	s.Close()
+	again := openBucket(t, bucket, dir)
+	if pins := again.Pins(); len(pins) != 1 || pins[0].CID != a {
+		t.Errorf("pins after reopening: %v", pins)
+	}
+}
+
+func TestAStoreInABucketReportsABucketThatFails(t *testing.T) {
+	bucket := newBucket()
+	s := openBucket(t, bucket, t.TempDir())
+	c := put(t, s, blobA)
+	bucket.fail = errBucket
+	if _, err := s.Put(ctx, bytes.NewReader([]byte("more"))); !errors.Is(err, errBucket) {
+		t.Errorf("Put: %v", err)
+	}
+	if _, err := s.Open(ctx, c); !errors.Is(err, errBucket) {
+		t.Errorf("Open: %v", err)
+	}
+	if _, err := s.Has(ctx, c); !errors.Is(err, errBucket) {
+		t.Errorf("Has: %v", err)
+	}
+	if _, err := s.GC(ctx, afterGrace()); !errors.Is(err, errBucket) {
+		t.Errorf("GC: %v", err)
+	}
+	if _, err := s.Size(ctx); !errors.Is(err, errBucket) {
+		t.Errorf("Size: %v", err)
+	}
+	held := bucketBlocks{bucket}
+	if _, err := held.GetSize(ctx, c); !errors.Is(err, errBucket) {
+		t.Errorf("GetSize: %v", err)
+	}
+	bucket.fail = nil
+	if size, err := held.GetSize(ctx, c); err != nil || size == 0 {
+		t.Errorf("GetSize = %d, %v", size, err)
+	}
+	// Several blocks at once are stored one after another, and stop at
+	// the first that cannot be.
+	block, _ := held.Get(ctx, c)
+	if err := held.PutMany(ctx, []blocks.Block{block, block}); err != nil {
+		t.Errorf("PutMany: %v", err)
+	}
+	bucket.fail = errBucket
+	if err := held.PutMany(ctx, []blocks.Block{block}); !errors.Is(err, errBucket) {
+		t.Errorf("PutMany into a bucket that fails: %v", err)
+	}
+}
+
+func TestOpeningAStoreInABucketFails(t *testing.T) {
+	bucket := newBucket()
+	file := filepath.Join(t.TempDir(), "a-file")
+	os.WriteFile(file, nil, 0o600)
+	if _, err := OpenBucket(bucket, filepath.Join(file, "store")); err == nil {
+		t.Error("opened a store beneath a regular file")
+	}
+	lockIsDir := t.TempDir()
+	os.Mkdir(filepath.Join(lockIsDir, "LOCK"), 0o700)
+	if _, err := OpenBucket(bucket, lockIsDir); err == nil {
+		t.Error("opened a store whose lock file cannot be created")
+	}
+	dir := t.TempDir()
+	first := openBucket(t, bucket, dir)
+	if second, err := OpenBucket(bucket, dir); err == nil {
+		second.Close()
+		t.Error("opened a store directory that is already open")
+	}
+	first.Close()
+	badPins := t.TempDir()
+	os.WriteFile(filepath.Join(badPins, "pins.json"), []byte("not json"), 0o600)
+	if _, err := OpenBucket(bucket, badPins); err == nil {
+		t.Error("opened a store whose pins cannot be read")
+	}
+	os.Remove(filepath.Join(badPins, "pins.json"))
+	openBucket(t, bucket, badPins)
+}