docs(research): firecracker image remote store

Investigates storage candidates, retention policy design, and
image-size reduction techniques in response to the PR #459 comment
on a secure remote store for committed Firecracker snapshots.

docs/research/firecracker-image-remote-store.md

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+# Firecracker Image Remote Store
+
+**Date:** 2026-07-22  
+**Context:** PR #459 (run-dir leak fix) surfaced that committed Firecracker snapshots
+currently live only on the host machine. Once a host is wiped or a run-dir is
+evicted, a user's preserved bottle is gone. This note investigates a secure remote
+store so committed images survive host turnover and can be restored without the
+user having to pre-flag which sessions to keep.
+
+## Verdict
+
+Backblaze B2 + Cloudflare CDN is the cost-optimal choice for most deployments.
+Cloudflare R2 is the simpler zero-config option at slightly higher storage cost.
+Self-hosted MinIO is the right call for air-gapped or on-premises installs.
+
+On the image-size front, zstd-compressing the committed tar before upload
+produces roughly a 60–70% reduction with negligible impact on restore latency.
+OverlayFS (for in-flight working rootfs, not for the committed artifact) cuts
+per-instance disk use to ~10–50 MB per extra bottle sharing the same base.
+
+---
+
+## What Gets Stored
+
+The Firecracker backend produces two artifact types:
+
+| Artifact | Created by | Size | Lifetime |
+|----------|-----------|------|---------|
+| `rootfs/agent-<digest>/` (dir) | `image_builder.py` → `mke2fs -d` | ~1 GB as ext4 | Cached per Dockerfile hash; evictable |
+| `committed/<slug>/rootfs.tar` | `FirecrackerFreezer._freeze` via SSH tar | 500 MB–1 GB | User-preserved; must survive host wipe |
+
+The committed artifact is a tar of the guest's live filesystem streamed out over
+SSH (`freezer.py:58–90`). At resume time `launch.py` calls `mke2fs -d` to
+rebuild a fresh ext4 from this tar. The tar — not the ext4 — is what needs to be
+pushed to remote storage and pulled back at restore time.
+
+The base image cache (`rootfs/agent-<digest>/`) is derivable from the Dockerfile
+and can be rebuilt on demand; it is lower priority for remote storage.
+
+---
+
+## Storage Candidates
+
+### Object storage
+
+| Provider | Storage | Egress | Notes |
+|----------|---------|--------|-------|
+| **Backblaze B2** | $0.006/GB | Free (via Cloudflare Bandwidth Alliance) | Cheapest storage; pairs with Cloudflare CDN to eliminate egress |
+| **Cloudflare R2** | $0.015/GB | $0 always | Zero-config egress; no lifecycle transitions (limitation) |
+| **Wasabi** | $0.0069/GB | Free (1:1 ratio) | 90-day minimum retention; good for archival; lifecycle evaluated daily |
+| **AWS S3** | $0.023/GB | $0.09/GB | Richest lifecycle support; expensive at scale; avoid unless already in AWS |
+| **MinIO** (self-hosted) | Host cost only | None | S3-compatible; best for private/on-prem deployments |
+
+**B2 + Cloudflare CDN** is effectively $0.006/GB with zero egress — about 18×
+cheaper than S3 for restore-heavy workloads. **R2** is the zero-config choice
+($0 egress by default, no Bandwidth Alliance pairing needed) at a slightly
+higher storage rate.
+
+**Cloudflare R2's missing lifecycle support** is the main caveat: auto-eviction
+rules (evict images older than N days) cannot currently be expressed natively in
+R2. Wasabi and S3 both support declarative lifecycle policies.
+
+### Retention policy recommendation
+
+The comment proposes:
+- Retain images for ~1 week by default
+- Warn when approaching a capacity threshold
+- Auto-evict oldest images once threshold is exceeded
+
+This maps cleanly to an application-level policy (not a provider lifecycle rule),
+which avoids the R2 limitation and works consistently across providers:
+
+1. On `commit`: upload tar, record `(slug, size_bytes, uploaded_at)` in a local
+   or remote manifest file.
+2. On startup / on `list`: scan the manifest, warn if total stored size exceeds
+   e.g. 80% of the configured threshold.
+3. On eviction run (CLI or cron): delete objects older than `retention_days`
+   (default 7) that push total over `max_capacity`; oldest-first.
+
+This keeps the policy logic in bot-bottle and the storage provider as a dumb
+object store — no vendor-specific lifecycle API required.
+
+---
+
+## Image Size Reduction
+
+### Current artifact sizes
+
+A typical committed tar for a Claude Code agent image is 500 MB–1 GB uncompressed.
+The per-run ext4 (copy of the base, written at `start`) adds another ~1 GB of
+local disk. The leak fix in this PR addresses the ext4 copies; the remote store
+addresses the committed tars.
+
+### Compression
+
+zstd compression of the committed tar before upload is the highest-leverage
+single change:
+
+| Codec | Typical size (1 GB rootfs) | Compress speed | Decompress speed |
+|-------|---------------------------|---------------|-----------------|
+| gzip | 350–430 MB | ~100 MB/s | ~500 MB/s |
+| **zstd (default)** | **330–360 MB** | **~400 MB/s** | **~2 GB/s** |
+| xz | 290–320 MB | ~20 MB/s | ~200 MB/s |
+
+**zstd is the best trade-off**: 65–67% size reduction, near-instantaneous
+decompression. The `tar` call in `freezer.py` could pipe through `zstd` before
+writing to disk and to the remote; `resume` decompresses on the way back. A
+`.tar.zst` suffix marks compressed artifacts so old tars remain restorable
+without the codec.
+
+### SquashFS for the base image cache
+
+The `rootfs/agent-<digest>/` directory (the buildah-exported tree) is rebuilt by
+`image_builder.py` and turned into per-run ext4 by `mke2fs -d`. Storing the
+base as a SquashFS image instead of a flat directory tree would reduce it from
+~1 GB to ~330–360 MB and make the cache remote-friendly. Firecracker does not
+directly boot SquashFS, but the existing `mke2fs -d` path reads a directory tree
+— a SquashFS mount could serve as the source. This is a larger change and lower
+priority than tar compression.
+
+### OverlayFS for per-run rootfs
+
+Multiple simultaneous bottles sharing the same agent image today each get a full
+`mke2fs -d` copy (~1 GB). OverlayFS (read-only base + writable sparse overlay)
+would reduce this to ~10–50 MB per instance beyond the first:
+
+- Mount the base image directory as read-only lower layer
+- Attach a sparse ext4 or tmpfs writable layer per bottle
+- Pass the merged overlay to Firecracker as the block device
+
+E2B's public write-up on Firecracker + OverlayFS confirms this approach works
+at scale. The `launch.py` changes would be non-trivial (device mapper or
+`fuse-overlayfs` plumbing), so this is a follow-up rather than a prerequisite
+for the remote store.
+
+---
+
+## Recommended Approach
+
+**Phase 1 — remote store with zstd (tight scope, actionable now)**
+
+1. Add `--zstd` to the `tar` call in `FirecrackerFreezer._freeze`; name the
+   artifact `rootfs.tar.zst`. Keep uncompressed restore path for legacy tars.
+2. Add a `bb firecracker upload <slug>` / `bb firecracker pull <slug>` pair that
+   pushes/fetches the compressed tar to the configured object store (S3-compatible
+   API, so B2, R2, MinIO, and Wasabi all work with the same client).
+3. Store a `manifest.json` in the bucket (or a local mirror) tracking slug →
+   `{size, uploaded_at}`. Use it for threshold warnings and eviction.
+4. Default retention: 7 days, configurable via `firecracker.image_retention_days`
+   in `~/.config/bot-bottle/config.toml` (or equivalent).
+5. Warn at 80% of `max_capacity` (default e.g. 50 GB); evict oldest on commit
+   once at 100%.
+
+**Storage recommendation:** Cloudflare R2 for hosted deployments (zero egress,
+zero config), MinIO for private/on-premises.
+
+**Phase 2 — base image cache compression**
+
+Compress the `agent-<digest>` cache dir as a `.tar.zst` to save ~65% on repeated
+image uploads. Low urgency since the base image is rebuildable.
+
+**Phase 3 — OverlayFS per-run disk**
+
+Replace the full per-run ext4 copy with an OverlayFS sparse layer. Largest disk
+impact (~90–95% savings per concurrent bottle) but highest implementation
+complexity. Track as a separate PRD.
+
+---
+
+## Open Questions
+
+- Does the host have a configured object-store credential path, or should the
+  remote store be an opt-in with an explicit `bb config set image-store.url ...`?
+- Should `commit` automatically upload, or should upload be an explicit step to
+  avoid surprise egress?
+- What is the acceptable cold-start latency for a restore from remote? A 330 MB
+  zstd tar at 100 Mbit/s takes ~26 s; at 1 Gbit/s, ~2.6 s. This bounds the
+  retention strategy (evict from local after successful upload vs keep local copy).