Draft restic-backup: giant fs trees to S3, option analysis, migration gate
pack-size 64 MiB (object count vs prune amplification), split repos for index RAM, cache on /backup, read-concurrency vs the seek-bound walk, nextcloud preview excludes, ARC floor before seeding, weekly rotating read-data-subset verification, and the analysis of a full plakar->restic migration (conclusion: the split is a reasonable end state, gated on PlakarKorp/plakar#2338). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Julien Lutran
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Claude Fable 5
Claude Fable 5
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# restic backups on ks4 — the giant fs trees
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Status: **drafted 2026-08-25, not yet deployed.**
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Why restic exists in this stack: plakar's incremental cost scales
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with tree size, not churn (~800 MiB of parent metadata re-read from
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S3 + hours of CPU per night on a 500k-file tree — measured, reported
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as [PlakarKorp/plakar#2338](https://github.com/PlakarKorp/plakar/issues/2338)),
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which priced `nextcloud-fs` and `seafile-fs` out of the plakar data
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leg. restic keeps its comparison state in a **local cache**, so a
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night costs a local stat-walk plus the churn — the rsync cost model.
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plakar keeps everything it is good at (dumps, small sources, small
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instances); restic covers only the two giants until #2338 is
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resolved (see §7).
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Scope:
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| source | size | files | churn/night (measured) |
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|---|---|---|---|
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| `/nextcloud` (live, nextcloud container) | 737 G | ~500 k | 90–430 MiB |
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| `/opt/seafile` (live, seafile container) | 933 G | millions of ~100 KiB block files | append-mostly |
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Read from the live rootfs paths on the host (crash-consistent, like
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the plakar fs sources; DBs are covered by the dump leg).
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## 1. Repository layout — one bucket, two repos
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Two **separate repositories** under one bucket
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(`s3:s3.sbg.io.cloud.ovh.net/restic/nextcloud` and `.../restic/seafile`):
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- restic loads the whole repo index into RAM per operation; the two
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datasets share no content, so a shared repo only inflates every
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run's memory for zero dedup gain.
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- prune/check run per repo — seafile's heavy maintenance never blocks
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or bloats nextcloud's.
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- one bucket keeps credential handling identical to the plakar legs.
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Same passphrase discipline as plakar: generate to
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`/root/.restic-passphrase` (mode 600), **copy to the password manager
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immediately**.
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## 2. Option analysis (our numbers, not defaults-worship)
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### `--pack-size` (default 16 MiB, max 128)
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Packs are the S3 objects; blobs (~1 MiB avg for data) live inside
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them. Pack size does **not** change index RAM (that scales with blob
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count) — it changes object count, request count, and rewrite
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amplification:
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| pack-size | S3 objects for ~1.2 T stored | trade-off |
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|---|---|---|
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| 16 MiB | ~75 000 | slow `check`/listing, more requests; prune rewrites are fine-grained |
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| **64 MiB (chosen)** | ~19 000 | 4× fewer objects/requests; assembly RAM ≈ pack×connections ≈ 64 M × 5 ≈ 320 MiB — fine |
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| 128 MiB | ~9 500 | fewer still, but prune rewrite amplification doubles (a pack with one dead blob rewrites 128 MiB) and upload buffers grow |
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`--pack-size 64` on **every backup/prune invocation** (it is not a
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repo property).
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### Memory (the real constraint on this 23/31 GiB-used box)
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- Index RAM ≈ 200–300 B/blob. Estimate: nextcloud ~1 M blobs,
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seafile ~2–4 M (tiny files = ≥1 blob each) → **~0.3 GiB and
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~0.6–1.2 GiB** respectively per backup run; prune peaks 2–3×.
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This is why the repos are split (§1).
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- `GOGC=20` env trades CPU for a substantially smaller Go heap —
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enable it if the seafile prune ever pressures the box.
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### Cache location
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`~/.cache/restic` would land on `/` (14 G free) and can reach a few
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GiB for repos this size → `--cache-dir /backup/restic-cache` (sdb
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pool, plenty of room; cache reads are small but ARC-warm after first
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use).
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### Concurrency
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- `--read-concurrency 8` (default 2): more outstanding reads lets the
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HDD elevator help on the seek-bound seafile tree — the one knob that
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directly targets our 275-IOPS wall.
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- `-o s3.connections=8` (default 5): mild upload parallelism bump;
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OVH same-region sustains it.
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### Compression
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Repo format v2 default `auto` is right; `max` only buys ~% on
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already-compressed content (seafile blocks, images) for real CPU.
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### Excludes — the cheapest optimisation of all
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Nextcloud's `appdata_*/preview` (and `dav-photocache`) are
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**regenerable caches holding a large fraction of the 500k files**;
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excluding them shrinks walk and churn dramatically:
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```
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# /root/scripts/restic-nextcloud-exclude
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/var/lib/incus/storage-pools/data/containers/nextcloud/rootfs/nextcloud/appdata_*/preview
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/var/lib/incus/storage-pools/data/containers/nextcloud/rootfs/nextcloud/appdata_*/dav-photocache
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```
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Seafile: no excludes — the block store *is* the data.
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## 3. Install + init (root on ks4)
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```sh
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apt install restic # Debian 13 ships ≥0.17 (repo v2, compression, pack-size ok)
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head -c 32 /dev/urandom | base64 > /root/.restic-passphrase && chmod 600 /root/.restic-passphrase
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# ⚠️ password manager, NOW.
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export AWS_ACCESS_KEY_ID=<AK> AWS_SECRET_ACCESS_KEY=<SK> # -> /root/.restic-env (mode 600)
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export RESTIC_PASSWORD_FILE=/root/.restic-passphrase
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restic -r s3:s3.sbg.io.cloud.ovh.net/restic/nextcloud init
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restic -r s3:s3.sbg.io.cloud.ovh.net/restic/seafile init
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mkdir -p /backup/restic-cache
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```
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## 4. Seed plan
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Order matters:
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1. **Bump the ARC floor first** so metadata walks stay warm between
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nightly runs (also helps the plakar legs):
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```sh
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echo $((8*1024*1024*1024)) > /sys/module/zfs/parameters/zfs_arc_min
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echo "options zfs zfs_arc_min=8589934592" > /etc/modprobe.d/zfs-arc.conf
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```
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2. **nextcloud** (proven ~22 MiB/s from the live pool → ~8 h), in a
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screen:
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```sh
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restic -r s3:.../restic/nextcloud --cache-dir /backup/restic-cache \
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backup --pack-size 64 --read-concurrency 8 -o s3.connections=8 \
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--exclude-file /root/scripts/restic-nextcloud-exclude \
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/var/lib/incus/storage-pools/data/containers/nextcloud/rootfs/nextcloud
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```
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3. **seafile** — the one-time debt. Expect 1–2 days on the live pool
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(warmer than sdb, and `--read-concurrency 8` helps); it is
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restartable at any point (committed packs dedup on retry).
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## 5. Nightly runs + retention + verification
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Cron (05:00 — after the 04:30 plakar leg's dump phase, sources don't
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overlap; wrap in a `restic-backup.sh` with flock + env, same style as
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the plakar drivers):
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```cron
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0 5 * * * /root/scripts/restic-backup.sh >> /var/log/restic-backup.log 2>&1
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```
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Per repo, the script does:
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```sh
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restic backup --pack-size 64 --read-concurrency 8 ... # as in §4
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restic forget --keep-daily 14 --keep-weekly 8 --keep-monthly 6
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restic unlock 2>/dev/null # clear stale locks after crashes
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```
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- `forget` without `--prune` is instant; actual **prune runs weekly**
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(Sunday, offset from plakar's check):
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```cron
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0 14 * * 0 ... restic prune --max-unused 10% --pack-size 64 ...
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```
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`--max-unused 10%` caps rewrite amplification (leaves ≤10 % dead
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data in place instead of rewriting packs eagerly).
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- **Verification**: weekly `restic check` (structure only, cheap) +
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`check --read-data-subset=1/52` rotating — a full data verification
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of every byte once a year, ~25 GiB read per week, for free.
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Expected steady state: nightly = local stat-walk (minutes once ARC is
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warm) + churn upload; no per-run S3 metadata storm — the exact
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property #2338 is about.
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## 6. Restore test (gate before trusting it)
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```sh
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restic -r s3:.../restic/nextcloud --cache-dir /backup/restic-cache \
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restore latest --target /backup/restore-test --include '/<some subtree>'
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diff -r ... && rm -rf /backup/restore-test
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```
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Never restore to `/tmp` — it is a 16 GiB tmpfs on ks4.
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## 7. If restic ever takes over ALL the S3 backups
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Decision gate: **the upstream answer to #2338.** Two futures:
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- **plakar fixes the growth problem** → consolidate the giants *back*
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onto plakar, retire restic (delete the `restic` bucket) — one tool,
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and the incus integration investment keeps compounding.
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- **No fix on a useful horizon** → migrate the data leg to restic:
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- dumps + 8 small fs sources are ~50 G total: re-seeding into a
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`restic/data` repo is an evening, and restic dedups the nightly
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dump tree (incl. the static `geo.sql`) exactly like plakar does.
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- **the incus leg is the sticking point**: the per-file instance
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backups ride our plakar-native integration. restic equivalents
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are all worse or heavier: backing up replica rootfs paths
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directly (fights incus's on-demand mounts), `incus export`
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tarballs (server-side materialization, no room), or `zfs send`
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streams into restic's stdin mode (works — `restic backup --stdin`
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— but chains restore through zfs+incus knowledge).
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- So a full migration really means "restic for all *file* data +
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keep plakar only for the incus leg" — two tools either way.
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Which is the same operational cost as today's split, minus the
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#2338 exposure. Conclusion: **full migration buys little; the
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current split is not a temporary hack but a reasonable end
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state.** Revisit only if plakar becomes unmaintained or the
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incus leg moves to zfs send anyway.
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