# restic backups on ks4 — the giant fs trees

Status: **drafted 2026-08-25, not yet deployed.**

Why restic exists in this stack: plakar's incremental cost scales
with tree size, not churn (~800 MiB of parent metadata re-read from
S3 + hours of CPU per night on a 500k-file tree — measured, reported
as [PlakarKorp/plakar#2338](https://github.com/PlakarKorp/plakar/issues/2338)),
which priced `nextcloud-fs` and `seafile-fs` out of the plakar data
leg. restic keeps its comparison state in a **local cache**, so a
night costs a local stat-walk plus the churn — the rsync cost model.
plakar keeps everything it is good at (dumps, small sources, small
instances); restic covers only the two giants until #2338 is
resolved (see §7).

Scope:

| source | size | files | churn/night (measured) |
|---|---|---|---|
| `/nextcloud` (live, nextcloud container) | 737 G | ~500 k | 90–430 MiB |
| `/opt/seafile` (live, seafile container) | 933 G | millions of ~100 KiB block files | append-mostly |

Read from the live rootfs paths on the host (crash-consistent, like
the plakar fs sources; DBs are covered by the dump leg).

## 1. Repository layout — one bucket, two repos

Two **separate repositories** under one bucket
(`s3:s3.sbg.io.cloud.ovh.net/restic/nextcloud` and `.../restic/seafile`):

- restic loads the whole repo index into RAM per operation; the two
  datasets share no content, so a shared repo only inflates every
  run's memory for zero dedup gain.
- prune/check run per repo — seafile's heavy maintenance never blocks
  or bloats nextcloud's.
- one bucket keeps credential handling identical to the plakar legs.

Same passphrase discipline as plakar: generate to
`/root/.restic-passphrase` (mode 600), **copy to the password manager
immediately**.

## 2. Option analysis (our numbers, not defaults-worship)

### `--pack-size` (default 16 MiB, max 128)

Packs are the S3 objects; blobs (~1 MiB avg for data) live inside
them. Pack size does **not** change index RAM (that scales with blob
count) — it changes object count, request count, and rewrite
amplification:

| pack-size | S3 objects for ~1.2 T stored | trade-off |
|---|---|---|
| 16 MiB | ~75 000 | slow `check`/listing, more requests; prune rewrites are fine-grained |
| **64 MiB (chosen)** | ~19 000 | 4× fewer objects/requests; assembly RAM ≈ pack×connections ≈ 64 M × 5 ≈ 320 MiB — fine |
| 128 MiB | ~9 500 | fewer still, but prune rewrite amplification doubles (a pack with one dead blob rewrites 128 MiB) and upload buffers grow |

`--pack-size 64` on **every backup/prune invocation** (it is not a
repo property).

### Memory (the real constraint on this 23/31 GiB-used box)

- Index RAM ≈ 200–300 B/blob. Estimate: nextcloud ~1 M blobs,
  seafile ~2–4 M (tiny files = ≥1 blob each) → **~0.3 GiB and
  ~0.6–1.2 GiB** respectively per backup run; prune peaks 2–3×.
  This is why the repos are split (§1).
- `GOGC=20` env trades CPU for a substantially smaller Go heap —
  enable it if the seafile prune ever pressures the box.

### Cache location

`~/.cache/restic` would land on `/` (14 G free) and can reach a few
GiB for repos this size → `--cache-dir /backup/restic-cache` (sdb
pool, plenty of room; cache reads are small but ARC-warm after first
use).

### Concurrency

- `--read-concurrency 8` (default 2): more outstanding reads lets the
  HDD elevator help on the seek-bound seafile tree — the one knob that
  directly targets our 275-IOPS wall.
- `-o s3.connections=8` (default 5): mild upload parallelism bump;
  OVH same-region sustains it.

### Compression

Repo format v2 default `auto` is right; `max` only buys ~% on
already-compressed content (seafile blocks, images) for real CPU.

### Excludes — the cheapest optimisation of all

Nextcloud's `appdata_*/preview` (and `dav-photocache`) are
**regenerable caches holding a large fraction of the 500k files**;
excluding them shrinks walk and churn dramatically:

```
# /root/scripts/restic-nextcloud-exclude
/var/lib/incus/storage-pools/data/containers/nextcloud/rootfs/nextcloud/data/appdata_*/preview
/var/lib/incus/storage-pools/data/containers/nextcloud/rootfs/nextcloud/data/appdata_*/dav-photocache
```

Seafile: no excludes — the block store *is* the data.

Verify excludes before the seed: count the win with
`find .../appdata_* -path '*/preview/*' | wc -l`, then
`restic backup --dry-run -vv --exclude-file ...` prints every
include/exclude decision without uploading. (Per-user `<user>/cache/`
dirs are a further regenerable candidate if the count justifies it.)

Cache on tmpfs was considered and rejected: the cache exists to avoid
*network* metadata re-fetches; tmpfs loses it at reboot (forcing a
full re-download), eats the RAM the ARC needs, and buys nothing —
disk-backed cache reads are ARC-warm after first touch anyway.

### Escape hatch: rustic

[rustic](https://github.com/rustic-rs/rustic) speaks the **same
repository format** — switching later is a binary swap on the same
repos, no re-seed. It would solve two specific pains if they
materialize: **lock-free prune** (no backup/prune scheduling
constraint) and a **lower RAM footprint** (the seafile-index
concern). Not chosen first because it isn't in Debian (manual
upgrades) and has a far smaller community than restic. Don't run
both tools concurrently on one repo (rustic ignores restic's locks);
a clean switchover is fine.

## 3. Install + init (root on ks4)

```sh
apt install restic            # Debian 13 ships ≥0.17 (repo v2, compression, pack-size ok)
head -c 32 /dev/urandom | base64 > /root/.restic-passphrase && chmod 600 /root/.restic-passphrase
# ⚠️ password manager, NOW.

export AWS_ACCESS_KEY_ID=<AK> AWS_SECRET_ACCESS_KEY=<SK>   # -> /root/.restic-env (mode 600)
export RESTIC_PASSWORD_FILE=/root/.restic-passphrase
restic -r s3:s3.sbg.io.cloud.ovh.net/restic/nextcloud init
restic -r s3:s3.sbg.io.cloud.ovh.net/restic/seafile  init
mkdir -p /backup/restic-cache
```

## 4. Seed plan

Order matters:

1. **Bump the ARC floor first** so metadata walks stay warm between
   nightly runs (also helps the plakar legs):
   ```sh
   echo 8589934592 > /sys/module/zfs/parameters/zfs_arc_min
   echo "options zfs zfs_arc_min=8589934592" > /etc/modprobe.d/zfs-arc.conf
   update-initramfs -u    # zfs loads from the initramfs
   awk '/^c_min/ {printf "%.1f GiB\n", $3/1073741824}' /proc/spl/kstat/zfs/arcstats
   ```
   ⚠️ the floor is defended even under memory pressure — real
   headroom becomes ~31G − services − 8G; revisit before adding any
   fat new service.
2. **nextcloud** (proven ~22 MiB/s from the live pool → ~8 h), in a
   screen:
   ```sh
   restic -r s3:.../restic/nextcloud --cache-dir /backup/restic-cache \
     backup --pack-size 64 --read-concurrency 8 -o s3.connections=8 \
     --exclude-file /root/scripts/restic-nextcloud-exclude \
     /var/lib/incus/storage-pools/data/containers/nextcloud/rootfs/nextcloud
   ```
3. **seafile** — the one-time debt. Expect 1–2 days on the live pool
   (warmer than sdb, and `--read-concurrency 8` helps); it is
   restartable at any point (committed packs dedup on retry).

## 5. Nightly runs + retention + verification

Cron (05:00 — after the 04:30 plakar leg's dump phase, sources don't
overlap; wrap in a `restic-backup.sh` with flock + env, same style as
the plakar drivers):

```cron
0 5 * * * /root/scripts/restic-backup.sh >> /var/log/restic-backup.log 2>&1
```

Per repo, the script does:

```sh
restic backup  --pack-size 64 --read-concurrency 8 ...   # as in §4
restic forget  --keep-daily 14 --keep-weekly 8 --keep-monthly 6
restic unlock 2>/dev/null                                  # clear stale locks after crashes
```

- `forget` without `--prune` is instant; actual **prune runs weekly**
  (Sunday, offset from plakar's check; prune takes an exclusive lock —
  never overlap the 05:00 backups):
  ```cron
  0 14 * * 0 ... restic prune --max-unused 10% --max-repack-size 4G --pack-size 64 ...
  ```
  Tuning rationale (our churn: ~0.1–0.4 GiB/day dead data vs ~700 GiB
  repos, seafile append-mostly ≈ nothing dies): `--max-unused 10%`
  lets prune skip repacking for months, carrying ≤10 % slack in cheap
  object storage; `--max-repack-size 4G` bounds any single Sunday's
  rewrite to ~10–15 min even after a mass deletion, deferring the
  rest to following weeks. (`--repack-small` only matters for repos
  with legacy sub-64 MiB packs — not us, we start at 64.)
- **Verification**: weekly `restic check` (structure only, cheap) +
  `check --read-data-subset=1/52` rotating — a full data verification
  of every byte once a year, ~25 GiB read per week, for free.

Expected steady state: nightly = local stat-walk (minutes once ARC is
warm) + churn upload; no per-run S3 metadata storm — the exact
property #2338 is about.

## 6. Restore test (gate before trusting it)

```sh
restic -r s3:.../restic/nextcloud --cache-dir /backup/restic-cache \
  restore latest --target /backup/restore-test --include '/<some subtree>'
diff -r ... && rm -rf /backup/restore-test
```

Never restore to `/tmp` — it is a 16 GiB tmpfs on ks4.

## 7. If restic ever takes over ALL the S3 backups

Decision gate: **the upstream answer to #2338.** Two futures:

- **plakar fixes the growth problem** → consolidate the giants *back*
  onto plakar, retire restic (delete the `restic` bucket) — one tool,
  and the incus integration investment keeps compounding.
- **No fix on a useful horizon** → migrate the data leg to restic:
  - dumps + 8 small fs sources are ~50 G total: re-seeding into a
    `restic/data` repo is an evening, and restic dedups the nightly
    dump tree (incl. the static `geo.sql`) exactly like plakar does.
  - **the incus leg is the sticking point**: the per-file instance
    backups ride our plakar-native integration. restic equivalents
    are all worse or heavier: backing up replica rootfs paths
    directly (fights incus's on-demand mounts), `incus export`
    tarballs (server-side materialization, no room), or `zfs send`
    streams into restic's stdin mode (works — `restic backup --stdin`
    — but chains restore through zfs+incus knowledge).
  - So a full migration really means "restic for all *file* data +
    keep plakar only for the incus leg" — two tools either way.
    Which is the same operational cost as today's split, minus the
    #2338 exposure. Conclusion: **full migration buys little; the
    current split is not a temporary hack but a reasonable end
    state.** Revisit only if plakar becomes unmaintained or the
    incus leg moves to zfs send anyway.