Add mergerfs + SnapRAID setup

server-stuff/mergerfs-snapraid-setup.md

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+# mergerfs + SnapRAID setup
+
+`/etc/fstab`:
+
+```txt
+# Data drives
+UUID=2eeb4386-e26e-4340-9747-74fb3d18dd57   /mnt/disk1  xfs           defaults 0 0
+UUID=e030fa2f-a9b4-4788-8d10-05cf5031f9c0   /mnt/disk2  xfs           defaults 0 0
+UUID=f9a79cef-3959-4aa8-a9c9-af54660cb755   /mnt/disk3  xfs           defaults 0 0
+UUID=c7283793-37c6-4b01-95e1-cca26af0da8f   /mnt/cache  btrfs         defaults 0 0
+UUID=8d8cb485-eb23-475b-a9c1-03b8309cdbaa   /mnt/parity xfs           defaults 0 0
+/mnt/disk1:/mnt/disk2:/mnt/disk3:/mnt/cache /mnt/user   fuse.mergerfs defaults,allow_other,use_ino,category.create=lfs,moveonenospc=true,minfreespace=512G,cache.files=partial,dropcacheonclose=true 0 0
+/mnt/disk1:/mnt/disk2:/mnt/disk3            /mnt/hdds   fuse.mergerfs defaults,allow_other,use_ino,category.create=mfs,moveonenospc=true,minfreespace=25M,cache.files=partial,dropcacheonclose=true  0 0```
+
+## mergerfs
+
+Command equivalent to fstab mergerfs mounts:
+
+```txt
+mergerfs -o defaults,allow_other,use_ino,category.create=lfs,moveonenospc=true,minfreespace=512G,cache.files=partial,dropcacheonclose=true /mnt/disk1:/mnt/disk2:/mnt/disk3:/mnt/cache /mnt/user
+mergerfs -o defaults,allow_other,use_ino,category.create=mfs,moveonenospc=true,minfreespace=25M,cache.files=partial,dropcacheonclose=true /mnt/disk1:/mnt/disk2:/mnt/disk3 /mnt/hdds
+```
+
+## Caching and mover
+
+### Topology
+
+- `/mnt/user` - Combined mergerfs pool
+- `/mnt/hdds` - HDD-only mergerfs pool
+- `/mnt/cache` - cache mount point
+- `/mnt/disk*` - Each HDD, starting from 1. `/mnt/disk1`, `/mnt/disk2`, etc.
+
+### Mover script
+
+Stolen from [here](https://raw.githubusercontent.com/trapexit/mergerfs/latest-release/tools/mergerfs.time-based-mover)
+
+It moves anything that hasn't been access in more than 3 days.
+
+Requires `rsync` - install with `apt update && apt install rsync`
+
+```sh
+#!/usr/bin/env sh
+
+if [ $# != 3 ]; then
+  echo "usage: $0 <cache-fs> <backing-pool> <days-old>"
+  exit 1
+fi
+
+CACHE="${1}"
+BACKING="${2}"
+N=${3}
+
+find "${CACHE}" -type f -atime +${N} -printf '%P\n' | \
+  rsync --files-from=- -axqHAXWES --preallocate --remove-source-files "${CACHE}/" "${BACKING}/"
+```
+
+Located at `/root/mergerfs.time-based-mover.sh`, runs daily at 4 AM:
+
+```cron
+0 4 * * * /root/mergerfs.time-based-mover.sh /mnt/cache /mnt/hdds 3
+```
+
+## SnapRAID
+
+### `/etc/snapraid.conf`
+
+The list of files is stored on multiple disks in `snapraid.content`, and the parity is just one file (`snapraid.parity`)
+
+```txt
+parity /mnt/parity/snapraid.parity
+content /var/snapraid/snapraid.content
+content /mnt/disk1/snapraid.content
+content /mnt/disk2/snapraid.content
+content /mnt/disk3/snapraid.content
+data d1 /mnt/disk1/
+data d2 /mnt/disk2/
+data d3 /mnt/disk3/
+```
+
+### Syncing
+
+Run daily at 2 AM (`/etc/cron.d/snapraid-sync`):
+
+```cron
+* 02 * * * /root/snapraid-sync.sh
+```
+
+`/root/snapraid-sync.sh`:
+
+```sh
+#!/usr/bin/env sh
+CONTAINERS=$(docker ps -q)
+docker stop $CONTAINERS
+snapraid sync
+docker start $CONTAINERS
+```