View source for SnapRAID / MergerFS

= SnapRAID / MergerFS =

== Setup ==
https://zackreed.me/setting-up-snapraid-on-ubuntu/

Note that if (like me) you use a dedicated snapraid content directory then you'll need to create that by hand for each disk with:

  mkdir /mnt/data/disk1/.snapraid

== Partitioning a new data disk ==
Note: "-m 2" here reserves 2% of the filesystem for root-owned files (eg. .../.snapraid/content)
  sudo parted -a optimal -s /dev/sdX -- mklabel gpt mkpart primary 0% 100%
  sudo mkfs.ext4 -m 2 -T largefile4 /dev/sdX1

== Partitioning a new parity disk ==
Note: "-m 0" here reserves 0% of the filesystem, ensuring that the parity disks are slightly larger than the data disks
  sudo parted -a optimal -s /dev/sdX -- mklabel gpt mkpart primary 0% 100%
  sudo mkfs.ext4 -m 0 -T largefile4 /dev/sdX1

== Adding a new data disk to mergerfs ==
From: https://zackreed.me/mergerfs-neat-tricks/
From within the root of the mergerfs filesystem (eg. /srv)
  xattr -w user.mergerfs.srcmounts '+>/mnt/data/disk4/srv' .mergerfs

== Removing a data disk from mergerfs ==
From within the root of the mergerfs filesystem (eg. /srv)
  xattr -w user.mergerfs.srcmounts '-/mnt/data/disk4/srv' .mergerfs

== Forcing a resync ==

  sudo snapraid sync

== Identifying/Fixing a bad block ==
* Run ddrescue to identify the failing bytes on the disk:
<pre>
ddrescue --ask --verbose --binary-prefixes --idirect --force /dev/sdX /dev/null sdX.map 
</pre>
* ddrescue will write out a map file containing start positions and sizes (all in bytes). Look for lines ending "-", and convert the hex positions to decimal (we'll call this '''P''')
* Get the block size for the partition with <code>tune2fs -l /dev/sdXY | grep "Block size"</code> (we'll call this '''B'''). In my case this is 4096.
* Get the sector size for the disk with <code>fdisk -l /dev/sdX | grep Units</code> (we'll call this '''S'''). In my case this is 512.
* Identify the starting sector for the /dev/sdXY volume (eg. /dev/sda1) with <code>fdisk -l /dev/sdX</code> (we'll call this '''T'''). In my case this is 2048.
* To work out the volume block from the ddrescue position we need to calculate <code>(P - (T * S)) / B</code>

More details:
* https://www.smartmontools.org/wiki/BadBlockHowto
* [[Mdadm#Repairing_failing_disk_on_degraded_array]]