Revision as of 08:31, 26 April 2022

SnapRAID / MergerFS

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

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

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

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

From within the root of the mergerfs filesystem (eg. /srv)

 xattr -w user.mergerfs.srcmounts '-/mnt/data/disk4/srv' .mergerfs

 sudo snapraid sync

ddrescue --ask --verbose --binary-prefixes --idirect --force /dev/sdX /dev/null sdX.map

ddrescue will write out a map file containing start positions and sizes (all in bytes) of good (+) and bad (-) byte ranges
Get the block size for the volume with tune2fs -l /dev/sdXY | grep "Block size" (we'll call this B). In my case this is 4096.
Get the sector size for the disk with fdisk -l /dev/sdX | grep Units (we'll call this S). In my case this is 512.
Identify the starting sector for the /dev/sdXY volume (eg. /dev/sda1) with fdisk -l /dev/sdX (we'll call this T). In my case this is 2048.
Run ddrescuelog to list out the bad block locations (using the block size B):

ddrescuelog -b B --list-blocks=- sdX.map

For each of these, convert to a block location in the volume (rather than the disk) by subtracting (T * S / B). In my case that's 256.

More details:

@@ Line 36: / Line 36: @@
 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''')
+* ddrescue will write out a map file containing start positions and sizes (all in bytes) of good (+) and bad (-) byte ranges
 * Get the block size for the volume 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>
+* Run ddrescuelog to list out the bad block locations (using the block size B):
+<pre>
+ddrescuelog -b B --list-blocks=- sdX.map
+</pre>
+* For each of these, convert to a block location in the volume (rather than the disk) by subtracting <code>(T * S / B)</code>. In my case that's 256.
 More details:
 * https://www.smartmontools.org/wiki/BadBlockHowto
 * [[Mdadm#Repairing_failing_disk_on_degraded_array]]