SnapRAID / MergerFS

Setup

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. Let's call each of these bad volume blocks BB
Start debugfs for the volume with debugfs /dev/sdXY
For each BB run:

testb BB

If debugfs returns "Block BB not in use", then that block isn't part of a file and can safely be overwritten. If it returns an inode number (we'll call it I) then you can convert that inode number to a file path with:

ncheck I

For any unused blocks, we need to do the following:

Check with dd that we’ve got the right block IDs. For each one of these reads we expect to see an error (and “0+0 records in”):

for block in `ddrescuelog –list-blocks=- sdX.map`
do
  dd if=/dev/sdX of=/dev/null count=1 bs=512 skip=$block
done

For each of the bad blocks, write zeros over the block to force it to be reallocated from spare space on the drive. Be careful here – getting it wrong will destroy data! Also note that when reading, “skip” is used to position the input stream, but here “seek” is used to position the output stream:

for block in `ddrescuelog –list-blocks=- sdX.map`
do
  dd if=/dev/zero of=/dev/sdX count=1 bs=512 seek=$block
done

It’s possible that dd will fail to write to the block, in which case try again with hdparm:

- First check that we’ve got the right sectors (we expect to see “SG_IO: bad/missing sense data” for each sector on stderr, so we pipe stdout to /dev/null to avoid noise):

for block in `ddrescuelog –list-blocks=- sdX.map`
do
  hdparm –read-sector $block /dev/sdX > /dev/null
done

- Assuming we’ve seen the expected errors, write zeros over each of the bad sectors. Be careful here – getting it wrong will destroy data! You may be asked to add a “—yes-i-know-what-i-am-doing” flag.

for block in `ddrescuelog –list-blocks=- sdX.map`
do
  hdparm –write-sector $block /dev/sdX
done

More details: