BTRFS Delete Edit

Here we should also have the following:

• Shell code to mount
• Mount options
• Standardized paths __active etc

Installation notes

enable scrubbing of root

systemctl --now enable btrfs-scrub@-.time

Useful commands

• Get readonly properties for a subvolume

  • btrfs property get -ts /path/to/subvolume

• Change a subvol to rw without making a new clone

  • btrfs property set -ts /path/to/subvolume ro false

Recovery

• look at btrfs detected errors in systemd-journald
• determine if zero-log is needed, otherwise proceed with recovering data
• mount -o ro,recovery to mount a filesystem with issues
  • if it’s mountable, you can copy data off the drive now
• btrfs restore will help you copy data off a broken btrfs filesystem
  • Haven’t had the pleasure of needing this tool yet

Mount Options

• rescue - Modes allowing mount with damaged filesystem structures, all requires the filesystem to be mounted read-only and doesn’t allow remount to read-write. This is supposed to provide unified and more fine grained tuning of errors that affect filesystem operation.
  • usebackuproot - Try to use backup root slots inside super block. Replaces standalone option usebackuproot
  • nologreplay - Do not replay any dirty logs. Replaces standalone option nologreplay
  • ignorebadroots, ibadroots - Ignore bad tree roots, greatly improve the chance for data salvage. DANGER
  • ignoredatacsums, idatacsums - Ignore data checksum verification. DANGER
  • ignoremetacsums, imetacsums - Ignore metadata checksum verification, useful for interrupted checksum conversion.
  • all - Enable all supported rescue options.

Dirty logs

One can determine whether zero-log is needed according to the kernel backtrace:

? replay_one_dir_item+0xb5/0xb5 [btrfs]
? walk_log_tree+0x9c/0x19d [btrfs]
? btrfs_read_fs_root_no_radix+0x169/0x1a1 [btrfs]
? btrfs_recover_log_trees+0x195/0x29c [btrfs]
? replay_one_dir_item+0xb5/0xb5 [btrfs]
? btree_read_extent_buffer_pages+0x76/0xbc [btrfs]
? open_ctree+0xff6/0x132c [btrfs]

• btrfs rescue zero-log <device> - clear the filesystem log tree, This command will clear the filesystem log tree. This may fix a specific set of problem when the filesystem mount fails due to the log replay. See below for sample stack traces that may show up in system log.
• btrfs rescue super-recovery <device> - Recover bad superblocks from good copies.

Last Resort

btrfsck

• -b|--backup
  • use the first valid set of backup roots stored in the superblock
• -p|--progress
  • indicate progress at various checking phases
• --readonly
  • (default) run in read-only mode, this option exists to calm potential panic when users are going to run the checker
• -s|--super <N>
  • use Nth superblock copy, valid values are 0, 1 or 2 if the respective superblock offset is within the device size
  • This can be used to use a different starting point if some of the primary superblock is damaged.
• --repair
  • try to repair the filesystem (dangerous, see warnings)