Marc's Public Blog - Linux Hacking

You mostly don't need fsck on btrfs

In real life, due to how btrfs is layed out, it's not succeptible to corruption issues or things that are out of sync between locations like ext2-3-4 are, so you'll find that in normal operation you would need fsck for ext2/3/4 to clean up a few things from time to time, but this is just not needed with btrfs. Now, for very unexpected things and bad corruption, obviously you have backups right? but if you'd like to know what was corrupted and whether a restore from backup recovery is needed, or recover new data since your last backup, read on.

Run online fsck nightly or weekly, and look for errors that btrfs is noticing and reporting in syslog:

Mounting filesystems with problems, try in order:

root@polgara:~# mount -o recovery /dev/mapper/crypt /mnt/mnt8
mount: /dev/mapper/crypt already mounted or /mnt/mnt8 busy
root@polgara:~# mount -o recovery,ro /dev/mapper/crypt /mnt/mnt8
root@polgara:~#

Getting data off a filesystem you can't mount:

How to configure sec, event correlator to report btrfs filesystem errors or warnings

After installing sec.pl (apt-get install sec on debian or http://simple-evcorr.sourceforge.net/ ), install the 2 config files below.

This is not foolproof, it relies on a regex of known messages that are ok, and reports all unknown ones. You can extend the forward looking negative regex as needed.

polgara:~# cat /etc/default/sec 
#Defaults for sec
RUN_DAEMON="yes"
DAEMON_ARGS="-conf=/etc/sec.conf -input=/var/log/syslog -pid=/var/run/sec.pid -detach -log=/var/log/sec.log"
polgara:~# cat /etc/sec.conf
# http://simple-evcorr.sourceforge.net/man.html
# http://sixshooter.v6.thrupoint.net/SEC-examples/article.html
# http://sixshooter.v6.thrupoint.net/SEC-examples/article-part2.html

type=SingleWithSuppress
ptype=RegExp
pattern=(?i)kernel.*btrfs: (?!disk space caching is enabled|use ssd allocation|use .* compression|unlinked .* orphans|turning on discard|device label .* devid .* transid|detected SSD devices, enabling SSD mode|has skinny extents|device label|creating UUID tree|checking UUID tree|setting .* feature flag|bdev.* flush 0, corrupt 0, gen 0)
window=60
desc=Btrfs unexpected log
action=pipe '%t: $0' /usr/bin/mail -s "sec: %s" root

Daily or weekly btrfs scrub

This is a must have with btrfs, btrfs scrub. Note that it is disk intensive since it checks everything for consistency (including data blocks), so on a server it does add load and it could take over a day ot run if you have terabytes of data.

If you don't have shlock, install inn, copy shlock out of it, and then delete it :) (or you can remove shlock from the script, it's not vital).

The up to date version of this script is at http://marc.merlins.org/linux/scripts/btrfs-scrub

#! /bin/bash

# By Marc MERLIN <marc_soft@merlins.org> 2014/03/20
# License: Apache-2.0

which btrfs >/dev/null || exit 0

export PATH=/usr/local/bin:/sbin:$PATH

# bash shortcut for `basename $0`
PROG=${0##*/}
lock=/var/run/$PROG

# shlock (from inn) does the right thing and grabs a lock for a dead process
# (it checks the PID in the lock file and if it's not there, it
# updates the PID with the value given to -p)
# You can replace this with another lock program if you prefer or even remove
# the lock.
if ! shlock -p $$ -f $lock; then
    echo "$lock held, quitting" >&2
    exit
fi

if which on_ac_power >/dev/null 2>&1; then
    ON_BATTERY=0
    on_ac_power >/dev/null 2>&1 || ON_BATTERY=$?
    if [ "$ON_BATTERY" -eq 1 ]; then
	exit 0
    fi
fi

FILTER='(^Dumping|balancing, usage)'
test -n "$DEVS" || DEVS=$(grep '\<btrfs\>' /proc/mounts | awk '{ print $1 }' | sort -u)
for btrfs in $DEVS
do
    tail -n 0 -f /var/log/syslog | grep "BTRFS: " | grep -Ev '(disk space caching is enabled|unlinked .* orphans|turning on discard|device label .* devid .* transid|enabling SSD mode|BTRFS: has skinny extents|BTRFS: device label)' &
    mountpoint="$(grep "$btrfs" /proc/mounts | awk '{ print $2 }' | sort | head -1)"
    logger -s "Quick Metadata and Data Balance of $mountpoint ($btrfs)" >&2
    # Even in 4.3 kernels, you can still get in places where balance
    # won't work (no place left, until you run a -m0 one first)
    btrfs balance start -musage=0 -v $mountpoint 2>&1 | grep -Ev "$FILTER"
    btrfs balance start -musage=20 -v $mountpoint 2>&1 | grep -Ev "$FILTER"
    # After metadata, let's do data:
    btrfs balance start -dusage=0 -v $mountpoint 2>&1 | grep -Ev "$FILTER"
    btrfs balance start -dusage=20 -v $mountpoint 2>&1 | grep -Ev "$FILTER"
    # And now we do scrub. Note that scrub can fail with "no space left
    # on device" if you're very out of balance.
    logger -s "Starting scrub of $mountpoint" >&2
    echo btrfs scrub start -Bd $mountpoint
    ionice -c 3 nice -10 btrfs scrub start -Bd $mountpoint
    pkill -f 'tail -n 0 -f /var/log/syslog'
    logger "Ended scrub of $mountpoint" >&2
done

rm $lock