• needs a non-btrfs boot partition - grub2 doesn't handle brtfs compression, etc.
• apt-get install apt-btrfs-snapshot - for automatic snapshots during apt actions

IMPORTANT:

do not run with an old kernel! jessie 3.16 is TOO OLD - crashes and rebootloops when balancing!!

install 4.0 from jessie backports https://backports.debian.org/Instructions/

btrfs is really tricky and needs a lot of maintaince:

• balance weekly
• use btrfs fi usage /home/ to see disc space! btrfs fi show /home/ shows *allocated* space, which is always almost everything
• leave 20% free :-0
• disc full can cause various crashes: even mysql may suffer and kill a connected slave db & so also php processes! (it seems)
• Use the new usage command, it has more information than btrfs fi show and btrfs fi df - see below

COW is not good for small files like database files:

disable it for a directory:

chattr -R +C /var/lib/mysql

check:

lsattr /var/lib/

balance reorders metadata - use after a lot data has changed or disc was full:

fast:

btrfs fi balance start -dusage=5 /

good:

btrfs bal start -v -dusage=51  /

..watch the process:

watch btrfs bal stat /

weekly cron “fsck”:

btrfs scrub start -c3 /

defrag and compress:

btrfs fi defrag -v -r -czlib /home/backup

or set mount-option “autodefrag”

remove dups:

duperemove  FIXME

If btrfs runs on a md raid or in raid mode, you can disable the metadata duplication for better performance

btrfs bal start -v -f -mconvert=single -sconvert=single /

or re-enable them

btrfs bal start -v -f  -mconvert=dup -sconvert=dup /

the computer may get slow and even freeze up! altough df shows free space :-0

Detailed description of the meanings of the command outputs: http://ram.kossboss.com/btrfs-fi-df-balance-btrfs-df-and-freespace-req/

Use the new usage command, it has more information than btrfs fi show and btrfs fi df

btrfs fi df /home/

this is quite confusing:

total shows the allocated space, not the space. Used shows the data-usage space. you need to add metadata usage.

btrfs fi df /home/

see, if the device was full:

btrfs filesystem show 

for example: here only 2.67TiB are *really* used, but because of shitty block usage 3.28TiB are full!! and only ~23GB free. balance required!

Total devices 2 FS bytes used 2.67TiB
devid    1 size 3.51TiB used 3.28TiB path /dev/sda4
devid    2 size 3.51TiB used 3.28TiB path /dev/sdb4

if the device was full, reorder chunks - kind of defrag:

btrfs fi balance start -v -dusage=5 /home/

increase dusage value up to 95

• https://coreos.com/os/docs/latest/btrfs-troubleshooting.html
• http://marc.merlins.org/perso/btrfs/post_2014-05-04_Fixing-Btrfs-Filesystem-Full-Problems.html

If you run a RAID1 and one device crashs, you have a problem! The device cannot be removed: Cannot go below the minimum number of raid devices (2)

Another problem is, that a degraded RAID can only be mounted RW ONCE! if you can only mount it RO, you cannot add/remove/replace a device. The solution is to replace failed device, after the failed disc has been replaced pysically.

Quote from the wiki:

In case of raidXX layout, you cannot go below the minimum number of the device required. So before removing a device (even the missing one) you may need to add a new one. For example if you have a raid1 layout with two device, and a device fails, you must:

1. partition the disc. use sgdisk to copy partition layout:

  sgdisk --replicate=/dev/$dest $source
  sgdisk --randomize-guids /dev/$dest  # NEW UUIDS!
  partprobe /dev/sda* /dev/sdb*  #..tell the kernel: partprobe /dev/sd*

Fix mdadm for /boot and swap partis: https://wiki.fr33.info/doku.php/linux/filesystems/raid?s[]=mdadm

Fix btrfs:

  mount btrfs in degraded mode # to get rw ("degraded" in /etc/default/grub) should mount / degraded
  (add a new device, remove the missing device)  # old, better replace
  
  btrfs replace start /dev/sdaX /dev/sdbX /

Useful articles:

• http://blog.programster.org/recover-btrfs-array-after-device-failure/
• https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices#Replacing_failed_devices

There may be problems, if btrfs runs on top of mdadm raid..maybe.. or maybe the reason is:

balance errors:

balance can be dangerous! cpu and fs may get locked by btrfs_transactions and kworker: check atop and top (not htop)

btrfs balance cancel /mn/xx   # may not work

balance is also restarted after reboot and the system hangs again!

mount without balance, maybe disable fstab line and hard reboot before

mount -o recovery,skip_balance /dev/sdaX /mnt/xx

be patient: this can take 1-2 hours!

after remounting with skip_balance, you can cancel the balance

btrfs balance cancel /mnt/xx

mount a partition with errors:

mount -o recovery /dev/sdaX /mnt/xx

recover a partition, that cannot be mounted

errors like these:

parent transid verify failed on 31302336512 wanted 62455 found 62456
Remounting read-write after error is not allowed 
can't read superblock

Try this:

mount -t btrfs -o nospace_cache /dev/mdX /home/

This will be very slow, but might work.

Maybe you need to reboot first.

Try this otherwise:

btrfs rescue zero-log /dev/md4

fsck:

btrfs check -p  /dev/sdaX

check and repair - last resort: may destroy files:

btrfs check -p  /dev/sdaX

• https://seravo.fi/2015/using-raid-btrfs-recovering-broken-disks
• https://btrfs.wiki.kernel.org/index.php/Problem_FAQ

• convert mdadm raid to btrfs raid: https://tech.feedyourhead.at/content/btrfs-turning-mdadm-array-btrfs