Btrfs Maintenance
Btrfs Maintenance
Btrfs needs no maintenance
This even may be true, when:
- You always keep 10% of your volume unallocated
I do not say "free", because free does mean nothing with btrfs. The value you have to watch most is "unallocated" !
Every filesystem needs some maintenance
Some filesystems do an automatic chkfs at every XX. mount Others need the user to do a chkfs manually. On most you have to watch not to fill them to 100%. Some reserve 5% for root. Some filesystems need to be defragmented. Others do not.
Every filesystem has its own needs and tools !
Automatic maintenance
Btrfs does some maintenance automatically.
Is the Volume in a clean state
At mount btrfs tests wehter the volume is in a clean state.
power down
Most file systems are not prepared for a sudden loss of power. If the computer suddenly loses power, transactions may not be written or only partially written to the volume. File systems that use journaling can often recover after a power failure.
Btrfs attempts to clean the volume by removing the last (uncompleted) action. This can result in losing the last changes you made before turning off. But you get a clean file system that doesn't need to be repaired.
error
If btrfs encounters an unrecoverable error, such as a checksum mismatch, the filesystem will be mounted read-only. (When using RAID2, such errors are auto-repaired)
clean
btrfs will mount the subvolume read-write.
Manual Maintenace
From time to time it may be advisable to check the health of the volume.
usage
Look if everything is OK with the unallocated space on your volume.
check unallocated space
Pay special attention to the unallocated space !
The commonly used term "free" is meaningless to btrfs users. Even if you have 50% of your volume "free", you may run out of storage space. **You must pay attention to the term unallocated**.
Try to keep at least 10% of your volume unallocated. If this is below 10%:
- expand your volume to double size (see: Btrfs)
- delete some unused data
- delete some old snapshots
- balance (Btrfs)
clean up unused snapshots
From time to time it is necessary to check if you have snapshots that you do not need any more, but that hold valuable space on your volume.
balance your free space
If you have some space "free" on your volume, but the unallocated space is below 10% (or close to 10%), you can help btrfs to rebalance some chunks.
Btrfs looks for chunks that contain more than 50% free space. It will take 2 of them and then move everything into a new chunk. After that, one chunk is released. This continues until each chunk is at least 50% full.
This will give you some unallocated(free) chunks that help btrfs **not** to get out of space.
If you have less than 20% unallocated space, please do the following:
Btrfs looks for chunks that are less than 75% full. It will take 4 of them and then move everything into 3 new chunks. After that, one chunk can be released. This continues until each chunk is at least 75% full.
If you still have less than 20% unallocated space, please do the following:
scrub
Check if everything is ok with the checksums and the readability of your data. **Only do this when in doubt**. This may take a long time as btrfs has to read ALL the data. Btrfs only scans the portion of your volume that actually contains data.
Tip:watch -d btrfs scrub status /
special cases
There seem to be some cases where additional maintenance is needed.
ssd
Read Rate Degradation (not confirmed)
This is not a problem of SSD's in general, but only of some rare cases. Replacing the firmware should solve the problem. A possibility to test the read-speed with btrfs is to read all data (scrub). If your ssd seems to have this RRD-problem, a balance will help because it does rewrite all data to a new place. This can be done while the system is in use ! But do it only once a year !
This will take a long time. You can shutdown your pc whenever you want. Dont't be surprised. the balance will restart after boot until it is finished !
If you want to keep your data rotating ;-) do:
This will only rewrite 10 chunks. This can be used weekly if you think it is necessary.
Rotating disks
On spinning disks, fragmentation can become an issue in some rare use cases. Then you can defragment your Btrfs volume.
Defragmentation (not recommended)
This is NOT the same as defragmentation in older Windows file systems. Btrfs does NOT require defragmentation as normal maintenance. Only do this if you can measure a huge read performance loss because a specific file/database is fragmented!
See Also
Btrfs Learn basics about btrfs