4 ms·

My experience with btrfs is from a few years ago, so it's possible they've changed the semantics here since then, but this might be applicable: Btrfs requires

by codys 6y ago

My experience with btrfs is from a few years ago, so it's possible they've changed the semantics here since then, but this might be applicable: Btrfs requires a regularly run administrative task via "btrfs balance". Balancing consolidates data between partially filled block groups, which will restore consumed space even if a block group is not entirely empty. Typically, one runs balance with a set of gradually increasing parameters to control the records affected (ie: bash script that starts with block groups that are more empty and proceeds towards ones with a higher percentage utilization). In my experience with btrfs a few years ago, running a balance in some situations would result is incredibly high system wide latency. The man page [1] does not appear to reference the free space issue directly, so I'm not sure if they've removed this need. 1: https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-balance https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-balanc...

ansible 6y ago
Interesting. I would assume that this is not a factor with RAID-1, where all system, metadata, and data is duplicated. I could see this as being very important for the higher RAID levels. We have since stopped deploying any sort of btrfs RAID (even RAID-1), and have gone back to using Linux MD.
- LanternLight83 6y ago
  Free space and the space taken up by metadata and snapshots can take a little attention to figure out https://ohthehugemanatee.org/blog/2019/02/11/btrfs-out-of-space-emergency-response/ https://ohthehugemanatee.org/blog/2019/02/11/btrfs-out-of-sp...
  - ansible 6y ago
    Now that is interesting. If I could mount the failed btrfs filesystem read-write for an appreciable length of time, I could try the balance operation. I'll need to set up a cron job on the other systems to run balance on a regular basis alongside scrub (obviously not at the exact same time).
- codys 6y ago
  To be clear: I was running a non-raid (single disk) setup where I found `btrfs balance` to be needed.
ac29 6y ago
> Btrfs requires a regularly run administrative task via "btrfs balance" This hasnt been my experience within the past few years, though I do remember it being somewhat necessary in the past. Both systems I'm running btrfs on have their free space within a few percent of the unallocated space (indicating most used blocks are fairly full and little space is wasted).
- codys 6y ago
  Are you using snapshots in btrfs? Perhaps trying to use this feature was what caused it to be needed for my use case.
cmurf 6y ago
What is the source of "Btrfs requires a regularly run administrative task...". No file system should require this kind of regular hand holding. If it does, it's a bug, and needs to be reported and fixed, not papered over with user initiated maintenance tasks. It is acceptable to have utilities for optimizing the layout from time to time, but they should be considered optimizations, not requirements. If it becomes required to use them as a work around, that's both suboptimal and neat. But there's still a bug that needs a proper fix. In the case a workaround is needed, it's recommended to do targeted filtered balances, not a full one. A full balance isn't going to solve a problem that a proper filtered balance can't.
- codys 6y ago
  There are whole repos[1] dedicated to scripts to perform the delicate work of running `btrfs balance` in various ways to ensure space is recovered. openSuse (a distro which notably defaults to btrfs) packages these btrfsmaintenance scripts [2], and it appears they may have included it in their default install (I can't find a list of packages). Their wiki page on disabling btrfsmaintenance [5] implies that if disabled, manual maintenance (presumably via running, among other commands, some form of balance) is needed. There's also an entry in the btrfs wiki [3] that indicates running `btrfs balance` will recover unused space in some cases. It helpfully notes that prior to "at least 3.14", balance was "sometimes" needed to recover free space in a file-system full state. (The lack of precision here doesn't inspire confidence) Another btrfs wiki page [4] indicates that running balance may be needed to recover space "after removing lots of files or deleting snapshots". 1: https://github.com/kdave/btrfsmaintenance https://github.com/kdave/btrfsmaintenance 2: https://software.opensuse.org/package/btrfsmaintenance https://software.opensuse.org/package/btrfsmaintenance 3: https://btrfs.wiki.kernel.org/index.php/FAQ#What_does_.22balance.22_do.3F https://btrfs.wiki.kernel.org/index.php/FAQ#What_does_.22bal... 4: https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-balance https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-balanc... 5: https://en.opensuse.org/SDB:Disable_btrfsmaintenance https://en.opensuse.org/SDB:Disable_btrfsmaintenance
  - cmurf 6y ago
    openSUSE has automatic snapshots with a fairly extensive retention policy. Kernel 3.14 is ancient. I can't bring myself to worry about it. I think your fourth paragraph cherry pick is disingenuous. A more complete excerpt, "There’s a special case when the block groups are completely unused, possibly left after removing lots of files or deleting snapshots. Removing empty block groups is automatic since 3.18." Fedora 33 users will be getting kernel 5.8 from day one, and 5.9 soon after release. I doubt my case is atypical. I haven't balanced my years old non-test real world used Btrfs file systems. I think you could switch your concern and criticism to the fact wikis get stale.

by codys 6y ago

ansible 6y ago

Interesting. I would assume that this is not a factor with RAID-1, where all system, metadata, and data is duplicated. I could see this as being very important for the higher RAID levels. We have since stopped deploying any sort of btrfs RAID (even RAID-1), and have gone back to using Linux MD.

LanternLight83 6y ago
Free space and the space taken up by metadata and snapshots can take a little attention to figure out https://ohthehugemanatee.org/blog/2019/02/11/btrfs-out-of-space-emergency-response/ https://ohthehugemanatee.org/blog/2019/02/11/btrfs-out-of-sp...
- ansible 6y ago
  Now that is interesting. If I could mount the failed btrfs filesystem read-write for an appreciable length of time, I could try the balance operation. I'll need to set up a cron job on the other systems to run balance on a regular basis alongside scrub (obviously not at the exact same time).
codys 6y ago
To be clear: I was running a non-raid (single disk) setup where I found `btrfs balance` to be needed.

ac29 6y ago

> Btrfs requires a regularly run administrative task via "btrfs balance" This hasnt been my experience within the past few years, though I do remember it being somewhat necessary in the past. Both systems I'm running btrfs on have their free space within a few percent of the unallocated space (indicating most used blocks are fairly full and little space is wasted).

codys 6y ago
Are you using snapshots in btrfs? Perhaps trying to use this feature was what caused it to be needed for my use case.

cmurf 6y ago

What is the source of "Btrfs requires a regularly run administrative task...". No file system should require this kind of regular hand holding. If it does, it's a bug, and needs to be reported and fixed, not papered over with user initiated maintenance tasks. It is acceptable to have utilities for optimizing the layout from time to time, but they should be considered optimizations, not requirements. If it becomes required to use them as a work around, that's both suboptimal and neat. But there's still a bug that needs a proper fix. In the case a workaround is needed, it's recommended to do targeted filtered balances, not a full one. A full balance isn't going to solve a problem that a proper filtered balance can't.

codys 6y ago
There are whole repos[1] dedicated to scripts to perform the delicate work of running `btrfs balance` in various ways to ensure space is recovered. openSuse (a distro which notably defaults to btrfs) packages these btrfsmaintenance scripts [2], and it appears they may have included it in their default install (I can't find a list of packages). Their wiki page on disabling btrfsmaintenance [5] implies that if disabled, manual maintenance (presumably via running, among other commands, some form of balance) is needed. There's also an entry in the btrfs wiki [3] that indicates running `btrfs balance` will recover unused space in some cases. It helpfully notes that prior to "at least 3.14", balance was "sometimes" needed to recover free space in a file-system full state. (The lack of precision here doesn't inspire confidence) Another btrfs wiki page [4] indicates that running balance may be needed to recover space "after removing lots of files or deleting snapshots". 1: https://github.com/kdave/btrfsmaintenance https://github.com/kdave/btrfsmaintenance 2: https://software.opensuse.org/package/btrfsmaintenance https://software.opensuse.org/package/btrfsmaintenance 3: https://btrfs.wiki.kernel.org/index.php/FAQ#What_does_.22balance.22_do.3F https://btrfs.wiki.kernel.org/index.php/FAQ#What_does_.22bal... 4: https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-balance https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-balanc... 5: https://en.opensuse.org/SDB:Disable_btrfsmaintenance https://en.opensuse.org/SDB:Disable_btrfsmaintenance
- cmurf 6y ago
  openSUSE has automatic snapshots with a fairly extensive retention policy. Kernel 3.14 is ancient. I can't bring myself to worry about it. I think your fourth paragraph cherry pick is disingenuous. A more complete excerpt, "There’s a special case when the block groups are completely unused, possibly left after removing lots of files or deleting snapshots. Removing empty block groups is automatic since 3.18." Fedora 33 users will be getting kernel 5.8 from day one, and 5.9 soon after release. I doubt my case is atypical. I haven't balanced my years old non-test real world used Btrfs file systems. I think you could switch your concern and criticism to the fact wikis get stale.