Re: migrating to space_cache=2 and btrfs userspace commands

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2021/7/13 下午11:38, DanglingPointer wrote:
> We're currently considering switching to "space_cache=v2" with noatime
> mount options for my lab server-workstations running RAID5.

Btrfs RAID5 is unsafe due to its write-hole problem.

>
>   * One has 13TB of data/metadata in a bunch of 6TB and 2TB disks
>     totalling 26TB.
>   * Another has about 12TB data/metadata in uniformly sized 6TB disks
>     totalling 24TB.
>   * Both of the arrays are on individually luks encrypted disks with
>     btrfs on top of the luks.
>   * Both have "defaults,autodefrag" turned on in fstab.
>
> We're starting to see large pauses during constant backups of millions
> of chunk files (using duplicacy backup) in the 24TB array.
>
> Pauses sometimes take up to 20+ seconds in frequencies after every
> ~30secs of the end of the last pause.  "btrfs-transacti" process
> consistently shows up as the blocking process/thread locking up
> filesystem IO.  IO gets into the RAID5 array via nfsd. There are no disk
> or btrfs errors recorded.  scrub last finished yesterday successfully.

Please provide the "echo l > /proc/sysrq-trigger" output when such pause
happens.

If you're using qgroup (may be enabled by things like snapper), it may
be the cause, as qgroup does its accounting when committing transaction.

If one transaction is super large, it can cause such problem.

You can test if qgroup is enabled by:

# btrfs qgroup show -prce <mnt>

>
> After doing some research around the internet, we've come to the
> consideration above as described.  Unfortunately the official
> documentation isn't clear on the following.
>
> Official documentation URL -
> https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs(5)
>
> 1. How to migrate from default space_cache=v1 to space_cache=v2? It
>     talks about the reverse, from v2 to v1!

Just mount with "space_cache=v2".

> 2. If we use space_cache=v2, is it indeed still the case that the
>     "btrfs" command will NOT work with the filesystem?

Why would you think "btrfs" won't work on a btrfs?

Thanks,
Qu

>  So will our
>     "btrfs scrub start /mount/point/..." cron jobs FAIL?  I'm guessing
>     the btrfs command comes from btrfs-progs which is currently v5.4.1-2
>     amd64, is that correct?
> 3. Any other ideas on how we can get rid of those annoying pauses with
>     large backups into the array?
>
> Thanks in advance!
>
> DP
>