Re: Adding a 4TB disk to a 2x4TB btrfs (data:single) filesystem and balancing takes extremely long (over a month). Filesystem has been deduped with bees

[Hugo Mills <hugo@carfax.org.uk>]

On Fri, Apr 01, 2022 at 02:13:58PM +0300, Konstantinos Skarlatos wrote:
> Hello,
> I am running btrfs on 2x 4TB HDDs (data: single, metadata: raid1) and i
> added another 4TB disk.
> According to btrfs wiki i should run balance after adding the new device.
> My problem is that this balance takes extremely long, it is running for 4
> days and it still has 91% left.
> Is this normal, and can i do anything to fix this?

   It's not normal for it to take that long, no. Do you have lots of
snapshots (like, thousands), and lots of small or heavily fragmented
files?

   Is the balance actually progressing, or has it got stuck? Are there
regular messages in dmesg about it balancing block groups? If not,
when was the last one?

   If your data is single, it's not really necessary to do the balance
anyway, so you may want to cancel it. The balance in this situation is
more about ensuring that all the space on the new disk is usable by
the higher RAID levels. For example, adding a single disk to a
nearly-full RAID-1 without balancing would leave you in a state where
you couldn't add more data chunks because there's only space on one
disk to do so, and RAID-1 needs two disks with space to allocate a
data chunk in. With single data, that's not a problem.

> kernel is linux-5.17.1, i have also tried with 5.16 kernels.
> mount options are: rw,relatime,compress-force=zstd:11,space_cache=v2
> I have been using bees for dedup, but it is disabled for the balance.
> I am not doing any IO on the disks, they have no smart errors, and none of
> them are SMR (2x WD40EFRX and 1x ST4000DM000)
> Autodefrag is disabled, and i also have checked that the disks are in stable
> drive cages in order to be sure i have no problems with vibration.
> Benchmarking them gives normal speeds. Quotas have never been enabled.

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