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

[Konstantinos Skarlatos <k.skarlatos@gmail.com>]

On 1/4/2022 4:17 μμ, Hugo Mills wrote:
> 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?
Hi, sorry for missing your reply.
I only have 12 subvolumes and no snapshots. There are about 10 million 
files in the filesystem.

>
>     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?
It is progressing without getting stuck, every few minutes (sometimes 
sooner sometimes longer) a new block gets balanced

>
>     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.
Thanks for the advice. I think this is something that should be better 
documented as the official wiki says that this must be done after adding 
a new disk


>
>> 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.