Re: btrfs metadata has reserved 1T of extra space and balances don't reclaim it

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

On 2021/9/29 10:23, Brandon Heisner wrote:
> I have a server running CentOS 7 on 4.9.5-1.el7.elrepo.x86_64 #1 SMP Fri Jan 20 11:34:13 EST 2017 x86_64 x86_64 x86_64 GNU/Linux.  It is version locked to that kernel.  The metadata has reserved a full 1T of disk space, while only using ~38G.  I've tried to balance the metadata to reclaim that so it can be used for data, but it doesn't work and gives no errors.  It just says it balanced the chunks but the size doesn't change.  The metadata total is still growing as well, as it used to be 1.04 and now it is 1.08 with only about 10G more of metadata used.  I've tried doing balances up to 70 or 80 musage I think, and the total metadata does not decrease.  I've done so many attempts at balancing, I've probably tried to move 300 chunks or more.  None have resulted in any change to the metadata total like they do on other servers running btrfs.  I first started with very low musage, like 10 and then increased it by 10 to try to see if that would balance any chunks out, but with no success.
>
> # /sbin/btrfs balance start -musage=60 -mlimit=30 /opt/zimbra
> Done, had to relocate 30 out of 2127 chunks

One question is, did -musage=0 resulted any change?

If there are empty metadata block groups, btrfs should be able to
reclaim without any extra commands.

And is there any dmesg during above -musage=0 output?

Thanks,
Qu

>
> I can do that command over and over again, or increase the mlimit, and it doesn't change the metadata total ever.
>
>
> # btrfs fi show /opt/zimbra/
> Label: 'Data'  uuid: ece150db-5817-4704-9e84-80f7d8a3b1da
>          Total devices 4 FS bytes used 1.48TiB
>          devid    1 size 1.46TiB used 1.38TiB path /dev/sde
>          devid    2 size 1.46TiB used 1.38TiB path /dev/sdf
>          devid    3 size 1.46TiB used 1.38TiB path /dev/sdg
>          devid    4 size 1.46TiB used 1.38TiB path /dev/sdh
>
> # btrfs fi df /opt/zimbra/
> Data, RAID10: total=1.69TiB, used=1.45TiB
> System, RAID10: total=64.00MiB, used=640.00KiB
> Metadata, RAID10: total=1.08TiB, used=37.69GiB
> GlobalReserve, single: total=512.00MiB, used=0.00B
>
>
> # btrfs fi us /opt/zimbra/ -T
> Overall:
>      Device size:                   5.82TiB
>      Device allocated:              5.54TiB
>      Device unallocated:          291.54GiB
>      Device missing:                  0.00B
>      Used:                          2.96TiB
>      Free (estimated):            396.36GiB      (min: 396.36GiB)
>      Data ratio:                       2.00
>      Metadata ratio:                   2.00
>      Global reserve:              512.00MiB      (used: 0.00B)
>
>              Data      Metadata  System
> Id Path     RAID10    RAID10    RAID10    Unallocated
> -- -------- --------- --------- --------- -----------
>   1 /dev/sde 432.75GiB 276.00GiB  16.00MiB   781.65GiB
>   2 /dev/sdf 432.75GiB 276.00GiB  16.00MiB   781.65GiB
>   3 /dev/sdg 432.75GiB 276.00GiB  16.00MiB   781.65GiB
>   4 /dev/sdh 432.75GiB 276.00GiB  16.00MiB   781.65GiB
> -- -------- --------- --------- --------- -----------
>     Total      1.69TiB   1.08TiB  64.00MiB     3.05TiB
>     Used       1.45TiB  37.69GiB 640.00KiB
>
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