Re: df free space not correct with raid1 pools with an odd number of devices

[Andrei Borzenkov <arvidjaar@gmail.com>]

25.07.2020 13:04, Jorge Bastos пишет:
> On Sat, Jul 25, 2020 at 8:43 AM Andrei Borzenkov <arvidjaar@gmail.com> wrote:
>>
> 
>>
>> OTOH, this is the correct if the most pessimistic estimation either. If
>> you have three 250G RAID1 devices and you allocate 250G data in one file
>> you consume two full devices and won't be able to allocate new data at
>> all (or for that matter no new metadata either).
>>
>>
>> So whatever value btrfs returns will be wrong for some allocation pattern.
> 
> I considered that but wouldn't a single file still be stripped and
> blocks allocated to all devices, on a most free space basis?
> 

Yes, I was unsure and stay corrected. It seems real allocation happens
per chunk and so gets distributed to all devices.

Sorry.

> E.g.: 3 x 500GB RAID1 pool:
> 
> $ btrfs fi usage -T /mnt/cache
> Overall:
>     Device size:                   1.36TiB
>     Device allocated:              2.13GiB
>     Device unallocated:            1.36TiB
>     Device missing:                  0.00B
>     Used:                        288.00KiB
>     Free (estimated):            697.61GiB      (min: 697.61GiB)
>     Data ratio:                       2.00
>     Metadata ratio:                   2.00
>     Global reserve:                3.25MiB      (used: 32.00KiB)
>     Multiple profiles:                  no
> 
>              Data     Metadata  System
> Id Path      RAID1    RAID1     RAID1    Unallocated
> -- --------- -------- --------- -------- -----------
>  1 /dev/sdb1 36.00MiB         - 32.00MiB   465.69GiB
>  2 /dev/sde1 36.00MiB   1.00GiB 32.00MiB   464.69GiB
>  3 /dev/sdf1        -   1.00GiB        -   464.76GiB
> -- --------- -------- --------- -------- -----------
>    Total     36.00MiB   1.00GiB 32.00MiB     1.36TiB
>    Used         0.00B 128.00KiB 16.00KiB
> 
> 
> 
> $ fallocate -l 690G /mnt/cache/file
> $ btrfs fi usage -T /mnt/cache
> Overall:
>     Device size:                   1.36TiB
>     Device allocated:              1.35TiB
>     Device unallocated:           13.15GiB
>     Device missing:                  0.00B
>     Used:                          1.35TiB
>     Free (estimated):              7.61GiB      (min: 7.61GiB)
>     Data ratio:                       2.00
>     Metadata ratio:                   2.00
>     Global reserve:                3.25MiB      (used: 0.00B)
>     Multiple profiles:                  no
> 
>              Data      Metadata  System
> Id Path      RAID1     RAID1     RAID1     Unallocated
> -- --------- --------- --------- --------- -----------
>  1 /dev/sdb1 461.04GiB         -  32.00MiB     4.69GiB
>  2 /dev/sde1 460.04GiB   1.00GiB  32.00MiB     4.69GiB
>  3 /dev/sdf1 461.00GiB   1.00GiB         -     3.76GiB
> -- --------- --------- --------- --------- -----------
>    Total     691.04GiB   1.00GiB  32.00MiB    13.15GiB
>    Used      690.00GiB 976.00KiB 112.00KiB
> 
> Jorge
>