Re: [PATCH 0/5] btrfs: scrub: improve the scrub performance

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

On 2023/8/2 10:15, Jani Partanen wrote:
> 
> On 02/08/2023 4.56, Qu Wenruo wrote:
>>
>> So the btrfs scrub report is doing the correct report using the values
>> from kernel.
>>
>> And considering the used space is around 600G, divided by 4 disks (aka,
>> 3 data stripes + 1 parity stripes), it's not that weird, as we would got
>> around 200G per device (parity doesn't contribute to the scrubbed bytes).
>>
>> Especially considering your metadata is RAID1C4, it means we should only
>> have more than 200G.
>> Instead it's the old report of less than 200G doesn't seem correct.
>>
>> Mind to provide the output of "btrfs fi usage <mnt>" to verify my
>> assumption?
>>
> btrfs fi usage /mnt/
> Overall:
>      Device size:                   1.16TiB
>      Device allocated:            844.25GiB
>      Device unallocated:          348.11GiB
>      Device missing:                  0.00B
>      Device slack:                    0.00B
>      Used:                        799.86GiB
>      Free (estimated):            289.58GiB      (min: 115.52GiB)
>      Free (statfs, df):           289.55GiB
>      Data ratio:                       1.33
>      Metadata ratio:                   4.00
>      Global reserve:              471.80MiB      (used: 0.00B)
>      Multiple profiles:                  no
> 
> Data,RAID5: Size:627.00GiB, Used:598.51GiB (95.46%)
>     /dev/sdb      209.00GiB
>     /dev/sdc      209.00GiB
>     /dev/sdd      209.00GiB
>     /dev/sde      209.00GiB

OK, my previous calculation is incorrect...

For each device there should be 209GiB used by RAID5 chunks, and only 
3/4 of them contributes to the scrubbed data bytes.

Thus there seems to be some double accounting.

Definitely needs extra digging for this situation.

Thanks,
Qu

> 
> Metadata,RAID1C4: Size:2.00GiB, Used:472.56MiB (23.07%)
>     /dev/sdb        2.00GiB
>     /dev/sdc        2.00GiB
>     /dev/sdd        2.00GiB
>     /dev/sde        2.00GiB
> 
> System,RAID1C4: Size:64.00MiB, Used:64.00KiB (0.10%)
>     /dev/sdb       64.00MiB
>     /dev/sdc       64.00MiB
>     /dev/sdd       64.00MiB
>     /dev/sde       64.00MiB
> 
> Unallocated:
>     /dev/sdb       87.03GiB
>     /dev/sdc       87.03GiB
>     /dev/sdd       87.03GiB
> 
>     /dev/sde       87.03GiB
> 
> 
> There is 1 extra 2GB file now so thats why it show little more usage now.
> 
> 
>> Sure, I'll CC you when refreshing the patchset, extra tests are always
>> appreciated.
>>
> Sound good, thanks!
>