Re: [Bug Report] Discard bios cannot be correctly merged in blk-mq

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Hi Miao

So you plan to fix this problem now? The plan is to submit the discard
bio directly to disk similar with raid0/raid10.
As we talked, it needs to consider the discard region. It should be
larger than chunk_sectors * nr_data_disks. It needs
to split the bio when its size not aligned with chunk_sectors *
nr_data_disks. And it needs to consider the start address
of the bio too. If it's not aligned with a start address of
chunk_sectors, it's better to split this part too.

I'm working on another job. So I don't have time to do this now. If
you submit the patches, I can help to review :)

Regards
Xiao

On Fri, Jun 18, 2021 at 2:28 PM Wang Shanker <shankerwangmiao@xxxxxxxxx> wrote:
>
> Hi, Xiao
>
> Any ideas on this issue?
>
> Cheers,
>
> Miao Wang
>
> > 2021年06月09日 17:03,Wang Shanker <shankerwangmiao@xxxxxxxxx> 写道:
> >
> >>
> >> 2021年06月09日 16:44,Xiao Ni <xni@xxxxxxxxxx> 写道:
> >>
> >> Hi all
> >>
> >> Thanks for reporting about this. I did a test in my environment.
> >> time blkdiscard /dev/nvme5n1  (477GB)
> >> real    0m0.398s
> >> time blkdiscard /dev/md0
> >> real    9m16.569s
> >>
> >> I'm not familiar with the block layer codes. I'll try to understand
> >> the codes related with discard request and
> >> try to fix this problem.
> >>
> >> I have a question for raid5 discard, it needs to consider more than
> >> raid0 and raid10. For example, there is a raid5 with 3 disks.
> >> D11 D21 P1 (stripe size is 4KB)
> >> D12 D22 P2
> >> D13 D23 P3
> >> D14 D24 P4
> >> ...  (chunk size is 512KB)
> >> If there is a discard request on D13 and D14, and there is no discard
> >> request on D23 D24. It can't send
> >> discard request to D13 and D14, right? P3 = D23 xor D13. If we discard
> >> D13 and disk2 is broken, it can't
> >> get the right data from D13 and P3. The discard request on D13 can
> >> write 0 to the discard region, right?
> >
> > Yes. It can be seen at the beginning of make_discard_request(), where
> > the requested range being discarded is aligned to ``stripe_sectors",
> > which should be chunk_sectors * nr_data_disks.
> >
> > Cheers,
> >
> > Miao Wang
>





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