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

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 [off-list ref] wrote:

Hi, Xiao

Any ideas on this issue?

Cheers,

Miao Wang

quoted

2021年06月09日 17:03，Wang Shanker [off-list ref] 写道：

quoted

2021年06月09日 16:44，Xiao Ni [off-list ref] 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