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

[Wang Shanker <shankerwangmiao@gmail.com>]

Hi, Xiao

Any ideas on this issue?

Cheers,

Miao Wang

> 2021年06月09日 17:03，Wang Shanker <shankerwangmiao@gmail.com> 写道：
> 
>> 
>> 2021年06月09日 16:44，Xiao Ni <xni@redhat.com> 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