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

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> 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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