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

On 05/06/2021 21:54, Wang Shanker wrote:

You may wonder the importance of merging discard operations. In the
implementation of RAID456, bios are committed in 4k trunks (they call
them as stripes in the code and the size is determined by DEFAULT_STRIPE_SIZE).
The proper merging of the bios is of vital importance for a reasonable
operating performance of RAID456 devices.

Note that I have seen reports (I'm not sure where or how true they are), 
that even when requests are sent as 512k or whatever, certain upper 
layers break them into 4k's, presumably expecting lower layers to merge 
them again.

You might have better luck looking for and suppressing the breaking up 
of large chunks.

Cheers,
Wol