On Fri, 7 Jan 2022, Martin K. Petersen wrote: > Eric, > > > Even new new RAID controlers that _do_ provide `io_opt` still do _not_ > > indicate partial_stripes_expensive (which is an mdraid feature, but Martin > > please correct me if I'm wrong here). > > partial_stripes_expensive is a bcache thing, I am not sure why it needs > a separate flag. It is implied, although I guess one could argue that > RAID0 is a special case since partial writes are not as painful as with > parity RAID. I'm guessing bcache used did some optimization for queue->limits.raid_partial_stripes_expensive because md raid5 code sets this flag. At least when using Linux md as the RAID5 implementation it gets configured automatically: raid5.c: mddev->queue->limits.raid_partial_stripes_expensive = 1; https://elixir.bootlin.com/linux/latest/source/drivers/md/raid5.c#L7729 Interestingly only bcache uses it, but md does set it. > The SCSI spec states that submitting an I/O that is smaller than io_min > "may incur delays in processing the command". And similarly, submitting > a command larger than io_opt "may incur delays in processing the > command". > > IOW, the spec says "don't write less than an aligned multiple of the > stripe chunk size" and "don't write more than an aligned full > stripe". That leaves "aligned multiples of the stripe chunk size but > less than the full stripe width" unaccounted for. And I guess that's > what the bcache flag is trying to capture. Maybe any time io_opt is provided then partial_stripes_expensive should be flagged too and any code to the contrary should be removed? Question: Does anyone have a reason to keep partial_stripes_expensive in the kernel at all? > SCSI doesn't go into details about RAID levels and other implementation > details which is why the wording is deliberately vague. But obviously > the expectation is that partial stripe writes are slower than full. > > In my book any component in the stack that sees either io_min or io_opt > should try very hard to send I/Os that are aligned multiples of those > values. I am not opposed to letting users manually twiddle the > settings. But I do think that we should aim for the stack doing the > right thing when it sees io_opt reported on a device. Agreed, thanks for the feedback! -Eric > > -- > Martin K. Petersen Oracle Linux Engineering >
