Re: [RFC 0/5] implement asynchronous BLKDISCARD via io_uring

Pavel Begunkov
2024-08-15T17:26:12Z

On 8/15/24 16:50, Jens Axboe wrote:
> On 8/14/24 4:45 AM, Pavel Begunkov wrote:
>> There is an interest in having asynchronous block operations like
>> discard. The patch set implements that as io_uring commands, which is
>> an io_uring request type allowing to implement custom file specific
>> operations.
>>
>> First 4 patches are simple preps, and the main part is in Patch 5.
>> Not tested with a real drive yet, hence sending as an RFC.
>>
>> I'm also going to add BLKDISCARDZEROES and BLKSECDISCARD, which should
>> reuse structures and helpers from Patch 5.
>>
>> liburing tests for reference:
>>
>> https://github.com/isilence/liburing.git discard-cmd-test
>
> FWIW, did a quick patch to wire it up for fio. Using this
> job file:
>
> [trim]
> filename=/dev/nvme31n1
> ioengine=io_uring
> iodepth=64
> rw=randtrim
> norandommap
> bs=4k
>
> the stock kernel gets:
>
> trim: IOPS=21.6k, BW=84.4MiB/s (88.5MB/s)(847MiB/10036msec); 0 zone resets
>
> using ~5% CPU, and with the process predictably stuck in D state all of
> the time, waiting on a sync trim.
>
> With the patches:
>
> trim: IOPS=75.8k, BW=296MiB/s (310MB/s)(2653MiB/8961msec); 0 zone resets
>
> using ~11% CPU.

Thanks for giving it a run. Can be further improved for particular use cases,
e.g. by adding a vectored version as an additional feature, i.e. multiple
discard ranges per request, and something like retry of short IO, but for
that we'd want getting -EAGAIN directly from submit_bio unlike failing via
the callback.

> Didn't verify actual functionality, but did check trims are going up and
> down. Drive used is:
>
> Dell NVMe PM1743 RI E3.S 7.68TB
>
> Outside of async trim being useful for actual workloads rather than the
> sync trim we have now, it'll also help characterizing real world mixed
> workloads that have trims with reads/writes.

--
Pavel Begunkov