Re: [LSF/MM TOPIC] More async operations for file systems - async discard?

[Ric Wheeler <ricwheeler@gmail.com>]

On 2/17/19 9:22 PM, Dave Chinner wrote:
> On Sun, Feb 17, 2019 at 06:42:59PM -0500, Ric Wheeler wrote:
>> On 2/17/19 4:09 PM, Dave Chinner wrote:
>>> On Sun, Feb 17, 2019 at 03:36:10PM -0500, Ric Wheeler wrote:
>>>> One proposal for btrfs was that we should look at getting discard
>>>> out of the synchronous path in order to minimize the slowdown
>>>> associated with enabling discard at mount time. Seems like an
>>>> obvious win for "hint" like operations like discard.
>>> We already have support for that. blkdev_issue_discard() is
>>> synchornous, yes, but __blkdev_issue_discard() will only build the
>>> discard bio chain - it is up to the caller to submit and wait for it.
>>>
>>> Some callers (XFS, dm-thinp, nvmet, etc) use a bio completion to
>>> handle the discard IO completion, hence allowing async dispatch and
>>> processing of the discard chain without blocking the caller. Others
>>> (like ext4) simply call submit_bio_wait() to do wait synchronously
>>> on completion of the discard bio chain.
>>>
>>>> I do wonder where we stand now with the cost of the various discard
>>>> commands - how painful is it for modern SSD's?
>>> AIUI, it still depends on the SSD implementation, unfortunately.
>> I think the variability makes life really miserable for layers above it.
> Yup, that it does.
>
>> Might be worth constructing some tooling that we can use to validate
>> or shame vendors over
> That doesn't seem to work.
>
>> - testing things like a full device discard,
>> discard of fs block size and big chunks, discard against already
>> discarded, etc.
> We did that many years ago because discard on SSDs sucked:
>
> https://people.redhat.com/lczerner/discard/test_discard.html
> https://sourceforge.net/projects/test-discard/files/
>
> And, really, that didn't changed a thing - discard still sucks...
>
> Cheers,
>
> Dave.

Totally forgot about that work - maybe it is time to try again and poke some 
vendors.

Ric