Re: [PATCH 2/2] xfs: add 'discard_sync' mount flag

[Jens Axboe <axboe@kernel.dk>]

On 4/30/18 12:25 PM, Luis R. Rodriguez wrote:
> On Mon, Apr 30, 2018 at 12:07:31PM -0600, Jens Axboe wrote:
>> On 4/30/18 11:19 AM, Brian Foster wrote:
>>> On Mon, Apr 30, 2018 at 09:32:52AM -0600, Jens Axboe wrote:
>>>> XFS recently added support for async discards. While this can be
>>>> a win for some workloads and devices, there are also cases where
>>>> async bursty discard will severly harm the latencies of reads
>>>> and writes.
>>>>
>>>> Add a 'discard_sync' mount flag to revert to using sync discard,
>>>> issuing them one at the time and waiting for each one. This fixes
>>>> a big performance regression we had moving to kernels that include
>>>> the XFS async discard support.
>>>>
>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>> ---
>>>
>>> Hm, I figured the async discard stuff would have been a pretty clear win
>>> all around, but then again I'm not terribly familiar with what happens
>>> with discards beneath the fs. I do know that the previous behavior would
>>> cause fs level latencies due to holding up log I/O completion while
>>> discards completed one at a time. My understanding is that this lead to
>>> online discard being pretty much universally "not recommended" in favor
>>> of fstrim.
>>
>> It's not a secret that most devices suck at discard.
> 
> How can we know if a device sucks at discard?

This test usually works well - does it support discard? Then it probably
sucks :-)

But seriously, synthetic test case with reads/writes (the actual workload)
and then mix in trims. If the performance suffers, then discards suck.
Just consider this series - it was a 25% latency win.

>> While the async
>> discard is nifty and I bet works well for some cases, it can also cause
>> a flood of discards on the device side which does not work well for
>> other cases.
> 
> Shouldn't async then be only enabled if the device used supports it well?
> Or should a blacklist per device be more suitable? Which is more popular?

You'd be left with nothing... My general recommendation is to not use
discards at all, unless there's a proven case that it makes a write
amplification difference for you - from at all, to "big enough that I'd
suffer the latency consequences".

-- 
Jens Axboe