linux-kernel - Re: [PATCH 3/3] block: by default, limit maximum discard size to 64MB

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Date:	Wed, 15 Jul 2015 07:46:48 -0400
From:	Austin S Hemmelgarn <ahferroin7@...il.com>
To:	Jens Axboe <axboe@...com>, Mike Snitzer <snitzer@...hat.com>
CC:	linux-kernel@...r.kernel.org, hch@....de
Subject: Re: [PATCH 3/3] block: by default, limit maximum discard size to
 64MB

On 2015-07-14 17:48, Jens Axboe wrote:
> On 07/14/2015 02:45 PM, Jens Axboe wrote:
>> On 07/14/2015 02:44 PM, Mike Snitzer wrote:
>>> On Tue, Jul 14 2015 at  2:48pm -0400,
>>> Jens Axboe <axboe@...com> wrote:
>>>
>>>> Lots of devices exhibit very high latencies for big discards, hurting
>>>> reads and writes. By default, limit the max discard we will build to
>>>> 64MB. This value has shown good results across a number of devices.
>>>>
>>>> This will potentially hurt discard throughput, from a provisioning
>>>> point of view (when the user does mkfs.xfs, for instance, and mkfs
>>>> issues a full device discard). If that becomes an issue, we could
>>>> have different behavior for provisioning vs runtime discards.
>>>>
>>>> Signed-off-by: Jens Axboe <axboe@...com>
>>>
>>> Christoph suggested you impose this default for the specific
>>> drivers/devices that benefit.  I'm not following why imposing a 64MB
>>> default is the right thing to do for all devices.
>>
>> I'd argue that's most of them... But the testing we did was on NVMe. I
>> can limit it to NVMe, no big deal.
>
> Oh, and LSI flash too, so not just NVMe.
>
While I don't have time to test it, I have a feeling that such a limit 
would help with many of the consumer SSD's out there.  Secondarily, once 
this gets in and discard is fixed for BTRFS, I'll have some performance 
testing to do WRT dm-thinp.


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