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SubjectRe: [PATCH 3/3] block: by default, limit maximum discard size to 64MB
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@fb.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@fb.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.

--
Jens Axboe



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