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

[Jens Axboe <axboe@fb.com>]

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.

> If this goes in I know for sure I'll need to work around it in DM-thinp.

In any case, that's a good point, dm/md should not have their own lower 
restrictions.

-- 
Jens Axboe