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

[Austin S Hemmelgarn]

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 <ax...@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 <ax...@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.

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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