On 07/14/2015 10:01 AM, Christoph Hellwig wrote:
On Tue, Jul 14, 2015 at 09:02:17AM -0600, Jens Axboe wrote:
Hi,
Most drivers use UINT_MAX (or some variant thereof) for max discard
size, since they don't have a real limit for a non-data transferring
command. This is fine from a throughput point of view, but for a lot
of devices (all?), it truly sucks on latency. We've seen cases of
hundreds of msec in latencies for reads/writes when deleting files
on an fs with discard enabled. For the problematic devices that we
have tested, artificially limiting the size of the discards issued
brings it down to a more manageable 1-2ms max latencies.
This looks reasonable to me. Any chance you could also come up
with reasonable start values for the hardware you've done this for
so that we can get a good out of the box experience?
Based on experimentation with two different vendors, 64MB would be a
good default. Question is how best to set that. At least with the
current patch, untouched, 'discard_max_bytes' will still show the hw max
value. If I default it to 64MB, we'd lose that information until people
started bumping it up in size. Maybe that's not such a big deal, however.
--
Jens Axboe
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