Re: [PATCH 0/2] Configurable max discard size

[Jens Axboe <axboe@fb.com>]

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