Date: Tue, 14 Jul 2015 18:01:47 +0200
From: Christoph Hellwig <hch@lst.de>
To: Jens Axboe <axboe@fb.com>
Cc: linux-kernel@vger.kernel.org, snitzer@redhat.com, hch@lst.de
Subject: Re: [PATCH 0/2] Configurable max discard size
Message-ID: <20150714160147.GA28031@lst.de>
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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?
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