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

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Message-ID: <20150714204419.GA7915@redhat.com>
Date:	Tue, 14 Jul 2015 16:44:20 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Jens Axboe <axboe@...com>
Cc:	linux-kernel@...r.kernel.org, hch@....de
Subject: Re: [PATCH 3/3] block: by default, limit maximum discard size to 64MB

On Tue, Jul 14 2015 at  2:48pm -0400,
Jens Axboe <axboe@...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@...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.

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