linux-kernel - Re: [PATCH 2/2] block: make /sys/block/<dev>/queue/discard_max

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Message-ID: <55A52ACA.3090201@fb.com>
Date:	Tue, 14 Jul 2015 09:29:14 -0600
From:	Jens Axboe <axboe@...com>
To:	Mike Snitzer <snitzer@...hat.com>
CC:	<linux-kernel@...r.kernel.org>, <hch@....de>
Subject: Re: [PATCH 2/2] block: make /sys/block/<dev>/queue/discard_max_bytes
 writeable

On 07/14/2015 09:23 AM, Mike Snitzer wrote:
> On Tue, Jul 14 2015 at 11:02am -0400,
> Jens Axboe <axboe@...com> wrote:
>
>> Lots of devices support huge discard sizes these days. Depending
>> on how the device handles them internally, huge discards can
>> introduce massive latencies (hundreds of msec) on the device side.
>>
>> We have a sysfs file, discard_max_bytes, that advertises the max
>> hardware supported discard size. Make this writeable, and split
>> the settings into a soft and hard limit. This can be set from
>> 'discard_granularity' and up to the hardware limit.
>
> Looks pretty good, but we'll lose the original discard_max_bytes once it
> is changed.  That information loss will prevent users from knowing what
> adjustments are possible over time.
>
> This may be OK, but figured i'd raise it.

That's true, I should have mentioned that. But if you write a higher 
value than the device supports, then it will be truncated to the max 
value that the device supports. So it's not really lost, and it was the 
best alternative I could think of.

-- 
Jens Axboe

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