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From: Jeff Moyer <jmoyer@redhat.com>
To: Jens Axboe <axboe@fb.com>
Cc: <linux-kernel@vger.kernel.org>, <snitzer@redhat.com>, <hch@lst.de>
Subject: Re: [PATCH 2/3] block: make /sys/block/<dev>/queue/discard_max_bytes writeable
References: <1436899703-31966-1-git-send-email-axboe@fb.com>
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Date: Thu, 16 Jul 2015 11:07:20 -0400
In-Reply-To: <1436899703-31966-3-git-send-email-axboe@fb.com> (Jens Axboe's
	message of "Tue, 14 Jul 2015 12:48:22 -0600")
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Jens Axboe <axboe@fb.com> writes:

> 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.
>
> Add a new sysfs file, 'discard_max_hw_bytes', that shows the hw
> set limit.
>
> Signed-off-by: Jens Axboe <axboe@fb.com>
> ---
>  Documentation/block/queue-sysfs.txt |  4 +++-
>  block/blk-settings.c                |  4 ++++
>  block/blk-sysfs.c                   | 40 ++++++++++++++++++++++++++++++++++++-
>  include/linux/blkdev.h              |  1 +
>  4 files changed, 47 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/block/queue-sysfs.txt b/Documentation/block/queue-sysfs.txt
> index 3a29f8914df9..3748cf827131 100644
> --- a/Documentation/block/queue-sysfs.txt
> +++ b/Documentation/block/queue-sysfs.txt
> @@ -20,7 +20,7 @@ This shows the size of internal allocation of the device in bytes, if
>  reported by the device. A value of '0' means device does not support
>  the discard functionality.
>  
> -discard_max_bytes (RO)
> +discard_max_bytes (RW)
>  ----------------------
>  Devices that support discard functionality may have internal limits on
>  the number of bytes that can be trimmed or unmapped in a single operation.
> @@ -28,6 +28,8 @@ The discard_max_bytes parameter is set by the device driver to the maximum
>  number of bytes that can be discarded in a single operation. Discard
>  requests issued to the device must not exceed this limit. A discard_max_bytes
>  value of 0 means that the device does not support discard functionality.
> +Writing a lower value to this file can limit the maximum discard size issued
> +to the device, which can help latencies.

You forgot to add a new entry for discard_max_hw_bytes.  Fix that and
you can slap my 'Reviewed-by: Jeff Moyer <jmoyer@redhat.com>' on there.

Cheers,
Jeff
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