linux-kernel - Re: [PATCH 2/3] block: make /sys/block/<dev>/queue/discard_max

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Message-ID: <55A7C9A0.5060506@fb.com>
Date:	Thu, 16 Jul 2015 09:11:28 -0600
From:	Jens Axboe <axboe@...com>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	<linux-kernel@...r.kernel.org>, <snitzer@...hat.com>, <hch@....de>
Subject: Re: [PATCH 2/3] block: make /sys/block/<dev>/queue/discard_max_bytes
 writeable

On 07/16/2015 09:07 AM, Jeff Moyer wrote:
> Jens Axboe <axboe@...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@...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@...hat.com>' on there.

I thought you meant sysfs, so I had to go back and check. But 
documentation, yep, I forgot that. I'll add it. Thanks!

-- 
Jens Axboe

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