linux-kernel - Re: [PATCH] blk: don't account discard request size

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Date:	Wed, 13 May 2015 11:32:02 -0400
From:	Jens Axboe <axboe@...com>
To:	Shaohua Li <shli@...com>
CC:	Jeff Moyer <jmoyer@...hat.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] blk: don't account discard request size

On 05/13/2015 11:22 AM, Shaohua Li wrote:
> On Wed, May 13, 2015 at 10:20:12AM -0400, Jens Axboe wrote:
>> On 05/13/2015 09:10 AM, Jeff Moyer wrote:
>>> Shaohua Li <shli@...com> writes:
>>>
>>>> In a workload with discard request, the IO throughput is generally much
>>>> higher than expected. This is quite confusing checking iostat. Discard
>>>> request doesn't really write data to drive, so don't account it.
>>>>
>>>> Signed-off-by: Shaohua Li <shli@...com>
>>>> ---
>>>>   block/blk-core.c | 6 +++++-
>>>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/block/blk-core.c b/block/blk-core.c
>>>> index fd154b9..0128d18 100644
>>>> --- a/block/blk-core.c
>>>> +++ b/block/blk-core.c
>>>> @@ -2138,7 +2138,11 @@ EXPORT_SYMBOL_GPL(blk_rq_err_bytes);
>>>>
>>>>   void blk_account_io_completion(struct request *req, unsigned int bytes)
>>>>   {
>>>> -	if (blk_do_io_stat(req)) {
>>>> +	/*
>>>> +	 * discard request doesn't really write @bytes to drive,
>>>> +	 * doesn't account it
>>>> +	 **/
>>>> +	if (blk_do_io_stat(req) && !(req->cmd_flags & REQ_DISCARD)) {
>>>>   		const int rw = rq_data_dir(req);
>>>>   		struct hd_struct *part;
>>>>   		int cpu;
>>>
>>> I think you want to modify __get_request to not set REQ_IO_STAT for
>>> discard requests.  This patch will still account the start of I/O, which
>>> means in_flight will be off.
>>
>> That would be better. But I'm still not sure we want to turn off
>> accounting for discards. For the mixed write/discard cases it's
>> definitely confusing. The better option would be to account it as a
>> discard and not a write. Preferably in a way that would not break
>> existing tools, but so that they could get updated to support it.
>
> It's intentional discard IO start gets accounted, so tools will show
> there is IO. I'm not sure if this is better though.
>
> Adding separate columns for discard (maybe flush too) is definitely
> preferred. Is breaking existing tools really ok?

We can't break then, I was just curious if adding a field to the end of 
the diskstats would potentially not break old applications. If not, they 
could just be updated to grab the new field too.

-- 
Jens Axboe

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