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

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Date:	Wed, 13 May 2015 10:20:12 -0400
From:	Jens Axboe <axboe@...com>
To:	Jeff Moyer <jmoyer@...hat.com>, Shaohua Li <shli@...com>
CC:	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] blk: don't account discard request size

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.


-- 
Jens Axboe

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