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

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Date:	Wed, 13 May 2015 09:10:25 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Shaohua Li <shli@...com>
Cc:	<linux-kernel@...r.kernel.org>, <axboe@...com>
Subject: Re: [PATCH] blk: don't account discard request size

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.

Cheers,
Jeff
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