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

From: Jens Axboe
Date: Wed May 13 2015 - 10:20:46 EST

On 05/13/2015 09:10 AM, Jeff Moyer wrote:
Shaohua Li <shli@xxxxxx> 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@xxxxxx>
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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