Re: [PATCH] block: add iostat counters for flush requests

From: Jens Axboe
Date: Thu Nov 21 2019 - 11:07:31 EST

On 11/21/19 9:04 AM, Darrick J. Wong wrote:
> On Thu, Nov 21, 2019 at 08:56:14AM -0700, Jens Axboe wrote:
>> On 11/21/19 3:40 AM, Konstantin Khlebnikov wrote:
>>> Requests that triggers flushing volatile writeback cache to disk (barriers)
>>> have significant effect to overall performance.
>>> Block layer has sophisticated engine for combining several flush requests
>>> into one. But there is no statistics for actual flushes executed by disk.
>>> Requests which trigger flushes usually are barriers - zero-size writes.
>>> This patch adds two iostat counters into /sys/class/block/$dev/stat and
>>> /proc/diskstats - count of completed flush requests and their total time.
>> This makes sense to me, and the "recent" discard addition already proved
>> that we're fine extending with more fields. Unless folks object, I'd be
>> happy to queue this up for 5.5.
> Looks like a good addition to /me... :)

That's all the encouragement I needed, added :-)

Jens Axboe