archive mirror
 help / color / mirror / Atom feed
* dax writes on ext4 slower than direct-i/o?
@ 2019-07-30 23:49 Dan Williams
  2019-08-02 14:43 ` Jan Kara
  0 siblings, 1 reply; 3+ messages in thread
From: Dan Williams @ 2019-07-30 23:49 UTC (permalink / raw)
  To: linux-ext4; +Cc: Jan Kara, Berrocal, Eduardo

Hi all,

Eduardo raised a puzzling question about why dax yields lower iops
than direct-i/o. The expectation is the reverse, i.e. that direct-i/o
should be slightly slower than dax due to block layer overhead. This
holds true for xfs, but on ext4 dax yields half the iops of direct-i/o
for an fio 4K random write workload.

Here is a relative graph of ext4: dax + direct-i/o vs xfs: dax + direct-i/o

A relative perf profile seems to show more time in
ext4_journal_start() which I thought may be due to atime or mtime
updates, but those do not seem to be the source of the extra journal

The urgency is a curiosity at this point, but I expect an end user
might soon ask whether this is an expected implementation side-effect
of dax.

Thanks in advance for any insight, and/or experiment ideas for us to go try.

Eduardo collected perf reports of these runs here:

...and the fio configuration is here:

^ permalink raw reply	[flat|nested] 3+ messages in thread

end of thread, other threads:[~2019-08-02 15:38 UTC | newest]

Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-07-30 23:49 dax writes on ext4 slower than direct-i/o? Dan Williams
2019-08-02 14:43 ` Jan Kara
2019-08-02 15:38   ` Dan Williams

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).