linux-ext4 - dax writes on ext4 slower than direct-i/o?

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Date:   Tue, 30 Jul 2019 16:49:41 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     linux-ext4 <linux-ext4@...r.kernel.org>
Cc:     Jan Kara <jack@...e.cz>,
        "Berrocal, Eduardo" <eduardo.berrocal@...el.com>
Subject: dax writes on ext4 slower than direct-i/o?

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

https://user-images.githubusercontent.com/56363/62172754-40c01e00-b2e8-11e9-8e4e-29e09940a171.jpg

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
I/O.

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:

https://github.com/pmem/ndctl/files/3449231/linux_5.3.2_perf.zip

...and the fio configuration is here:

https://gist.github.com/djbw/e5e69cbccbaaf0f43ecde127393c305c