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From: Jan Kara <>
To: Dan Williams <>
Cc: linux-ext4 <>, Jan Kara <>,
	"Berrocal, Eduardo" <>
Subject: Re: dax writes on ext4 slower than direct-i/o?
Date: Fri, 2 Aug 2019 16:43:04 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Hi Dan!

On Tue 30-07-19 16:49:41, Dan Williams wrote:
> 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
> 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.

Yeah, I think the reason is that ext4_iomap_begin() currently starts a
transaction unconditionally for each write whereas ext4_direct_IO_write()
is more clever and starts a transaction only when needing to allocate any
blocks. We could put similar smarts into ext4_iomap_begin() and it's
probably a good idea, just at this moment I'm working with one guy on
moving ext4 direct IO code to iomap infrastructure which overhauls
ext4_iomap_begin() anyway, so let's do this after that work.


Jan Kara <>

  reply	other threads:[~2019-08-02 14:43 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-30 23:49 Dan Williams
2019-08-02 14:43 ` Jan Kara [this message]
2019-08-02 15:38   ` Dan Williams

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