linux-ext4 - Re: getdents - ext4 vs btrfs performance

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Date:	Wed, 29 Feb 2012 15:55:07 +0100
From:	Jacek Luczak <difrost.kernel@...il.com>
To:	Chris Mason <chris.mason@...cle.com>,
	Jacek Luczak <difrost.kernel@...il.com>,
	linux-ext4@...r.kernel.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-btrfs@...r.kernel.org,
	lczerner@...hat.com
Subject: Re: getdents - ext4 vs btrfs performance

2012/2/29 Chris Mason <chris.mason@...cle.com>:
> On Wed, Feb 29, 2012 at 03:07:45PM +0100, Jacek Luczak wrote:
>
> [ btrfs faster than ext for find and cp -a ]
>
>> 2012/2/29 Jacek Luczak <difrost.kernel@...il.com>:
>>
>> I will try to answer the question from the broken email I've sent.
>>
>> @Lukas, it was always a fresh FS on top of LVM logical volume. I've
>> been cleaning cache/remounting to sync all data before (re)doing
>> tests.
>
> The next step is to get cp -a out of the picture, in this case you're
> benchmarking both the read speed and the write speed (what are you
> copying to btw?).

It's simple cp -a Jenkins{,.bak} so dir to dir copy on same volume.

> Using tar cf /dev/zero <some_dir> is one way to get a consistent picture
> of the read speed.

IMO the problem is not - only - in read speed. The directory order hit
here. There's a difference in the sequential tests that place btrfs as
the winner but still this should not have that huge influence on
getdents. I know a bit on the difference between ext4 and btrfs
directory handling and I would not expect that huge difference. On the
production system where the issue has been observed doing some real
work in the background copy takes up to 4h.

For me btrfs looks perfect here, what could be worth checking is the
change of timing in syscall between 39.4 and 3.2.7. Before getdents
was not that high on the list while now it jumps to second position
but without huge impact on the timings.

> You can confirm the theory that it is directory order causing problems
> by using acp to read the data.
>
> http://oss.oracle.com/~mason/acp/acp-0.6.tar.bz2

Will check this still today and report back.

-jacek
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