linux-kernel - Re: Performance of ext4

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Message-ID: <Pine.LNX.4.64.0806200905070.3383@diagnostix.dwd.de>
Date:	Fri, 20 Jun 2008 09:21:48 +0000 (GMT)
From:	Holger Kiehl <Holger.Kiehl@....de>
To:	Theodore Tso <tytso@....edu>
Cc:	Eric Sandeen <sandeen@...hat.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Jan Kara <jack@...e.cz>, Solofo.Ramangalahy@...l.net,
	Nick Dokos <nicholas.dokos@...com>, linux-ext4@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Performance of ext4

On Fri, 20 Jun 2008, Theodore Tso wrote:

> On Fri, Jun 20, 2008 at 08:32:52AM +0000, Holger Kiehl wrote:
>>> It sounds like i_size is actually dropping in
>>> size at some pointer long after the file was written.  If I had to
>
> sorry, "at some point"...
>
>>> guess the value in the inode cache is correct; and perhaps so is the
>>> value on the journal.  But somehow, the wrong value is getting written
>>> to disk
>
> Or, "the right value is never getting written to disk".  (Which as I
> think about it is more likely; it's likely that an update to i_size is
> getting *lost*, perhaps because the delalloc code is possibly
> modifying i_size without starting a transaction first.  Again this is
> just a guess.)
>
>> What I find strange is that the missing parts of the file are not for
>> example exactly 512 or 1024 or 4096 bytes it is mostly some odd number
>> of bytes.
>
> Is there any chance the truncation point is related to how the program
> is writing its output file?  i.e., if it is a text file, is the
> truncation happening after a new-line or when the stdio library might
> have done an explicit or implicit fflush()?
>
When the benchmark runs it writes to stdout and with tee to the result
file. It first writes some information about the system, prepares the
test files (creates lots of small files), calls sync and then starts
the test. Then every minute one line gets written to the result file.
Often I have seen that everything after the sync was missing. But
sometimes it happened that some parts at the end are missing. But it
was always a clean cut, that is there where no lines that where cut
partially. The lines where always complete.

Holger

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