linux-kernel - Re: Performance of ext4

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Message-ID: <20080620085922.GH9119@mit.edu>
Date:	Fri, 20 Jun 2008 04:59:22 -0400
From:	Theodore Tso <tytso@....edu>
To:	Holger Kiehl <Holger.Kiehl@....de>
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, 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()?

							- Ted
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