linux-ext4 - Re: Question on fallocate/ftruncate sequence

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Message-Id: <1248279877.14463.7.camel@bobble.smo.corp.google.com>
Date:	Wed, 22 Jul 2009 09:24:37 -0700
From:	Frank Mayhar <fmayhar@...gle.com>
To:	Andreas Dilger <adilger@....com>
Cc:	Eric Sandeen <sandeen@...hat.com>,
	Curt Wohlgemuth <curtw@...gle.com>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: Question on fallocate/ftruncate sequence

On Tue, 2009-07-21 at 15:54 -0600, Andreas Dilger wrote:
> On Jul 21, 2009  14:29 -0700, Frank Mayhar wrote:
> > I've spent a little while today digging into this.  My guess (only a
> > guess at this point until I have a chance to prove it) is that
> > i_disksize should be updated by fallocate() even when KEEP_SIZE is
> > specified.  It's currently not updated in that case.
> 
> No, that isn't correct.  The intent of KEEP_SIZE is to allow fallocate
> to preallocate blocks beyond the EOF, so that it doesn't affect the
> file data visible to userspace, but can avoid fragmentation from e.g.
> log files or mbox files.
> 
> The i_disksize variable is just to handle the lag in updating the on-disk
> file size during truncate, because the VFS updates i_size to indicate a
> truncate, but in order to handle the truncation of files within finite
> transaction sizes the on-disk file size needs to be shrunk incrementally.

Okay, thanks, this makes this much more clear.

It does sound like there needs to be a flag somewhere (probably in the
on-disk inode) that indicates that there are allocated blocks beyond
EOF, as you say.  Then use that in ftruncate().

We would really like to avoid your workaround for performance reasons.
-- 
Frank Mayhar <fmayhar@...gle.com>
Google, Inc.

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