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

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Date:	Thu, 23 Jul 2009 09:27:02 -0700
From:	Frank Mayhar <fmayhar@...gle.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Andreas Dilger <adilger@....com>,
	Curt Wohlgemuth <curtw@...gle.com>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: Question on fallocate/ftruncate sequence

On Wed, 2009-07-22 at 22:05 -0500, Eric Sandeen wrote:
> Frank Mayhar wrote:
> > On Tue, 2009-07-21 at 15:54 -0600, Andreas Dilger wrote:
> 
> ...
> 
> >> That said, we might need to have some kind of flag in the on-disk
> >> inode to indicate that it was preallocated beyond EOF.  Otherwise,
> >> e2fsck will try and extend the file size to match the block count,
> >> which isn't correct.  We could also use this flag to determine if
> >> truncate needs to be run on the inode even if the new size is the
> >> same.
> > 
> > After chatting with Curt about this today, it sounds like this needs two
> > things.  One is your flag in the on-disk inode, set in fallocate() to
> > indicate that it has an allocation past EOF.  E2fsck would use this to
> > avoid "fixing" the file size to match the block count.  Truncate would
> > use this to notice that there are blocks allocated past i_size and get
> > rid of them.  It would be cleared by truncate or by ext4_get_blocks when
> > using the last block of such an allocation.
> > 
> > Does this make sense?  Have I missed anything?
> 
> I guess I'm not totally sold on the new on-disk flag; we can work out
> blocks past EOF w/o needing a new flag can't we?

It's on-disk because e2fsck needs it to know when not to extend i_size
to the actual allocated length of the file.  Were it not for that we
could easily solve the fallocate/trucate problem with an in-memory flag
only.
-- 
Frank Mayhar <fmayhar@...gle.com>
Google, Inc.

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