linux-ext4 - Re: extent counting fun

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Message-Id: <AF4CA4E6-4ADE-41E3-843D-02266228E2CB@dilger.ca>
Date:	Mon, 5 Jul 2010 17:46:36 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: extent counting fun

On 2010-07-05, at 13:24, Eric Sandeen wrote:
> [root@...t ~]# filefrag -B /mnt/test/file
> /mnt/test/file: 34 extents found
> [root@...t ~]# filefrag /mnt/test/file
> /mnt/test/file: 1058 extents found, perfection would be 1 extent
> 
> Hum, is it 34 or 1058? :)

What do the extents look like on disk?  Is this just because it is running on a block-mapped file and is skipping a singleton block periodically for indirect blocks, or is there a bug in the way the extents are being reported?

> Older filefrag counted contiguous metadata as part of a contiguous
> extent... newer filefrag works in fiemap query-only mode by default,
> and just takes what fiemap tells it.  The inconsistency is weird
> though, and led to a Red Hat bug that I'm inclined to NOTABUG... but
> do people think this needs to be made any more consistent?
> 
> Should we hack ext3_fiemap() to include the checks for contiguous
> metadata?  Or was that too shady/clever to start with ...? :)

I wouldn't object to having FIEMAP add a flag for metadata blocks.  I've always thought it would be useful to be able to query/dump metadata blocks (e.g. indirect/index blocks) and the inode itself.

Cheers, Andreas





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