linux-kernel - Re: [RFC 2/8] Aufs2: structure

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Message-ID: <49A26ACC.90804@slax.org>
Date:	Mon, 23 Feb 2009 10:22:20 +0100
From:	Tomas M <tomas@...x.org>
To:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 2/8] Aufs2: structure

> The struct of a xino file is simple, just a sequence of aufs inode
> numbers which is indexed by the lower inode number.
> In the above sample, assume the inode number of /ro/fileA is i111 and
> aufs assigns the inode number i999 for fileA. Then aufs writes 999 as
> 4(8) bytes at 111 * 4(8) bytes offset in the xino file.

I think it is worth mentioning that the xino file, if I understand it correctly, is a 'sparse file', that means it is full of 'holes' and doesn't consume as much disk space as it might appear.

In my opinion, the current xino-file approach is not much usable on filesystems which do not support sparse files (for example, if you wish to union two vfats), since some 'seeks' would probably write a lot of nulls. But I am not any kernel developer so I don't even know if there exists any filesystem which would be unable to support sparse files (except the mentioned VFAT, of course).

Tomas M
slax.org
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