linux-ext4 - Re: [PATCH 1/4] vfs: vfs-level fiemap interface

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Date:	Wed, 17 Sep 2008 16:02:12 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Jörn Engel <joern@...fs.org>
Cc:	Theodore Tso <tytso@....edu>, Andreas Dilger <adilger@....com>,
	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	akpm@...uxfoundation.org, Mark Fasheh <mfasheh@...e.com>,
	mtk.manpages@...il.com
Subject: Re: [PATCH 1/4] vfs: vfs-level fiemap interface

Jörn Engel wrote:
> Apart from the typo above, here is a more discouraging version:
> 
>   In general, accessing the block device directly is strongly discouraged.
>   Exceptions exist mainly in the form of boot loaders like lilo and grub,
>   at a time when the filesystem is not (cannot be) mounted.
> 
>   If the flag DATA_ENCODED is set, however, even this exception is no
>   longer valid.  The content is encoded in some form.  Details are
>   unknown, it could be compressed, encrypted or something else.

I'm not clear about something from the above description.

If I were writing a journalling / tree-like filesystem, and I did
store data in blocks without encoding, but fsync() only waits for them
to be committed to journal, not their final destination, and also they
might be moved around - should I set DATA_ENCODED or not?  (And should
I return the temporary location in the long-running journal since
that's the only place the data is committed at the time of the call?)

Assume that even reading after unmounting is not 100% safe, because
the data blocks could be relocated after calling FIEMAP (when the
filesystem must be mounted), and before the unmount.

-- Jamie
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