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Date: Wed, 17 Sep 2008 16:18:42 +0200 From: Jörn Engel <joern@...fs.org> To: Theodore Tso <tytso@....edu> Cc: 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 On Tue, 16 September 2008 18:03:46 -0400, Theodore Tso wrote: > On Mon, Sep 15, 2008 at 11:49:43PM -0700, Andreas Dilger wrote: > > The intent of this flag was a "catch-all" to indicate it isn't safe > > to try and read this block from disk, either because it is encrypted, > > compressed, on a remote system (HSM or over a network), or maybe not > > even written to disk yet (delalloc). > > > > In some cases (e.g. dump on a snapshot, or boot with LILO) it IS ok to > > read directly from a block device underneath the filesystem, but that > > would completely fail for the above cases. > > Indeed, I thought it was pretty clear and obvious, but let me give an > quick but formal definition, and a potential name: DATA_ENCODED > > If this flag is not set, then applications that who wish to access the ^^^^^^^^ > file data may do so by accessing the block device at the indicated > offset when the filesystem is unmounted. If the filesystem is > mounted, it is undefined whether accessing via the block device will > return valid data. If the flag DATA_ENCODED flag is set, it is almost > certain that an application will never be able to access the file data > via the block device. > > Would this make people happy? 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 If the flag DATA_ENCODED is set, however, even this exception is no longer valid.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. Jörn -- Man darf nicht das, was uns unwahrscheinlich und unnatürlich erscheint, mit dem verwechseln, was absolut unmöglich ist. -- Carl Friedrich Gauß -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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