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Re: fs change on read-only mount

To: Bgs <bgs@xxxxxx>
Subject: Re: fs change on read-only mount
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Thu, 13 Aug 2009 09:14:07 -0400
Cc: SGI Project XFS mailing list <xfs@xxxxxxxxxxx>
In-reply-to: <4A84090A.6040601@xxxxxx>
References: <4A84090A.6040601@xxxxxx>
User-agent: Mutt/1.5.19 (2009-01-05)
On Thu, Aug 13, 2009 at 02:37:30PM +0200, Bgs wrote:
> 
>  Greetings,
> 
> I don't know xfs' superblock handling well enough, so I'm asking for
> advice here:
> 
> Does xfs write anything on the disk when mounting read-only? Is it
> possible to use a partitions hash (or some well defined portion of the
> partition) for integrity checks?

It should not write anything to disk [1], but older versions did so due to
bugs.  Doing a hash of a read-only filesystem should be fine.

> The partitions are used read-only and hash would be re-generated  if any
> rw action was done (after remount ro or full reboot of course). Can this
> be done? I'm concerned about 'mount count' like writes...

There is no mount-count like write in XFS.

[1] the only exception is that log reocvery is performed when we attempt 
    a read-only mount with an unclean log, that is the box crashed while
    writing the filesystems.  To make sure you never do this always mark
    the underlying device readonly, using blkdev --ro.

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