fs change on read-only mount

fs change on read-only mount

[Bgs]

 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?

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...

Thanks in advance
Bgs

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Re: fs change on read-only mount

[Christoph Hellwig]

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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Re: fs change on read-only mount

[Bgs]

 Thanks for the fast answer.

So the following scenarios should be ok:

1) Normal boot (no crash, previous mount was ro as well, journal empty)
-> mount ro from the beginning => partition hash stays the same

2) Mount rw at any point -> make changes -> remount ro -> sync -> create
 new hash -> next boot with ro mount has this new hash.

Regards
Bgs


Christoph Hellwig wrote:
> 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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Re: fs change on read-only mount

[Christoph Hellwig]

On Thu, Aug 13, 2009 at 03:47:08PM +0200, Bgs wrote:
> 
>  Thanks for the fast answer.
> 
> So the following scenarios should be ok:
> 
> 1) Normal boot (no crash, previous mount was ro as well, journal empty)
> -> mount ro from the beginning => partition hash stays the same
> 
> 2) Mount rw at any point -> make changes -> remount ro -> sync -> create
>  new hash -> next boot with ro mount has this new hash.

Yes.

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