linux-kernel - Re: [PATCH 7/7][TAKE5] ext4: support new modes

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Date:	Wed, 27 Jun 2007 10:04:56 +1000
From:	David Chinner <dgc@....com>
To:	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org, David Chinner <dgc@....com>,
	suparna@...ibm.com, cmm@...ibm.com, xfs@....sgi.com
Subject: Re: [PATCH 7/7][TAKE5] ext4: support new modes

On Wed, Jun 27, 2007 at 12:59:08AM +0530, Amit K. Arora wrote:
> On Tue, Jun 26, 2007 at 12:14:00PM -0400, Andreas Dilger wrote:
> > On Jun 26, 2007  17:37 +0530, Amit K. Arora wrote:
> > > > I also thought another proposed flag was to determine whether mtime (and
> > > > maybe ctime) is changed when doing prealloc/dealloc space?  Default should
> > > > probably be to change mtime/ctime, and have FA_FL_NO_MTIME.  Someone else
> > > > should decide if we want to allow changing the file w/o changing ctime, if
> > > > that is required even though the file is not visibly changing.  Maybe the
> > > > ctime update should be implicit if the size or mtime are changing?
> > > 
> > > Is it really required ? I mean, why should we allow users not to update
> > > ctime/mtime even if the file metadata/data gets updated ? It sounds
> > > a bit "unnatural" to me.
> > > Is there any application scenario in your mind, when you suggest of
> > > giving this flexibility to userspace ?
> > 
> > One reason is that XFS does NOT update the mtime/ctime when doing the
> > XFS_IOC_* allocation ioctls.

Not totally correct.

XFS_IOC_ALLOCSP/FREESP change timestamps if they change
the file size (via the truncate call made to change the file size).
If they don't change the file size, then they are a no-op and should
not change the file size.

XFS_IOC_RESVSP/UNRESVSP don't change timestamps just like they don't
change file size. That is by design AFAICT so these calls can be
used by HSM-type applications that don't want to change timestamps
when punching out data blocks or preallocating new ones.

> Hmm.. I personally will call it a bug in XFS code then. :)

No, I'd call it useful. :)

> > > I think, modifying ctime/mtime should be dependent on the other flags.
> > > E.g., if we do not zero out data blocks on allocation/deallocation,
> > > update only ctime. Otherwise, update ctime and mtime both.
> > 
> > I'm only being the advocate for requirements David Chinner has put
> > forward due to existing behaviour in XFS.  This is one of the reasons
> > why I think the "flags" mechanism we now have - we can encode the
> > various different behaviours in any way we want and leave it to the
> > caller.
> 
> I understand. May be we can confirm once more with David Chinner if this
> is really required. Will it really be a compatibility issue if new XFS
> preallocations (ie. via fallocate) update mtime/ctime?

It should be left up to the filesystem to decide. Only the
filesystem knows whether something changed and the timestamp should
or should not be updated.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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