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

To: linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, David Chinner <dgc@xxxxxxx>, suparna@xxxxxxxxxx, cmm@xxxxxxxxxx, xfs@xxxxxxxxxxx
Subject: Re: [PATCH 7/7][TAKE5] ext4: support new modes
From: "Amit K. Arora" <aarora@xxxxxxxxxxxxxxxxxx>
Date: Wed, 27 Jun 2007 00:59:08 +0530
In-reply-to: <20070626161400.GE6652@schatzie.adilger.int>
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On Tue, Jun 26, 2007 at 12:14:00PM -0400, Andreas Dilger wrote:
> On Jun 26, 2007  17:37 +0530, Amit K. Arora wrote:
> > Hmm.. I am thinking of a scenario when the file system supports some
> > individual flags, but does not support a particular combination of them.
> > Just for example sake, assume we have FA_ZERO_SPACE mode also. Now, if a
> > file system supports FA_ZERO_SPACE, FA_ALLOCATE, FA_DEALLOCATE and
> > FA_RESV_SPACE; and no other mode (i.e. FA_UNRESV_SPACE is not supported
> > for some reason). This means that although we support FA_FL_DEALLOC,
> > FA_FL_KEEP_SIZE and FA_FL_DEL_DATA flags, but we do not support the
> > combination of all these flags (which is nothing but FA_UNRESV_SPACE).
> That is up to the filesystem to determine then.  I just thought it should
> be clear to return an error for flags (or as you say combinations thereof)
> that the filesystem doesn't understand.
> That said, I'd think in most cases the flags are orthogonal, so if you
> support some combination of the flags (e.g. FA_FL_DEL_DATA, FA_FL_DEALLOC)
> then you will also support other combinations of those flags just from
> the way it is coded.

> > > 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.

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

> > 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 ? Will old
applications really get affected ? If yes, then it might be worth
implementing - even though I personally don't like it.

David, can you please confirm ? Thanks!

Amit Arora

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