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