linux-ext4 - Re: rfc: [patch] change attribute for ext3

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Date:	Thu, 14 Sep 2006 15:01:48 -0600
From:	Andreas Dilger <adilger@...sterfs.com>
To:	Alexandre Ratchov <alexandre.ratchov@...l.net>
Cc:	linux-ext4@...r.kernel.org, nfsv4@...ux-nfs.org
Subject: Re: rfc: [patch] change attribute for ext3

On Sep 14, 2006  15:21 +0200, Alexandre Ratchov wrote:
> IMHO, the natural place to do this stuff is the VFS, because it can be
> common to all file-systems supporting this feature. Currently it's the same
> with ctime, mtime and atime. These are in the VFS even if there are
> file-systems that don't support all of them.

Well, that is only partly true.  I see lots of places in ext3 that are
setting i_ctime and i_mtime...

> > Ugh, this would definitely hurt performance, because ext3_dirty_inode()
> > packs-for-disk the whole inode each time.  I believe Stephen had patches
> > at one time to do the inode packing at transaction commit time instead
> > of when it is changed, so we only do the packing once.  Having a generic
> > mechanism to do pre-commit callbacks from jbd (i.e. to pack a dirty
> > inode to the buffer) would also be useful for other things like doing
> > the inode or group descriptor checksum only once per transaction...
> 
> afaik, for an open file, there is always a copy of the inode in memory,
> because there is a reference to it in the file structure. So, in principle,
> we shouldn't need to make dirty the inode. I don't know if this is feasable
> perhaps am i missing something here.

The in-memory inode needs to be copied into the buffer so that it is
part of the transaction being committed to disk, or updates are lost.
This was a common bug with early ext3 - marking the inode dirty and
then changing a field in the in-core inode - which would not be saved
to disk.  In other filesystems this is only a few-cycle race, but in
ext3 the in-core inode is not written to disk unless the inode is
again marked dirty.

The potential benefit of making this a callback from the JBD layer is
it avoids copying the inode for EVERY dirty, and only doing it once
per transaction.  Add a list of callbacks hooked onto the transaction
to be called before it is committed, and the callback data is the
inode pointer which does a single ext3_do_update_inode() call if the
inode is still marked dirty.

> it's not strictly necessary; it's not more necessary that the interface to
> ctime or other attributes. It's here for completness, in my opinion the
> change attribute is the same as the ctime time-stamp

Then makes sense to just improve the ctime mechanism instead of adding
new code and interfaces...

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.

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