linux-kernel - Re: [EXT4 set 4][PATCH 1/5] i

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Message-ID: <20070711172623.GE4138@fieldses.org>
Date:	Wed, 11 Jul 2007 13:26:23 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Neil Brown <neilb@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, nfsv4@...ux-nfs.org,
	linux-ext4@...r.kernel.org, cmm@...ibm.com,
	linux-kernel@...r.kernel.org
Subject: Re: [EXT4 set 4][PATCH 1/5] i_version:64 bit inode version

On Wed, Jul 11, 2007 at 01:21:55PM +1000, Neil Brown wrote:
> And just by-the-way, the server doesn't really have the option of not
> sending the attribute.  If i_version isn't defined, it has to fake
> something using mtime, and hope that is good enough.

ctime, actually--the change attribute is also supposed to be updated on
attribute updates.

> Alternately we could mandate that i_version is always kept up-to-date
> and if a filesystem doesn't have anything to load from storage, it
> just sets it to the current time in nanoseconds.
> 
> That would mean that a client would need to flush it's cache whenever
> the inode fell out of cache on the server, but I don't think we can
> reliably do better than that.
> 
> I think I like that approach.
> 
> So my vote is to increment i_version in common code every time any
> change is made to the file, and alloc_inode should initialise it to
> current time, which might be changed by the filesystem before it calls
> unlock_new_inode. 

So the client would be invalidating its cache more often than necessary,
rather than failing to invalidate it when it should.  I agree that
that's probably the better tradeoff, although I wish I had a better idea
of the downside.  I don't know, for example, whether users might see
unpleasant results if every client has to reread its cached data on a
reboot.

The currently proposed change--just providing a model change attribute
implementation for ext4 and leaving other filesystems untouched--is a
more conservative step.

So I'm inclined to just do this ext4 thing first, and then look into
further change attribute experiments next time around....

--b.
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