linux-kernel - Re: [rfc git patch] union directory

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Message-Id: <E1L6Skx-0007uK-3H@pomaz-ex.szeredi.hu>
Date:	Sat, 29 Nov 2008 17:33:19 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	bharata@...ux.vnet.ibm.com
CC:	miklos@...redi.hu, linux-fsdevel@...r.kernel.org, jblunck@...e.de,
	dlezcano@...ibm.com, linux-kernel@...r.kernel.org,
	viro@...IV.linux.org.uk
Subject: Re: [rfc git patch] union directory

On Fri, 28 Nov 2008, Bharata B Rao wrote:
> On Fri, Nov 28, 2008 at 12:01:32PM +0100, Miklos Szeredi wrote:
> > I've been doing some small fixing/cleanup work on the union directory
> > patches by Jan, and just noticed there's a thread about the union
> > mounts on LKML, so I thought publicizing won't hurt.
> 
> Interesting that you call it "union directory", do you have plans to go
> the Plan 9's way of union directories ?

I think yes, although I never tried Plan9 and don't know the details
of the union directory semantics.

At first we thought of providing completely read-only unioning (no
whiteouts, no object creation/removal).  This gets rid of a _lot_ of
complexity.

> > It's still a work in progress, notably the readdir code currently only
> > works on a few specific filesystem types.
> 
> readdir was one of the things on which we couldn't reach a consensus
> on how to do it the right way. We were suggested that we move the
> duplicate elimination into user space and an effort towards this was
> also done (Ref: http://lkml.org/lkml/2008/4/29/248) by moving this
> to glibc readdir. But we weren't sure how this could work for NFS and
> were told that it is required to get the NFS side of things
> sorted out first. So that's where readdir effort stands now afaik.
> Do you have any ideas/plans on this front ?

The plan is to get a simple kernel implementation first which caches
the directory in 'struct file'.

Long term we'll see.  Maybe the userspace implementation is worth
persuing as well for a more "kernel memory friendly" implementation.

Thanks,
Miklos
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