linux-kernel - Re: [PATCH 0/7] overlay filesystem: request for inclusion

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Date:	Thu, 9 Jun 2011 15:57:48 +0200
From:	Michal Suchanek <hramrach@...trum.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	NeilBrown <neilb@...e.de>, Miklos Szeredi <miklos@...redi.hu>,
	viro@...iv.linux.org.uk, torvalds@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	apw@...onical.com, nbd@...nwrt.org, jordipujolp@...il.com,
	ezk@....cs.sunysb.edu, mszeredi@...e.cz
Subject: Re: [PATCH 0/7] overlay filesystem: request for inclusion

On 9 June 2011 05:52, Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Thu, 9 Jun 2011 11:59:34 +1000 NeilBrown <neilb@...e.de> wrote:
>
>> On Wed, 8 Jun 2011 15:32:08 -0700 Andrew Morton <akpm@...ux-foundation.org>
>> wrote:
>>
>> > On Wed,  1 Jun 2011 14:46:13 +0200
>> > Miklos Szeredi <miklos@...redi.hu> wrote:
>> >
>> > > I'd like to ask for overlayfs to be merged into 3.1.
>> >
>> > Dumb questions:
>> >
>> > I've never really understood the need for fs overlaying.  Who wants it?
>> > What are the use-cases?
>>
>> https://lwn.net/Articles/324291/
>>
>> I think the strongest use case is that LIVE-DVD's want it to have a write-able
>> root filesystem which is stored on the DVD.
>
> Well, these things have been around for over 20 years.  What motivated
> the developers of other OS's to develop these things and how are their
> users using them?

FWIW there is an union solution in NetBSD. I am not sure it is used in
the LiveCD but you can definitely use it to build a piece of software
without actually touching the source directory.

>
>> >
>> > This sort of thing could be implemented in userspace and wired up via
>> > fuse, I assume.  Has that been attempted and why is it inadequate?
>>
>> I think that would be a valid question if the proposal was large and
>> complex.  But overlayfs is really quite small and self-contained.
>
> Not merging it would be even smaller and simpler.  If there is a
> userspace alternative then that option should be evaluated and compared
> in a rational manner.

The problem with the userspace alternative is that it does not work. I
tried to run my live CD on top of unionfs-fuse and the filesystem
would fail intermittently leading to random errors during boot.

>
>
>
> Another issue: there have been numerous attempts at Linux overlay
> filesystems from numerous parties.  Does (or will) this implementation
> satisfy all their requirements?

No implementation will satisfy all needs. There is always some
compromise between availability (userspace/in-tree/easy to patch in)
feature completeness (eg. AuFS is not so easy to forward-port to new
kernels but has numerous features) performance, reliability.

>
> Because if not, we're in a situation where the in-kernel code is
> unfixably inadequate so we end up merging another similar-looking
> thing, or the presence of this driver makes it harder for them to get
> other drivers merged and the other parties' requirements remain
> unsatisfied.

One of the major use cases is building live CDs.

That and other things can be done with overlayfs.

Thanks

Michal
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