linux-kernel - Re: [PATCH 0/9] overlay filesystem: request for inclusion (v17)

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Message-ID: <20130315044411.GW21522@ZenIV.linux.org.uk>
Date:	Fri, 15 Mar 2013 04:44:11 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"J. R. Okajima" <hooanon05@...oo.co.jp>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, hch@...radead.org, apw@...onical.com,
	nbd@...nwrt.org, neilb@...e.de, jordipujolp@...il.com,
	ezk@....cs.sunysb.edu, dhowells@...hat.com,
	sedat.dilek@...glemail.com, mszeredi@...e.cz
Subject: Re: [PATCH 0/9] overlay filesystem: request for inclusion (v17)

On Fri, Mar 15, 2013 at 01:15:36PM +0900, J. R. Okajima wrote:

> +- whiteout is hardlinked in order to reduce the consumption of inodes
> +  on branch

*blink* Whiteouts have no inodes at all.  Filesystem has an additional
kind of directory entries, recognizable as whiteouts.  How they are
done is up to filesystem in question.

> +- kernel thread for removing the dir who has a plenty of whiteouts

Again, implementation detail for individual filesystem.

> +The whiteout in aufs is very similar to Unionfs's. That is represented
> +by its filename. UnionMount takes an approach of a file mode, but I am
> +afraid several utilities (find(1) or something) will have to support it.

Why the devil should find(1) even see them?

I really don't believe that it's a good idea to try making them fs-agnostic;
it's an implementation detail of filesystem, with things like "is this
directory empty?" very much belonging to the fs in question...

I don't know; maybe it's my experience of dealing with umsdos that has badly
soured me on that kind of approach, but IME this kind of schemes tend to
be brittle as hell ;-/
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