linux-kernel - Re: [PATCH 01/22][RFC] Unionfs: Documentation

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Date:	Fri, 1 Sep 2006 09:42:34 +0200 (MEST)
From:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
To:	Josef Sipek <jsipek@...sunysb.edu>
cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	hch@...radead.org, akpm@...l.org, viro@....linux.org.uk
Subject: Re: [PATCH 01/22][RFC] Unionfs: Documentation

Hi,


nice to see that unionfs finally tries to get in :)


>+Whiteouts:
>+==========
>+
>+A whiteout removes a file name from the name-space. Whiteouts are needed when
>+one attempts to remove a file on a read-only branch.

"namespace".

>+Suppose we have a two branch union, where branch 0 is read-write and branch 1

I'd go for "two-branch".

>+Copyup:
>+=======
>+
>+When a change is made to the contents of a file's data or meta-data, they
>+have to be stored somewhere. The best way is to create a copy of the
>+original file on a branch that is writable, and then redirect the write
>+though to this copy. The copy must be made on a higher priority branch so
>+that lookup & readdir return this newer "version" of the file rather than
>+the original (see duplicate elimination).

Apropos copyup, sparse copyup would probably a nice feature in future, but it
also has its effects.

>--- linux-2.6-git/Documentation/filesystems/unionfs/usage.txt	1969-12-31 19:00:00.000000000 -0500
>+++ linux-2.6-git-unionfs/Documentation/filesystems/unionfs/usage.txt	2006-08-31 19:25:19.000000000 -0400
>+
>+mount -t unionfs -o branch-option[,union-options[,...]] none unionfs

should read
mount -t unionfs -o branch-option[,union-options[,...]] none MOUNTPOINT

>+KNOWN ISSUES:
>+=============
>+
>+The NFS server returns -EACCES for read-only exports, instead of -EROFS.  This

Will the NFS code ever be changed to return EROFS instead?

>+nfs-mouted branch.

mounted

>+Modifying a Unionfs branch directly, while the union is mounted is currently
>+unsupported.  Any such change can cause Unionfs to oops, however it could even
>+BRESULT IN DATA LOSS.

RESULT




Jan Engelhardt
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