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Date: Fri, 03 Sep 2010 09:36:38 +0300
From: Andriy Gapon <avg@icyb.net.ua>
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To: Phillip Nordwall <phillip.nordwall@gmail.com>
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Cc: freebsd-fs@freebsd.org
Subject: Re: ZFS mount_unionfs or ZFS overlay
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on 03/09/2010 00:45 Phillip Nordwall said the following:
> I'm trying to do a mount_unionfs of a zfs file system over another
> filesystem and am getting an Operation not supported. If I try to do a zfs
> mount -O (for an overlay) I get the following error: no overlay mounts
> support on FreeBSD, ignoring.
> 
> I'm thinking about digging into the code (guided by The Design and
> Implementation of the FreeBSD Operating Sytem) to find out how to make zfs
> union mounts work, but I figured I should ask here if there is a good reason
> why it is not currently implemented.

To be able to work as upper layer in unionfs a filesystem should have support
for "whiteouts".  This hint should get you started.

-- 
Andriy Gapon