Date: Wed, 23 Mar 2011 17:30:00 +1100 From: Lawrence Stewart <lastewart@swin.edu.au> To: Jeremy Chadwick <freebsd@jdc.parodius.com> Cc: freebsd-fs@freebsd.org Subject: Re: mount -o union doesn't allow changes to sub directories? Message-ID: <4D899368.9090709@swin.edu.au> In-Reply-To: <20110323061641.GA74977@icarus.home.lan> References: <4D8989A5.1020203@swin.edu.au> <20110323061641.GA74977@icarus.home.lan>
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Hi Jeremy, On 03/23/11 17:16, Jeremy Chadwick wrote: > On Wed, Mar 23, 2011 at 04:48:21PM +1100, Lawrence Stewart wrote: >> Hi again, >> >> If I run something like: >> >> mount -o union /dev/md0 /etc >> >> where md0 is a memory backed fs device, and /etc is fully populated but >> mounted RO off a CD, I can touch a new file in /etc, but can't in any >> subdir of etc (fails with "Read-only filesystem" reported). >> >> As far as I can tell this is a bug, but wanted to check if I'm >> misunderstanding how the "-o union" mount option is supposed to work. > > Please see mount_unionfs(8), specifically the explanations of "lower" > and "upper" layers, along with EROFS (errno 30, "Read-only filesystem"). I don't think 'mount -o union' is the same thing as doing a unionfs mount. Earlier today I was playing with unionfs and it worked perfectly for the scenario I describe above. 'mount -o union' seems to do something similar, but subtly different to unionfs. The key difference from the UI perspective is that unionfs can be used to mount a directory on top of another directory, whereas using 'mount -o union' requires you to mount a device with an fs on it on top of a directory. The semantics of 'mount -o union' are far more useful for my situation that unionfs, so I'm keen to try and see if I can get it working properly, which I suspect will involve a kernel patch. Cheers, Lawrence
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