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Re: union-mounting mfs



On Sat, Jun 28, 2008 at 02:35:47PM +0200, Frank Wille wrote:
> Hi,

Sorry for the delay.

> I wonder if this is a bug, or am I doing something wrong?
> For a Compact Flash disk I want to mount an mfs with option union on top of
> /var to allow writing of log files, etc. But strangely only files created in
> the /var directory will appear in the mfs. When creating a file in a sub
> directory, like /var/db, it will go directly to the ffs on the CF disk!
> 
> This behaviour can easily be reproduced with a union-mfs on a CD-ROM. Example:
> 
> # mount_cd9660 /dev/cd0a /cdrom
> # mount_mfs -o union -s 32m test_mfs /cdrom/i386
> # cp /netbsd /cdrom/i386/
> # cp /netbsd /cdrom/i386/binary/kernel/
> cp: /cdrom/i386/binary/kernel/netbsd: Read-only file system
> 
> The first cp works, the second doesn't, because it bypasses the mfs! Why?

Because that's how -o union works. It only applies to the top-level 
directory. When you looked up "/cdrom/i386/binary" you got a directory 
vnode on the CD ROM. Thus when you looked up "kernel" you got another 
vnode on the CD ROM. You then were unable to write.

-o union != unionfs.

Take care,

Bill

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