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Date: Sat, 10 Oct 2009 14:48:37 -0700
From: David Ehrmann <ehrmann@gmail.com>
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Subject: Re: Strange md/unionfs issue
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David Ehrmann wrote:
> I have a system running on a flash disk.  To prevent it from wearing 
> out quickly, I don't run with a swap and mount /tmp and /var from 
> memory.  Because /var needs some directories and contains files I 
> probably should save from time to time, I mount it from flash, mount 
> /var to /var_real so I can access the underlying files, and then I 
> mount an md device on top of /var as a unionfs.  Initially, it seems 
> to work, but seconds later, the unionfs seems to fail.
>
> server# cd /var
> server# ls
> .snap           crash           heimdal         preserve        yp
> account         cron            log             run
> at              db              mail            rwho
> audit           empty           msgs            spool
> backups         games           named           tmp
> server# mount
> /dev/ad0s1a on / (ufs, NFS exported, local)
> devfs on /dev (devfs, local, multilabel)
> /dev/ad0s1f on /usr (ufs, local, soft-updates)
> /dev/ad0s1e on /var (ufs, local, soft-updates)
> /var on /var_real (nullfs, local, noatime)
> /dev/md0 on /var (ufs, local, union, soft-updates)
> /dev/md1 on /tmp (ufs, local, soft-updates)
> server# cd /var_real/
> server# ls
> .snap           crash           heimdal         preserve        yp
> account         cron            log             run
> at              db              mail            rwho
> audit           empty           msgs            spool
> backups         games           named           tmp
> server# cd /var
> server# ls
> .snap
> server#
>
> Removing /var_real from the fstab didn't help.  Here's the full version:
>
> # Device                Mountpoint      FStype  Options         
> Dump    Pass#
> /dev/ad0s1a             /               ufs     rw              1       1
> /dev/ad0s1f             /usr            ufs     rw              2       2
>
> /dev/ad0s1e             /var            ufs     rw              2       2
> /var                    /var_real       nullfs  rw,noatime
> md                      /var            mfs     rw,union,-s128M
>
> md                      /tmp            mfs     rw,-s128M
>
> Ideas?
>
It looks like I don't have the problem if I mount the md on top of /var 
if /var is really on /, i.e. not a mount point.