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List:       freebsd-fs
Subject:    How to use zfs send -R without risking union mounts?
From:       Darren Pilgrim <list_freebsd () bluerosetech ! com>
Date:       2014-01-16 19:54:49
Message-ID: 52D83909.3080809 () bluerosetech ! com
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When you send -R a filesystem, it very nicely retains all of the 
properties.  That also includes the mountpoint property.  Setting 
canmount=off is the only safeguard against mounting a filesystem 
accidentally and it can't be inherited.  That means it's rather 
dangerous to send -R the filesystems on which the OS reside.

I want to create a backup using a process like:

Create the initial full backup:

zpool create backup /dev/gpt/backup
zfs create backup/tank
zfs send -R tank@yesterday | zfs recv -F backup/tank
zpool export backup

Then do incremental backups:

zpool import -N backup
zfs send -R -I tank@yesterday tank@today | zfs recv -F backup/tank
zpool export backup

The problem I ran into is zfs can mount the contents of backup/tank. 
Normally if you try to mount a ZFS filesystem at a non-empty directory, 
it gives the error:

mountpoint '/foo' exists and is not empty

During testing, I inadvertently dropped the -N flag to zpool import and 
ZFS successfully mounted everything on the backup drive over top of the 
live systems!  I had two mounts for /, /var, /usr, /home, etc. 
Imagining the hell of that happening in production, with active 
filesystems, is an exercise for the reader.

How do you force ZFS to never automatically mount a filesystem or any of 
its descendants?  You can't recursively set properties and canmount 
can't be inherited, so I'm stuck on how to enforce this critical bit of 
safety.
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