How to use zfs send -R without risking union mounts?
Darren Pilgrim
list_freebsd at bluerosetech.com
Thu Jan 16 19:54:46 UTC 2014
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 at yesterday | zfs recv -F backup/tank
zpool export backup
Then do incremental backups:
zpool import -N backup
zfs send -R -I tank at yesterday tank at 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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