Mounting

Mounting ZFS datasets

TLDR: Always use --clear-mountpoint unless you have good reasons not to.

When a target dataset is mounted, in some cases ZFS might modify a parent dataset.

On the next backup run you'll get this dreaded message:

 cannot receive incremental stream: destination has been modified since most recent snapshot

If you then proceed to use --rollback or --force things might get weird: ZFS will yank away the underlying directory, while its still mounted. If you use ls on a folder you wont see the mountpoint, but you can still access it.

In version 3.3 of zfs-autobackup this will seem to be more prominent, because mounting has improved. Normally this problem would happen later after you reboot or zfs mount -a

Possible causes

Consider this pool:

dataset:         mounted at:
pool             /pool
pool/sub1        /pool/sub1
pool/sub1/sub2   /pool/sub1/sub2

This means the following mount-point directories where created:

• The root filesystem has a directory named pool
• Dataset pool has a directory named sub1
• Dataset pool/sub1 has a directory named sub2

Target side

Now if we only select pool and pool/sub1/sub2 to backup, then the target server will look like this:

dataset:                mounted at:
backup/pool             /pool
backup/pool/sub1/sub2   /pool/sub1/sub2

This means the following mount-point directories are needed:

• The root filesystem has a directory named pool
• Dataset backup/pool has a directory named sub1/sub2 !

So sub2 will be created in sub1, thus modifying the dataset!

Solution

The best solution is to use the --clear-mountpoint option of zfs-autobackup. This will set canmount=noauto on newly received datasets.

When you need to access the data you can use zfs mount to mount just one dataset and leave the rest untouched.

Note that this option also prevents bootproblems, in case a dataset has a mountpoint that conflicts with existing mountpoints.

Fix the problem manually

If you forgot to use --clear-mount, you can use some shell magic to fix it, for example for test_target1:

 # zfs list -H -oname -r test_target1|xargs zfs set canmount=noauto
 # zfs list -H -oname -r test_target1|xargs -n1 zfs umount

After this you will have to use the --rollback option once to remove all the changes.

Workarounds

These are some workarounds if cant use the method above, but they are not a perfect solution:

• Create the needed directories on the source side.
• Just use --rollback (or --force) all the time. This is kind of ugly and will confuse mountpoints and give various problems.
• Set the readonly=on property on the target dataset. This prevents modification, but doesn't allow you to mount some datasets.
• Manualy change the mountpoint of a target dataset to something like /mnt.