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Mounting.md

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+## Mounting ZFS datasets
+
+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.
+
+#### 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`:
+
+```console
+ # 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, 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 as 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.
+