Add 'zfs rename -u' to rename without remounting

Allow to rename file systems without remounting if it is possible.
It is possible for file systems with 'mountpoint' property set to
'legacy' or 'none' - we don't have to change mount directory for them.
Currently such file systems are unmounted on rename and not even
mounted back.

This introduces layering violation, as we need to update
'f_mntfromname' field in statfs structure related to mountpoint (for
the dataset we are renaming and all its children).

In my opinion it is worth it, as it allow to update FreeBSD in even
cleaner way - in ZFS-only configuration root file system is ZFS file
system with 'mountpoint' property set to 'legacy'. If root dataset is
named system/rootfs, we can snapshot it (system/rootfs@upgrade), clone
it (system/oldrootfs), update FreeBSD and if it doesn't boot we can
boot back from system/oldrootfs and rename it back to system/rootfs
while it is mounted as /. Before it was not possible, because
unmounting / was not possible.

Authored by: Pawel Jakub Dawidek <[email protected]>
Reviewed-by: Allan Jude <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Ported by: Matt Macy <[email protected]>
Signed-off-by: Ryan Moeller <[email protected]>
Closes #10839 

1 files changed, 31 insertions, 3 deletions

diff --git a/man/man8/zfs-rename.8 b/man/man8/zfs-rename.8
index 9d650709a..d8d9f49d7 100644
--- a/man/man8/zfs-rename.8
+++ b/man/man8/zfs-rename.8
@@ -44,9 +44,16 @@
 .Ar filesystem Ns | Ns Ar volume Ns | Ns Ar snapshot
 .Nm
 .Cm rename
-.Op Fl fp
+.Fl p
+.Op Fl f
 .Ar filesystem Ns | Ns Ar volume
 .Ar filesystem Ns | Ns Ar volume
+.Nm
+.Cm rename
+.Fl u
+.Op Fl f
+.Ar filesystem
+.Ar filesystem
 .Sh DESCRIPTION
 .Bl -tag -width ""
 .It Xo
@@ -59,10 +66,19 @@
 .It Xo
 .Nm
 .Cm rename
-.Op Fl fp
+.Fl p
+.Op Fl f
 .Ar filesystem Ns | Ns Ar volume
 .Ar filesystem Ns | Ns Ar volume
 .Xc
+.It Xo
+.Nm
+.Cm rename
+.Fl u
+.Op Fl f
+.Ar filesystem
+.Ar filesystem
+.Xc
 Renames the given dataset.
 The new target can be located anywhere in the ZFS hierarchy, with the exception
 of snapshots.
@@ -73,12 +89,24 @@ Renamed file systems can inherit new mount points, in which case they are
 unmounted and remounted at the new mount point.
 .Bl -tag -width "-a"
 .It Fl f
-Force unmount any filesystems that need to be unmounted in the process.
+Force unmount any file systems that need to be unmounted in the process.
+This flag has no effect if used together with the
+.Fl u
+flag.
 .It Fl p
 Creates all the nonexistent parent datasets.
 Datasets created in this manner are automatically mounted according to the
 .Sy mountpoint
 property inherited from their parent.
+.It Fl u
+Do not remount file systems during rename.
+If a file system's
+.Sy mountpoint
+property is set to
+.Sy legacy
+or
+.Sy none ,
+the file system is not unmounted even if this option is not given.
 .El
 .It Xo
 .Nm