Add 'zfs wait' command

Add a mechanism to wait for delete queue to drain.

When doing redacted send/recv, many workflows involve deleting files 
that contain sensitive data. Because of the way zfs handles file 
deletions, snapshots taken quickly after a rm operation can sometimes 
still contain the file in question, especially if the file is very 
large. This can result in issues for redacted send/recv users who 
expect the deleted files to be redacted in the send streams, and not 
appear in their clones.

This change duplicates much of the zpool wait related logic into a 
zfs wait command, which can be used to wait until the internal
deleteq has been drained.  Additional wait activities may be added 
in the future. 

Reviewed-by: Matthew Ahrens <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: John Gallagher <[email protected]>
Signed-off-by: Paul Dagnelie <[email protected]>
Closes #9707 

1 files changed, 71 insertions, 0 deletions

diff --git a/man/man8/zfs-wait.8 b/man/man8/zfs-wait.8
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+.Dd August 9, 2019
+.Dt ZFS-WAIT 8
+.Os Linux
+.Sh NAME
+.Nm zfs Ns Pf - Cm wait
+.Nd Wait for background activity to stop in a ZFS filesystem
+.Sh SYNOPSIS
+.Nm
+.Cm wait
+.Op Fl t Ar activity Ns Oo , Ns Ar activity Ns Oc Ns ...
+.Ar fs
+.Sh DESCRIPTION
+.Bl -tag -width Ds
+.It Xo
+.Nm
+.Cm wait
+.Op Fl t Ar activity Ns Oo , Ns Ar activity Ns Oc Ns ...
+.Ar fs
+.Xc
+Waits until all background activity of the given types has ceased in the given
+filesystem.
+The activity could cease because it has completed or because the filesystem has
+been destroyed or unmounted.
+If no activities are specified, the command waits until background activity of
+every type listed below has ceased.
+If there is no activity of the given types in progress, the command returns
+immediately.
+.Pp
+These are the possible values for
+.Ar activity ,
+along with what each one waits for:
+.Bd -literal
+        deleteq       The filesystem's internal delete queue to empty
+.Ed
+.Pp
+Note that the internal delete queue does not finish draining until
+all large files have had time to be fully destroyed and all open file
+handles to unlinked files are closed.
+.El
+.El
+.Sh SEE ALSO
+.Xr lsof  8