Allow rewrite skip cloned and snapshotted blocks

Rewrite of cloned and snapshotted blocks can allocate additional
space, that may be undesired.  In some cases it may have sense
to still rewrite snapshotted blocks, expecting the snapshots to
rotate with time, freeing space.  In other cases rewrite of cloned
blocks may be acceptable, despite persistent space usage increase.
For this reason add them as separate flags to `zfs rewrite`.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #18179

man/man8/zfs-rewrite.8

@@ -32,7 +32,7 @@
 .Sh SYNOPSIS
 .Nm zfs
 .Cm rewrite
-.Oo Fl Prvx Ns Oc
+.Oo Fl CPSrvx Ns Oc
 .Op Fl l Ar length
 .Op Fl o Ar offset
 .Ar file Ns | Ns Ar directory Ns …
@@ -45,6 +45,11 @@ as if they were atomically read and written back.
 .No See Sx NOTES .
 for more information about property changes that may be applied during rewrite.
 .Bl -tag -width "-r"
+.It Fl C
+Skip blocks that are shared via block cloning (BRT).
+Cloned blocks are referenced by multiple files or datasets.
+Rewriting these blocks would create separate copies and increase space usage.
+This flag prevents such expansion by skipping cloned blocks.
 .It Fl P
 Perform physical rewrite, preserving logical birth time of blocks.
 By default, rewrite updates logical birth times, making blocks appear
@@ -54,6 +59,12 @@ inclusion in incremental streams.
 Physical rewrite requires the
 .Sy physical_rewrite
 feature to be enabled on the pool.
+.It Fl S
+Skip blocks that are shared with snapshots.
+Blocks created before the most recent snapshot are shared with that snapshot.
+Rewriting these blocks would create new copies, leaving the old copies for
+the snapshot and increasing space usage.
+This flag prevents such expansion by skipping snapshot-shared blocks.
 .It Fl l Ar length
 Rewrite at most this number of bytes.
 .It Fl o Ar offset
@@ -82,6 +93,12 @@ will have no effect.
 .Pp
 Rewrite of cloned blocks and blocks that are part of any snapshots,
 same as some property changes may increase pool space usage.
+Use the
+.Fl C
+and
+.Fl S
+flags to skip cloned and snapshot-shared blocks respectively to prevent
+this expansion.
 Holes that were never written or were previously zero-compressed are
 not rewritten and will remain holes even if compression is disabled.
 .Pp