Fast Clone Deletion

Deleting a clone requires finding blocks are clone-only, not shared
with the snapshot. This was done by traversing the entire block tree
which results in a large performance penalty for sparsely
written clones.

This is new method keeps track of clone blocks when they are
modified in a "Livelist" so that, when it’s time to delete,
the clone-specific blocks are already at hand.

We see performance improvements because now deletion work is
proportional to the number of clone-modified blocks, not the size
of the original dataset.

Reviewed-by: Sean Eric Fagan <sef@ixsystems.com>
Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Signed-off-by: Sara Hartse <sara.hartse@delphix.com>
Closes #8416

man/man5/zfs-module-parameters.5

@@ -1909,6 +1909,98 @@ Pattern written to vdev free space by \fBzpool initialize\fR.
 Default value: \fB16,045,690,984,833,335,022\fR (0xdeadbeefdeadbeee).
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_livelist_max_entries\fR (ulong)
+.ad
+.RS 12n
+The threshold size (in block pointers) at which we create a new sub-livelist.
+Larger sublists are more costly from a memory perspective but the fewer
+sublists there are, the lower the cost of insertion.
+.sp
+Default value: \fB500,000\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_livelist_min_percent_shared\fR (int)
+.ad
+.RS 12n
+If the amount of shared space between a snapshot and its clone drops below
+this threshold, the clone turns off the livelist and reverts to the old deletion
+method. This is in place because once a clone has been overwritten enough
+livelists no long give us a benefit.
+.sp
+Default value: \fB75\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_livelist_condense_new_alloc\fR (int)
+.ad
+.RS 12n
+Incremented each time an extra ALLOC blkptr is added to a livelist entry while
+it is being condensed.
+This option is used by the test suite to track race conditions.
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_livelist_condense_sync_cancel\fR (int)
+.ad
+.RS 12n
+Incremented each time livelist condensing is canceled while in
+spa_livelist_condense_sync.
+This option is used by the test suite to track race conditions.
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_livelist_condense_sync_pause\fR (int)
+.ad
+.RS 12n
+When set, the livelist condense process pauses indefinitely before
+executing the synctask - spa_livelist_condense_sync.
+This option is used by the test suite to trigger race conditions.
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_livelist_condense_zthr_cancel\fR (int)
+.ad
+.RS 12n
+Incremented each time livelist condensing is canceled while in
+spa_livelist_condense_cb.
+This option is used by the test suite to track race conditions.
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_livelist_condense_zthr_pause\fR (int)
+.ad
+.RS 12n
+When set, the livelist condense process pauses indefinitely before
+executing the open context condensing work in spa_livelist_condense_cb.
+This option is used by the test suite to trigger race conditions.
+.sp
+Default value: \fB0\fR.
+.RE
+
 .sp
 .ne 2
 .na

man/man5/zpool-features.5

@@ -547,6 +547,26 @@ allow more data to be stored in the bonus buffer, thus potentially
 improving performance by avoiding the use of spill blocks.
 .RE
 
+.sp
+.ne 2
+.na
+\fB\fBlivelist\fR\fR
+.ad
+.RS 4n
+.TS
+l l .
+GUID	com.delphix:livelist
+READ\-ONLY COMPATIBLE	yes
+DEPENDENCIES	none
+.TE
+This feature allows clones to be deleted faster than the traditional method
+when a large number of random/sparse writes have been made to the clone.
+All blocks allocated and freed after a clone is created are tracked by the
+the clone's livelist which is referenced during the deletion of the clone.
+The feature is activated when a clone is created and remains active until all
+clones have been destroyed.
+.RE
+
 .sp
 .ne 2
 .na
@@ -882,7 +902,6 @@ This feature becomes \fBactive\fR when the \fBzpool checkpoint\fR subcommand
 is used to checkpoint the pool.
 The feature will only return back to being \fBenabled\fR when the pool
 is rewound or the checkpoint has been discarded.
-.RE
 
 .SH "SEE ALSO"
 zpool(8)