Illumos #3552, #3564

3552 condensing one space map burns 3 seconds of CPU in spa_sync() thread
3564 spa_sync() spends 5-10% of its time in metaslab_sync() (when not condensing)
Reviewed by: Adam Leventhal <[email protected]>
Reviewed by: Dan Kimmel <[email protected]>
Reviewed by: Matthew Ahrens <[email protected]>
Approved by: Richard Lowe <[email protected]>

References:
  illumos/illumos-gate@16a4a8074274d2d7cc408589cf6359f4a378c861
  https://www.illumos.org/issues/3552
  https://www.illumos.org/issues/3564

Ported-by: Tim Chase <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #1513

1 files changed, 25 insertions, 7 deletions

diff --git a/include/sys/metaslab_impl.h b/include/sys/metaslab_impl.h
index 386f6240c..a36baedd4 100644
--- a/include/sys/metaslab_impl.h
+++ b/include/sys/metaslab_impl.h
@@ -67,20 +67,38 @@ struct metaslab_group {
 };
 
 /*
- * Each metaslab's free space is tracked in space map object in the MOS,
- * which is only updated in syncing context.  Each time we sync a txg,
+ * Each metaslab maintains an in-core free map (ms_map) that contains the
+ * current list of free segments. As blocks are allocated, the allocated
+ * segment is removed from the ms_map and added to a per txg allocation map.
+ * As blocks are freed, they are added to the per txg free map. These per
+ * txg maps allow us to process all allocations and frees in syncing context
+ * where it is safe to update the on-disk space maps.
+ *
+ * Each metaslab's free space is tracked in a space map object in the MOS,
+ * which is only updated in syncing context. Each time we sync a txg,
  * we append the allocs and frees from that txg to the space map object.
  * When the txg is done syncing, metaslab_sync_done() updates ms_smo
- * to ms_smo_syncing.  Everything in ms_smo is always safe to allocate.
+ * to ms_smo_syncing. Everything in ms_smo is always safe to allocate.
+ *
+ * To load the in-core free map we read the space map object from disk.
+ * This object contains a series of alloc and free records that are
+ * combined to make up the list of all free segments in this metaslab. These
+ * segments are represented in-core by the ms_map and are stored in an
+ * AVL tree.
+ *
+ * As the space map objects grows (as a result of the appends) it will
+ * eventually become space-inefficient. When the space map object is
+ * zfs_condense_pct/100 times the size of the minimal on-disk representation,
+ * we rewrite it in its minimized form.
  */
 struct metaslab {
 	kmutex_t	ms_lock;	/* metaslab lock		*/
 	space_map_obj_t	ms_smo;		/* synced space map object	*/
 	space_map_obj_t	ms_smo_syncing;	/* syncing space map object	*/
-	space_map_t	ms_allocmap[TXG_SIZE];  /* allocated this txg	*/
-	space_map_t	ms_freemap[TXG_SIZE];	/* freed this txg	*/
-	space_map_t	ms_defermap[TXG_DEFER_SIZE]; /* deferred frees	*/
-	space_map_t	ms_map;		/* in-core free space map	*/
+	space_map_t	*ms_allocmap[TXG_SIZE];	/* allocated this txg	*/
+	space_map_t	*ms_freemap[TXG_SIZE];	/* freed this txg	*/
+	space_map_t	*ms_defermap[TXG_DEFER_SIZE];	/* deferred frees */
+	space_map_t	*ms_map;	/* in-core free space map	*/
 	int64_t		ms_deferspace;	/* sum of ms_defermap[] space	*/
 	uint64_t	ms_weight;	/* weight vs. others in group	*/
 	metaslab_group_t *ms_group;	/* metaslab group		*/