X-Git-Url: https://git.camperquake.de/gitweb.cgi?a=blobdiff_plain;f=include%2Fsys%2Fmetaslab_impl.h;h=a36baedd442e9243779788fcfeea55fe28f67cf3;hb=e51be06697762215dc3b679f8668987034a5a048;hp=386f6240cbe228584b130194b8c08588cf91d5bd;hpb=c99c90015ece64746e20b74245caca41d1dbefe1;p=zfs.git

diff --git a/include/sys/metaslab_impl.h b/include/sys/metaslab_impl.h
index 386f624..a36baed 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		*/