Expand fragmentation table size to reflect larger possibility space for allocation sizes

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Signed-off-by: Paul Dagnelie <[email protected]>

Author: Paul Dagnelie

man/man4/zfs.4

@@ -1778,7 +1778,7 @@ Normally disabled because these datasets may be missing key data.
 .It Sy zfs_min_metaslabs_to_flush Ns = Ns Sy 1 Pq u64
 Minimum number of metaslabs to flush per dirty TXG.
 .
-.It Sy zfs_metaslab_fragmentation_threshold Ns = Ns Sy 70 Ns % Pq uint
+.It Sy zfs_metaslab_fragmentation_threshold Ns = Ns Sy 85 Ns % Pq uint
 Allow metaslabs to keep their active state as long as their fragmentation
 percentage is no more than this value.
 An active metaslab that exceeds this threshold

module/zfs/metaslab.c

@@ -146,7 +146,7 @@ static uint_t zfs_mg_fragmentation_threshold = 95;
  * active metaslab that exceeds this threshold will no longer keep its active
  * status allowing better metaslabs to be selected.
  */
-static uint_t zfs_metaslab_fragmentation_threshold = 70;
+static uint_t zfs_metaslab_fragmentation_threshold = 85;
 
 /*
  * When set will load all metaslabs when pool is first opened.
@@ -2889,8 +2889,6 @@ metaslab_fini(metaslab_t *msp)
 	kmem_free(msp, sizeof (metaslab_t));
 }
 
-#define	FRAGMENTATION_TABLE_SIZE	17
-
 /*
  * This table defines a segment size based fragmentation metric that will
  * allow each metaslab to derive its own fragmentation value. This is done
@@ -2901,33 +2899,41 @@ metaslab_fini(metaslab_t *msp)
  * us the fragmentation metric. This means that a high fragmentation metric
  * equates to most of the free space being comprised of small segments.
  * Conversely, if the metric is low, then most of the free space is in
- * large segments. A 10% change in fragmentation equates to approximately
- * double the number of segments.
+ * large segments.
  *
- * This table defines 0% fragmented space using 16MB segments. Testing has
- * shown that segments that are greater than or equal to 16MB do not suffer
- * from drastic performance problems. Using this value, we derive the rest
- * of the table. Since the fragmentation value is never stored on disk, it
- * is possible to change these calculations in the future.
+ * This table defines 0% fragmented space using 1G segments. Using this value,
+ * we derive the rest of the table. This table originally went up to 16MB, but
+ * with larger recordsizes, larger ashifts, and use of raidz3, it is possible
+ * to have significantly larger allocations than were previously possible.
+ * Since the fragmentation value is never stored on disk, it is possible to
+ * change these calculations in the future.
  */
-static const int zfs_frag_table[FRAGMENTATION_TABLE_SIZE] = {
+static const int zfs_frag_table[] = {
 	100,	/* 512B	*/
 	100,	/* 1K	*/
 	98,	/* 2K	*/
 	95,	/* 4K	*/
-	90,	/* 8K	*/
-	80,	/* 16K	*/
-	70,	/* 32K	*/
-	60,	/* 64K	*/
-	50,	/* 128K	*/
-	40,	/* 256K	*/
-	30,	/* 512K	*/
-	20,	/* 1M	*/
-	15,	/* 2M	*/
-	10,	/* 4M	*/
-	5,	/* 8M	*/
-	0	/* 16M	*/
+	92,	/* 8K	*/
+	90,	/* 16K	*/
+	85,	/* 32K	*/
+	80,	/* 64K	*/
+	75,	/* 128K	*/
+	70,	/* 256K	*/
+	60,	/* 512K	*/
+	50,	/* 1M	*/
+	40,	/* 2M	*/
+	35,	/* 4M	*/
+	30,	/* 8M	*/
+	25,	/* 16M	*/
+	20,	/* 32M	*/
+	15,	/* 64M	*/
+	10,	/* 128M	*/
+	5,	/* 256M	*/
+	2,	/* 512M	*/
+	0,	/* 1G	*/
 };
+#define	FRAGMENTATION_TABLE_SIZE \
+    (sizeof (zfs_frag_table)/(sizeof (zfs_frag_table[0])))
 
 /*
  * Calculate the metaslab's fragmentation metric and set ms_fragmentation.