Make metaslab_aliquot a module parameter.

This seems generally useful. metaslab_aliquot is the ZFS allocation
granularity, which is roughly equivalent to what is called the stripe
size in traditional RAID arrays. It seems relevant to performance
tuning.

Signed-off-by: Etienne Dechamps <etienne@edechamps.fr>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

man/man5/zfs-module-parameters.5

@@ -134,6 +134,20 @@ Max write bytes per interval
 Default value: \fB8,388,608\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBmetaslab_aliquot\fR (ulong)
+.ad
+.RS 12n
+Metaslab granularity, in bytes. This is roughly similar to what would be
+referred to as the "stripe size" in traditional RAID arrays. In normal
+operation, ZFS will try to write this amount of data to a top-level vdev
+before moving on to the next one.
+.sp
+Default value: \fB524,288\fR.
+.RE
+
 .sp
 .ne 2
 .na

module/zfs/metaslab.c

@@ -59,7 +59,7 @@
  * operation, we will try to write this amount of data to a top-level vdev
  * before moving on to the next one.
  */
-uint64_t metaslab_aliquot = 512ULL << 10;
+unsigned long metaslab_aliquot = 512 << 10;
 
 uint64_t metaslab_gang_bang = SPA_MAXBLOCKSIZE + 1;	/* force gang blocks */
 
@@ -2707,6 +2707,7 @@ metaslab_check_free(spa_t *spa, const blkptr_t *bp)
 }
 
 #if defined(_KERNEL) && defined(HAVE_SPL)
+module_param(metaslab_aliquot, ulong, 0644);
 module_param(metaslab_debug_load, int, 0644);
 module_param(metaslab_debug_unload, int, 0644);
 module_param(metaslab_preload_enabled, int, 0644);
@@ -2717,6 +2718,8 @@ module_param(metaslab_fragmentation_factor_enabled, int, 0644);
 module_param(metaslab_lba_weighting_enabled, int, 0644);
 module_param(metaslab_bias_enabled, int, 0644);
 
+MODULE_PARM_DESC(metaslab_aliquot,
+	"allocation granularity (a.k.a. stripe size)");
 MODULE_PARM_DESC(metaslab_debug_load,
 	"load all metaslabs when pool is first opened");
 MODULE_PARM_DESC(metaslab_debug_unload,