Make zil_slog_limit a tunable module parameter.

zil_slog_limit specifies the maximum commit size to be written to the separate
log device. Larger commits bypass the separate log device and go directly to
the data devices.

The optimal value for zil_slog_limit directly depends on the latency and
throughput characteristics of both the separate log device and the data disks.
Small synchronous writes are faster on low-latency separate log devices (e.g.
SSDs) whereas large synchronous writes are faster on high-latency data disks
(e.g. spindles) because of higher throughput, especially with a large array.
The point is, the line between "small" and "large" synchronous writes in this
scenario is heavily dependent on the hardware used. That's why it should be
made configurable.

Signed-off-by: Brian Behlendorf <[email protected]>
Closes #783

module/zfs/zil.c

@@ -859,7 +859,7 @@ uint64_t zil_block_buckets[] = {
  * is less than the limit or the total list size is less than 2X the limit.
  * Limit checking is disabled by setting zil_slog_limit to UINT64_MAX.
  */
-uint64_t zil_slog_limit = 1024 * 1024;
+unsigned long zil_slog_limit = 1024 * 1024;
 #define	USE_SLOG(zilog) (((zilog)->zl_logbias == ZFS_LOGBIAS_LATENCY) && \
 	(((zilog)->zl_cur_used < zil_slog_limit) || \
 	((zilog)->zl_itx_list_sz < (zil_slog_limit << 1))))
@@ -2010,4 +2010,7 @@ MODULE_PARM_DESC(zil_replay_disable, "Disable intent logging replay");
 
 module_param(zfs_nocacheflush, int, 0644);
 MODULE_PARM_DESC(zfs_nocacheflush, "Disable cache flushes");
+
+module_param(zil_slog_limit, ulong, 0644);
+MODULE_PARM_DESC(zil_slog_limit, "Max commit bytes to separate log device");
 #endif