OpenZFS 9963 - Separate tunable for disabling ZIL vdev flush

Porting Notes:
* Add options to zfs-module-parameters(5) man page.
* zfs_nocacheflush move to vdev.c instead of vdev_disk.c, since
  the latter doesn't get built for user space.

Authored by: Prakash Surya <[email protected]>
Reviewed by: Matt Ahrens <[email protected]>
Reviewed by: Brad Lewis <[email protected]>
Reviewed by: Patrick Mooney <[email protected]>
Reviewed by: Tom Caputi <[email protected]>
Reviewed by: George Melikov <[email protected]>
Approved by: Dan McDonald <[email protected]>
Ported-by: Signed-off-by: Brian Behlendorf <[email protected]>

OpenZFS-issue: https://www.illumos.org/issues/9963
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/f8fdf68125
Closes #8186

2 files changed, 19 insertions, 8 deletions

diff --git a/module/zfs/vdev.c b/module/zfs/vdev.c
index ff5a15365..8273e7907 100644
--- a/module/zfs/vdev.c
+++ b/module/zfs/vdev.c
@@ -99,6 +99,13 @@ int zfs_scan_ignore_errors = 0;
  */
 int vdev_standard_sm_blksz = (1 << 17);
 
+/*
+ * Tunable parameter for debugging or performance analysis. Setting this
+ * will cause pool corruption on power loss if a volatile out-of-order
+ * write cache is enabled.
+ */
+int zfs_nocacheflush = 0;
+
 /*PRINTFLIKE2*/
 void
 vdev_dbgmsg(vdev_t *vd, const char *fmt, ...)
@@ -4650,5 +4657,8 @@ MODULE_PARM_DESC(zfs_scan_ignore_errors,
 module_param(vdev_validate_skip, int, 0644);
 MODULE_PARM_DESC(vdev_validate_skip,
 	"Bypass vdev_validate()");
+
+module_param(zfs_nocacheflush, int, 0644);
+MODULE_PARM_DESC(zfs_nocacheflush, "Disable cache flushes");
 /* END CSTYLED */
 #endif
diff --git a/module/zfs/zil.c b/module/zfs/zil.c
index d5ebf7561..a453c26c3 100644
--- a/module/zfs/zil.c
+++ b/module/zfs/zil.c
@@ -118,11 +118,12 @@ static kstat_t *zil_ksp;
 int zil_replay_disable = 0;
 
 /*
- * Tunable parameter for debugging or performance analysis.  Setting
- * zfs_nocacheflush will cause corruption on power loss if a volatile
- * out-of-order write cache is enabled.
+ * Disable the DKIOCFLUSHWRITECACHE commands that are normally sent to
+ * the disk(s) by the ZIL after an LWB write has completed. Setting this
+ * will cause ZIL corruption on power loss if a volatile out-of-order
+ * write cache is enabled.
  */
-int zfs_nocacheflush = 0;
+int zil_nocacheflush = 0;
 
 /*
  * Limit SLOG write size per commit executed with synchronous priority.
@@ -1041,7 +1042,7 @@ zil_lwb_add_block(lwb_t *lwb, const blkptr_t *bp)
 	int ndvas = BP_GET_NDVAS(bp);
 	int i;
 
-	if (zfs_nocacheflush)
+	if (zil_nocacheflush)
 		return;
 
 	mutex_enter(&lwb->lwb_vdev_lock);
@@ -1065,7 +1066,7 @@ zil_lwb_add_txg(lwb_t *lwb, uint64_t txg)
 /*
  * This function is a called after all VDEVs associated with a given lwb
  * write have completed their DKIOCFLUSHWRITECACHE command; or as soon
- * as the lwb write completes, if "zfs_nocacheflush" is set.
+ * as the lwb write completes, if "zil_nocacheflush" is set.
  *
  * The intention is for this function to be called as soon as the
  * contents of an lwb are considered "stable" on disk, and will survive
@@ -3513,8 +3514,8 @@ MODULE_PARM_DESC(zfs_commit_timeout_pct, "ZIL block open timeout percentage");
 module_param(zil_replay_disable, int, 0644);
 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_nocacheflush, int, 0644);
+MODULE_PARM_DESC(zil_nocacheflush, "Disable ZIL cache flushes");
 
 module_param(zil_slog_bulk, ulong, 0644);
 MODULE_PARM_DESC(zil_slog_bulk, "Limit in bytes slog sync writes per commit");