OpenZFS 9617 - too-frequent TXG sync causes excessive write inflation

Porting notes:
* Renamed zfs_dirty_data_sync_pct to zfs_dirty_data_sync_percent and
  changed the type to be consistent with the other dirty module params.
* Updated zfs-module-parameters.5 accordingly.

Authored by: Matthew Ahrens <[email protected]>
Reviewed by: Serapheim Dimitropoulos <[email protected]>
Reviewed by: Brad Lewis <[email protected]>
Reviewed by: George Wilson <[email protected]>
Reviewed by: Andrew Stormont <[email protected]>
Reviewed-by: George Melikov <[email protected]>
Approved by: Robert Mustacchi <[email protected]>
Ported-by: Brian Behlendorf <[email protected]>

OpenZFS-issue: https://illumos.org/issues/9617
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/7928f4ba
Closes #7976

4 files changed, 18 insertions, 11 deletions

diff --git a/include/sys/dsl_pool.h b/include/sys/dsl_pool.h
index 01870e867..56317cf73 100644
--- a/include/sys/dsl_pool.h
+++ b/include/sys/dsl_pool.h
@@ -57,7 +57,7 @@ struct dsl_crypto_params;
 
 extern unsigned long zfs_dirty_data_max;
 extern unsigned long zfs_dirty_data_max_max;
-extern unsigned long zfs_dirty_data_sync;
+extern int zfs_dirty_data_sync_percent;
 extern int zfs_dirty_data_max_percent;
 extern int zfs_dirty_data_max_max_percent;
 extern int zfs_delay_min_dirty_percent;
diff --git a/man/man5/zfs-module-parameters.5 b/man/man5/zfs-module-parameters.5
index b02d6aae7..d7b53d161 100644
--- a/man/man5/zfs-module-parameters.5
+++ b/man/man5/zfs-module-parameters.5
@@ -1225,12 +1225,14 @@ Default value: \fB10\fR%, subject to \fBzfs_dirty_data_max_max\fR.
 .sp
 .ne 2
 .na
-\fBzfs_dirty_data_sync\fR (int)
+\fBzfs_dirty_data_sync_percent\fR (int)
 .ad
 .RS 12n
-Start syncing out a transaction group if there is at least this much dirty data.
+Start syncing out a transaction group if there's at least this much dirty data
+as a percentage of \fBzfs_dirty_data_max\fR.  This should be less than
+\fBzfs_vdev_async_write_active_min_dirty_percent\fR.
 .sp
-Default value: \fB67,108,864\fR.
+Default value: \fB20\fR% of \fBzfs_dirty_data_max\fR.
 .RE
 
 .sp
diff --git a/module/zfs/dsl_pool.c b/module/zfs/dsl_pool.c
index bd2055710..b940382d1 100644
--- a/module/zfs/dsl_pool.c
+++ b/module/zfs/dsl_pool.c
@@ -106,9 +106,11 @@ int zfs_dirty_data_max_percent = 10;
 int zfs_dirty_data_max_max_percent = 25;
 
 /*
- * If there is at least this much dirty data, push out a txg.
+ * If there's at least this much dirty data (as a percentage of
+ * zfs_dirty_data_max), push out a txg.  This should be less than
+ * zfs_vdev_async_write_active_min_dirty_percent.
  */
-unsigned long zfs_dirty_data_sync = 64 * 1024 * 1024;
+int zfs_dirty_data_sync_percent = 20;
 
 /*
  * Once there is this amount of dirty data, the dmu_tx_delay() will kick in
@@ -879,10 +881,12 @@ dsl_pool_need_dirty_delay(dsl_pool_t *dp)
 {
 	uint64_t delay_min_bytes =
 	    zfs_dirty_data_max * zfs_delay_min_dirty_percent / 100;
+	uint64_t dirty_min_bytes =
+	    zfs_dirty_data_max * zfs_dirty_data_sync_percent / 100;
 	boolean_t rv;
 
 	mutex_enter(&dp->dp_lock);
-	if (dp->dp_dirty_total > zfs_dirty_data_sync)
+	if (dp->dp_dirty_total > dirty_min_bytes)
 		txg_kick(dp);
 	rv = (dp->dp_dirty_total > delay_min_bytes);
 	mutex_exit(&dp->dp_lock);
@@ -1345,7 +1349,7 @@ module_param(zfs_dirty_data_max_max, ulong, 0444);
 MODULE_PARM_DESC(zfs_dirty_data_max_max,
 	"zfs_dirty_data_max upper bound in bytes");
 
-module_param(zfs_dirty_data_sync, ulong, 0644);
+module_param(zfs_dirty_data_sync_percent, int, 0644);
 MODULE_PARM_DESC(zfs_dirty_data_sync, "sync txg when this much dirty data");
 
 module_param(zfs_delay_scale, ulong, 0644);
diff --git a/module/zfs/txg.c b/module/zfs/txg.c
index cfc1d0a35..aad1a8e57 100644
--- a/module/zfs/txg.c
+++ b/module/zfs/txg.c
@@ -517,7 +517,8 @@ txg_sync_thread(void *arg)
 		clock_t timeout = zfs_txg_timeout * hz;
 		clock_t timer;
 		uint64_t txg;
-		txg_stat_t *ts;
+		uint64_t dirty_min_bytes =
+		    zfs_dirty_data_max * zfs_dirty_data_sync_percent / 100;
 
 		/*
 		 * We sync when we're scanning, there's someone waiting
@@ -529,7 +530,7 @@ txg_sync_thread(void *arg)
 		    !tx->tx_exiting && timer > 0 &&
 		    tx->tx_synced_txg >= tx->tx_sync_txg_waiting &&
 		    !txg_has_quiesced_to_sync(dp) &&
-		    dp->dp_dirty_total < zfs_dirty_data_sync) {
+		    dp->dp_dirty_total < dirty_min_bytes) {
 			dprintf("waiting; tx_synced=%llu waiting=%llu dp=%p\n",
 			    tx->tx_synced_txg, tx->tx_sync_txg_waiting, dp);
 			txg_thread_wait(tx, &cpr, &tx->tx_sync_more_cv, timer);
@@ -561,7 +562,7 @@ txg_sync_thread(void *arg)
 		tx->tx_quiesced_txg = 0;
 		tx->tx_syncing_txg = txg;
 		DTRACE_PROBE2(txg__syncing, dsl_pool_t *, dp, uint64_t, txg);
-		ts = spa_txg_history_init_io(spa, txg, dp);
+		txg_stat_t *ts = spa_txg_history_init_io(spa, txg, dp);
 		cv_broadcast(&tx->tx_quiesce_more_cv);
 
 		dprintf("txg=%llu quiesce_txg=%llu sync_txg=%llu\n",