Add txgs-<pool> kstat file

Create a kstat file which contains useful statistics about the
last N txgs processed.  This can be helpful when analyzing pool
performance.  The new KSTAT_TYPE_TXG type was added for this
purpose and it tracks the following statistics per-txg.

  txg          - Unique txg number
  state        - State (O)pen/(Q)uiescing/(S)yncing/(C)ommitted
  birth;       - Creation time
  nread        - Bytes read
  nwritten;    - Bytes written
  reads        - IOPs read
  writes       - IOPs write
  open_time;   - Length in nanoseconds the txg was open
  quiesce_time - Length in nanoseconds the txg was quiescing
  sync_time;   - Length in nanoseconds the txg was syncing

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/dsl_pool.c

@@ -44,6 +44,7 @@
 int zfs_no_write_throttle = 0;
 int zfs_write_limit_shift = 3;			/* 1/8th of physical memory */
 int zfs_txg_synctime_ms = 1000;		/* target millisecs to sync a txg */
+int zfs_txg_history = 60;		/* statistics for the last N txgs */
 
 unsigned long zfs_write_limit_min = 32 << 20;	/* min write limit is 32MB */
 unsigned long zfs_write_limit_max = 0;		/* max data payload per txg */
@@ -54,6 +55,143 @@ kmutex_t zfs_write_limit_lock;
 
 static pgcnt_t old_physmem = 0;
 
+static int
+dsl_pool_txg_history_update(kstat_t *ksp, int rw)
+{
+	dsl_pool_t *dp = ksp->ks_private;
+	txg_history_t *th;
+	int i = 0;
+
+	if (rw == KSTAT_WRITE)
+		return (EACCES);
+
+	if (ksp->ks_data)
+		kmem_free(ksp->ks_data, ksp->ks_data_size);
+
+	mutex_enter(&dp->dp_lock);
+
+	ksp->ks_ndata = dp->dp_txg_history_size;
+	ksp->ks_data_size = dp->dp_txg_history_size * sizeof(kstat_txg_t);
+	if (ksp->ks_data_size > 0)
+		ksp->ks_data = kmem_alloc(ksp->ks_data_size, KM_PUSHPAGE);
+
+	/* Traversed oldest to youngest for the most readable kstat output */
+	for (th = list_tail(&dp->dp_txg_history); th != NULL;
+	     th = list_prev(&dp->dp_txg_history, th)) {
+		mutex_enter(&th->th_lock);
+		ASSERT3S(i + sizeof(kstat_txg_t), <=, ksp->ks_data_size);
+		memcpy(ksp->ks_data + i, &th->th_kstat, sizeof(kstat_txg_t));
+		i += sizeof(kstat_txg_t);
+		mutex_exit(&th->th_lock);
+	}
+
+	mutex_exit(&dp->dp_lock);
+
+	return (0);
+}
+
+static void
+dsl_pool_txg_history_init(dsl_pool_t *dp, uint64_t txg)
+{
+	char name[KSTAT_STRLEN];
+
+	list_create(&dp->dp_txg_history, sizeof (txg_history_t),
+	    offsetof(txg_history_t, th_link));
+	dsl_pool_txg_history_add(dp, txg);
+
+	(void) snprintf(name, KSTAT_STRLEN, "txgs-%s", spa_name(dp->dp_spa));
+	dp->dp_txg_kstat = kstat_create("zfs", 0, name, "misc",
+	    KSTAT_TYPE_TXG, 0, KSTAT_FLAG_VIRTUAL);
+	if (dp->dp_txg_kstat) {
+		dp->dp_txg_kstat->ks_data = NULL;
+		dp->dp_txg_kstat->ks_private = dp;
+		dp->dp_txg_kstat->ks_update = dsl_pool_txg_history_update;
+		kstat_install(dp->dp_txg_kstat);
+	}
+}
+
+static void
+dsl_pool_txg_history_destroy(dsl_pool_t *dp)
+{
+	txg_history_t *th;
+
+	if (dp->dp_txg_kstat) {
+		if (dp->dp_txg_kstat->ks_data)
+			kmem_free(dp->dp_txg_kstat->ks_data,
+			    dp->dp_txg_kstat->ks_data_size);
+
+		kstat_delete(dp->dp_txg_kstat);
+	}
+
+	mutex_enter(&dp->dp_lock);
+	while ((th = list_remove_head(&dp->dp_txg_history))) {
+		dp->dp_txg_history_size--;
+		mutex_destroy(&th->th_lock);
+		kmem_free(th, sizeof(txg_history_t));
+	}
+
+	ASSERT3U(dp->dp_txg_history_size, ==, 0);
+	list_destroy(&dp->dp_txg_history);
+	mutex_exit(&dp->dp_lock);
+}
+
+txg_history_t *
+dsl_pool_txg_history_add(dsl_pool_t *dp, uint64_t txg)
+{
+	txg_history_t *th, *rm;
+
+	th = kmem_zalloc(sizeof(txg_history_t), KM_SLEEP);
+	mutex_init(&th->th_lock, NULL, MUTEX_DEFAULT, NULL);
+	th->th_kstat.txg = txg;
+	th->th_kstat.state = TXG_STATE_OPEN;
+	th->th_kstat.birth = gethrtime();
+
+	mutex_enter(&dp->dp_lock);
+
+	list_insert_head(&dp->dp_txg_history, th);
+	dp->dp_txg_history_size++;
+
+	while (dp->dp_txg_history_size > zfs_txg_history) {
+		dp->dp_txg_history_size--;
+		rm = list_remove_tail(&dp->dp_txg_history);
+		mutex_destroy(&rm->th_lock);
+		kmem_free(rm, sizeof(txg_history_t));
+	}
+
+	mutex_exit(&dp->dp_lock);
+
+	return (th);
+}
+
+/*
+ * Traversed youngest to oldest because lookups are only done for open
+ * or syncing txgs which are guaranteed to be at the head of the list.
+ * The txg_history_t structure will be returned locked.
+ */
+txg_history_t *
+dsl_pool_txg_history_get(dsl_pool_t *dp, uint64_t txg)
+{
+	txg_history_t *th;
+
+	mutex_enter(&dp->dp_lock);
+	for (th = list_head(&dp->dp_txg_history); th != NULL;
+	     th = list_next(&dp->dp_txg_history, th)) {
+		if (th->th_kstat.txg == txg) {
+			mutex_enter(&th->th_lock);
+			break;
+		}
+	}
+	mutex_exit(&dp->dp_lock);
+
+	return (th);
+}
+
+void
+dsl_pool_txg_history_put(txg_history_t *th)
+{
+	mutex_exit(&th->th_lock);
+}
+
 int
 dsl_pool_open_special_dir(dsl_pool_t *dp, const char *name, dsl_dir_t **ddp)
 {
@@ -96,6 +234,8 @@ dsl_pool_open_impl(spa_t *spa, uint64_t txg)
 	dp->dp_iput_taskq = taskq_create("zfs_iput_taskq", 1, minclsyspri,
 	    1, 4, 0);
 
+	dsl_pool_txg_history_init(dp, txg);
+
 	return (dp);
 }
 
@@ -213,6 +353,7 @@ dsl_pool_close(dsl_pool_t *dp)
 	arc_flush(dp->dp_spa);
 	txg_fini(dp);
 	dsl_scan_fini(dp);
+	dsl_pool_txg_history_destroy(dp);
 	rw_destroy(&dp->dp_config_rwlock);
 	mutex_destroy(&dp->dp_lock);
 	taskq_destroy(dp->dp_iput_taskq);
@@ -864,6 +1005,9 @@ MODULE_PARM_DESC(zfs_write_limit_shift, "log2(fraction of memory) per txg");
 module_param(zfs_txg_synctime_ms, int, 0644);
 MODULE_PARM_DESC(zfs_txg_synctime_ms, "Target milliseconds between txg sync");
 
+module_param(zfs_txg_history, int, 0644);
+MODULE_PARM_DESC(zfs_txg_history, "Historic statistics for the last N txgs");
+
 module_param(zfs_write_limit_min, ulong, 0444);
 MODULE_PARM_DESC(zfs_write_limit_min, "Min txg write limit");
 

module/zfs/txg.c

@@ -29,6 +29,7 @@
 #include <sys/dsl_pool.h>
 #include <sys/dsl_scan.h>
 #include <sys/callb.h>
+#include <sys/spa_impl.h>
 
 /*
  * Pool-wide transaction groups.
@@ -279,6 +280,8 @@ txg_rele_to_sync(txg_handle_t *th)
 static void
 txg_quiesce(dsl_pool_t *dp, uint64_t txg)
 {
+	hrtime_t start;
+	txg_history_t *th;
 	tx_state_t *tx = &dp->dp_tx;
 	int g = txg & TXG_MASK;
 	int c;
@@ -292,6 +295,15 @@ txg_quiesce(dsl_pool_t *dp, uint64_t txg)
 	ASSERT(txg == tx->tx_open_txg);
 	tx->tx_open_txg++;
 
+	/*
+	 * Measure how long the txg was open and replace the kstat.
+	 */
+	th = dsl_pool_txg_history_get(dp, txg);
+	th->th_kstat.open_time = gethrtime() - th->th_kstat.birth;
+	th->th_kstat.state = TXG_STATE_QUIESCING;
+	dsl_pool_txg_history_put(th);
+	dsl_pool_txg_history_add(dp, tx->tx_open_txg);
+
 	/*
 	 * Now that we've incremented tx_open_txg, we can let threads
 	 * enter the next transaction group.
@@ -302,6 +314,8 @@ txg_quiesce(dsl_pool_t *dp, uint64_t txg)
 	/*
 	 * Quiesce the transaction group by waiting for everyone to txg_exit().
 	 */
+	start = gethrtime();
+
 	for (c = 0; c < max_ncpus; c++) {
 		tx_cpu_t *tc = &tx->tx_cpu[c];
 		mutex_enter(&tc->tc_lock);
@@ -309,6 +323,13 @@ txg_quiesce(dsl_pool_t *dp, uint64_t txg)
 			cv_wait(&tc->tc_cv[g], &tc->tc_lock);
 		mutex_exit(&tc->tc_lock);
 	}
+
+	/*
+	 * Measure how long the txg took to quiesce.
+	 */
+	th = dsl_pool_txg_history_get(dp, txg);
+	th->th_kstat.quiesce_time = gethrtime() - start;
+	dsl_pool_txg_history_put(th);
 }
 
 static void
@@ -395,6 +416,8 @@ txg_sync_thread(dsl_pool_t *dp)
 
 	start = delta = 0;
 	for (;;) {
+		hrtime_t hrstart;
+		txg_history_t *th;
 		uint64_t timer, timeout;
 		uint64_t txg;
 
@@ -441,11 +464,17 @@ txg_sync_thread(dsl_pool_t *dp)
 		tx->tx_syncing_txg = txg;
 		cv_broadcast(&tx->tx_quiesce_more_cv);
 
+		th = dsl_pool_txg_history_get(dp, txg);
+		th->th_kstat.state = TXG_STATE_SYNCING;
+		vdev_get_stats(spa->spa_root_vdev, &th->th_vs1);
+		dsl_pool_txg_history_put(th);
+
 		dprintf("txg=%llu quiesce_txg=%llu sync_txg=%llu\n",
 		    txg, tx->tx_quiesce_txg_waiting, tx->tx_sync_txg_waiting);
 		mutex_exit(&tx->tx_sync_lock);
 
 		start = ddi_get_lbolt();
+		hrstart = gethrtime();
 		spa_sync(spa, txg);
 		delta = ddi_get_lbolt() - start;
 
@@ -458,6 +487,23 @@ txg_sync_thread(dsl_pool_t *dp)
 		 * Dispatch commit callbacks to worker threads.
 		 */
 		txg_dispatch_callbacks(dp, txg);
+
+		/*
+		 * Measure the txg sync time determine the amount of I/O done.
+		 */
+		th = dsl_pool_txg_history_get(dp, txg);
+		vdev_get_stats(spa->spa_root_vdev, &th->th_vs2);
+		th->th_kstat.sync_time = gethrtime() - hrstart;
+		th->th_kstat.nread = th->th_vs2.vs_bytes[ZIO_TYPE_READ] -
+		    th->th_vs1.vs_bytes[ZIO_TYPE_READ];
+		th->th_kstat.nwritten = th->th_vs2.vs_bytes[ZIO_TYPE_WRITE] -
+		    th->th_vs1.vs_bytes[ZIO_TYPE_WRITE];
+		th->th_kstat.reads = th->th_vs2.vs_ops[ZIO_TYPE_READ] -
+		    th->th_vs1.vs_ops[ZIO_TYPE_READ];
+		th->th_kstat.writes = th->th_vs2.vs_ops[ZIO_TYPE_WRITE] -
+		    th->th_vs1.vs_ops[ZIO_TYPE_WRITE];
+		th->th_kstat.state = TXG_STATE_COMMITTED;
+		dsl_pool_txg_history_put(th);
 	}
 }