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4045 zfs write throttle & i/o scheduler performance work
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
@@ -44,23 +44,95 @@
#include <sys/bptree.h>
#include <sys/zfeature.h>
#include <sys/zil_impl.h>
#include <sys/dsl_userhold.h>
-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 */
+/*
+ * ZFS Write Throttle
+ * ------------------
+ *
+ * ZFS must limit the rate of incoming writes to the rate at which it is able
+ * to sync data modifications to the backend storage. Throttling by too much
+ * creates an artificial limit; throttling by too little can only be sustained
+ * for short periods and would lead to highly lumpy performance. On a per-pool
+ * basis, ZFS tracks the amount of modified (dirty) data. As operations change
+ * data, the amount of dirty data increases; as ZFS syncs out data, the amount
+ * of dirty data decreases. When the amount of dirty data exceeds a
+ * predetermined threshold further modifications are blocked until the amount
+ * of dirty data decreases (as data is synced out).
+ *
+ * The limit on dirty data is tunable, and should be adjusted according to
+ * both the IO capacity and available memory of the system. The larger the
+ * window, the more ZFS is able to aggregate and amortize metadata (and data)
+ * changes. However, memory is a limited resource, and allowing for more dirty
+ * data comes at the cost of keeping other useful data in memory (for example
+ * ZFS data cached by the ARC).
+ *
+ * Implementation
+ *
+ * As buffers are modified dsl_pool_willuse_space() increments both the per-
+ * txg (dp_dirty_pertxg[]) and poolwide (dp_dirty_total) accounting of
+ * dirty space used; dsl_pool_dirty_space() decrements those values as data
+ * is synced out from dsl_pool_sync(). While only the poolwide value is
+ * relevant, the per-txg value is useful for debugging. The tunable
+ * zfs_dirty_data_max determines the dirty space limit. Once that value is
+ * exceeded, new writes are halted until space frees up.
+ *
+ * The zfs_dirty_data_sync tunable dictates the threshold at which we
+ * ensure that there is a txg syncing (see the comment in txg.c for a full
+ * description of transaction group stages).
+ *
+ * The IO scheduler uses both the dirty space limit and current amount of
+ * dirty data as inputs. Those values affect the number of concurrent IOs ZFS
+ * issues. See the comment in vdev_queue.c for details of the IO scheduler.
+ *
+ * The delay is also calculated based on the amount of dirty data. See the
+ * comment above dmu_tx_delay() for details.
+ */
-uint64_t zfs_write_limit_min = 32 << 20; /* min write limit is 32MB */
-uint64_t zfs_write_limit_max = 0; /* max data payload per txg */
-uint64_t zfs_write_limit_inflated = 0;
-uint64_t zfs_write_limit_override = 0;
+/*
+ * zfs_dirty_data_max will be set to zfs_dirty_data_max_percent% of all memory,
+ * capped at zfs_dirty_data_max_max. It can also be overridden in /etc/system.
+ */
+uint64_t zfs_dirty_data_max;
+uint64_t zfs_dirty_data_max_max = 4ULL * 1024 * 1024 * 1024;
+int zfs_dirty_data_max_percent = 10;
-kmutex_t zfs_write_limit_lock;
+/*
+ * If there is at least this much dirty data, push out a txg.
+ */
+uint64_t zfs_dirty_data_sync = 64 * 1024 * 1024;
-static pgcnt_t old_physmem = 0;
+/*
+ * Once there is this amount of dirty data, the dmu_tx_delay() will kick in
+ * and delay each transaction.
+ * This value should be >= zfs_vdev_async_write_active_max_dirty_percent.
+ */
+int zfs_delay_min_dirty_percent = 60;
+/*
+ * This controls how quickly the delay approaches infinity.
+ * Larger values cause it to delay less for a given amount of dirty data.
+ * Therefore larger values will cause there to be more dirty data for a
+ * given throughput.
+ *
+ * For the smoothest delay, this value should be about 1 billion divided
+ * by the maximum number of operations per second. This will smoothly
+ * handle between 10x and 1/10th this number.
+ *
+ * Note: zfs_delay_scale * zfs_dirty_data_max must be < 2^64, due to the
+ * multiply in dmu_tx_delay().
+ */
+uint64_t zfs_delay_scale = 1000 * 1000 * 1000 / 2000;
+
+
+/*
+ * XXX someday maybe turn these into #defines, and you have to tune it on a
+ * per-pool basis using zfs.conf.
+ */
+
+
hrtime_t zfs_throttle_delay = MSEC2NSEC(10);
hrtime_t zfs_throttle_resolution = MSEC2NSEC(10);
int
dsl_pool_open_special_dir(dsl_pool_t *dp, const char *name, dsl_dir_t **ddp)
@@ -85,11 +157,10 @@
dp = kmem_zalloc(sizeof (dsl_pool_t), KM_SLEEP);
dp->dp_spa = spa;
dp->dp_meta_rootbp = *bp;
rrw_init(&dp->dp_config_rwlock, B_TRUE);
- dp->dp_write_limit = zfs_write_limit_min;
txg_init(dp, txg);
txg_list_create(&dp->dp_dirty_datasets,
offsetof(dsl_dataset_t, ds_dirty_link));
txg_list_create(&dp->dp_dirty_zilogs,
@@ -98,10 +169,11 @@
offsetof(dsl_dir_t, dd_dirty_link));
txg_list_create(&dp->dp_sync_tasks,
offsetof(dsl_sync_task_t, dst_node));
mutex_init(&dp->dp_lock, NULL, MUTEX_DEFAULT, NULL);
+ cv_init(&dp->dp_spaceavail_cv, NULL, CV_DEFAULT, NULL);
dp->dp_vnrele_taskq = taskq_create("zfs_vn_rele_taskq", 1, minclsyspri,
1, 4, 0);
return (dp);
@@ -212,13 +284,13 @@
}
void
dsl_pool_close(dsl_pool_t *dp)
{
- /* drop our references from dsl_pool_open() */
-
/*
+ * Drop our references from dsl_pool_open().
+ *
* Since we held the origin_snap from "syncing" context (which
* includes pool-opening context), it actually only got a "ref"
* and not a hold, so just drop that here.
*/
if (dp->dp_origin_snap)
@@ -344,99 +416,119 @@
dsl_deadlist_t *dl = arg;
dsl_deadlist_insert(dl, bp, tx);
return (0);
}
+static void
+dsl_pool_sync_mos(dsl_pool_t *dp, dmu_tx_t *tx)
+{
+ zio_t *zio = zio_root(dp->dp_spa, NULL, NULL, ZIO_FLAG_MUSTSUCCEED);
+ dmu_objset_sync(dp->dp_meta_objset, zio, tx);
+ VERIFY0(zio_wait(zio));
+ dprintf_bp(&dp->dp_meta_rootbp, "meta objset rootbp is %s", "");
+ spa_set_rootblkptr(dp->dp_spa, &dp->dp_meta_rootbp);
+}
+
+static void
+dsl_pool_dirty_delta(dsl_pool_t *dp, int64_t delta)
+{
+ ASSERT(MUTEX_HELD(&dp->dp_lock));
+
+ if (delta < 0)
+ ASSERT3U(-delta, <=, dp->dp_dirty_total);
+
+ dp->dp_dirty_total += delta;
+
+ /*
+ * Note: we signal even when increasing dp_dirty_total.
+ * This ensures forward progress -- each thread wakes the next waiter.
+ */
+ if (dp->dp_dirty_total <= zfs_dirty_data_max)
+ cv_signal(&dp->dp_spaceavail_cv);
+}
+
void
dsl_pool_sync(dsl_pool_t *dp, uint64_t txg)
{
zio_t *zio;
dmu_tx_t *tx;
dsl_dir_t *dd;
dsl_dataset_t *ds;
objset_t *mos = dp->dp_meta_objset;
- hrtime_t start, write_time;
- uint64_t data_written;
- int err;
list_t synced_datasets;
list_create(&synced_datasets, sizeof (dsl_dataset_t),
offsetof(dsl_dataset_t, ds_synced_link));
- /*
- * We need to copy dp_space_towrite() before doing
- * dsl_sync_task_sync(), because
- * dsl_dataset_snapshot_reserve_space() will increase
- * dp_space_towrite but not actually write anything.
- */
- data_written = dp->dp_space_towrite[txg & TXG_MASK];
-
tx = dmu_tx_create_assigned(dp, txg);
- dp->dp_read_overhead = 0;
- start = gethrtime();
-
+ /*
+ * Write out all dirty blocks of dirty datasets.
+ */
zio = zio_root(dp->dp_spa, NULL, NULL, ZIO_FLAG_MUSTSUCCEED);
- while (ds = txg_list_remove(&dp->dp_dirty_datasets, txg)) {
+ while ((ds = txg_list_remove(&dp->dp_dirty_datasets, txg)) != NULL) {
/*
* We must not sync any non-MOS datasets twice, because
* we may have taken a snapshot of them. However, we
* may sync newly-created datasets on pass 2.
*/
ASSERT(!list_link_active(&ds->ds_synced_link));
list_insert_tail(&synced_datasets, ds);
dsl_dataset_sync(ds, zio, tx);
}
- DTRACE_PROBE(pool_sync__1setup);
- err = zio_wait(zio);
+ VERIFY0(zio_wait(zio));
- write_time = gethrtime() - start;
- ASSERT(err == 0);
- DTRACE_PROBE(pool_sync__2rootzio);
+ /*
+ * We have written all of the accounted dirty data, so our
+ * dp_space_towrite should now be zero. However, some seldom-used
+ * code paths do not adhere to this (e.g. dbuf_undirty(), also
+ * rounding error in dbuf_write_physdone).
+ * Shore up the accounting of any dirtied space now.
+ */
+ dsl_pool_undirty_space(dp, dp->dp_dirty_pertxg[txg & TXG_MASK], txg);
/*
* After the data blocks have been written (ensured by the zio_wait()
* above), update the user/group space accounting.
*/
- for (ds = list_head(&synced_datasets); ds;
- ds = list_next(&synced_datasets, ds))
+ for (ds = list_head(&synced_datasets); ds != NULL;
+ ds = list_next(&synced_datasets, ds)) {
dmu_objset_do_userquota_updates(ds->ds_objset, tx);
+ }
/*
* Sync the datasets again to push out the changes due to
* userspace updates. This must be done before we process the
* sync tasks, so that any snapshots will have the correct
* user accounting information (and we won't get confused
* about which blocks are part of the snapshot).
*/
zio = zio_root(dp->dp_spa, NULL, NULL, ZIO_FLAG_MUSTSUCCEED);
- while (ds = txg_list_remove(&dp->dp_dirty_datasets, txg)) {
+ while ((ds = txg_list_remove(&dp->dp_dirty_datasets, txg)) != NULL) {
ASSERT(list_link_active(&ds->ds_synced_link));
dmu_buf_rele(ds->ds_dbuf, ds);
dsl_dataset_sync(ds, zio, tx);
}
- err = zio_wait(zio);
+ VERIFY0(zio_wait(zio));
/*
* Now that the datasets have been completely synced, we can
* clean up our in-memory structures accumulated while syncing:
*
* - move dead blocks from the pending deadlist to the on-disk deadlist
* - release hold from dsl_dataset_dirty()
*/
- while (ds = list_remove_head(&synced_datasets)) {
+ while ((ds = list_remove_head(&synced_datasets)) != NULL) {
objset_t *os = ds->ds_objset;
bplist_iterate(&ds->ds_pending_deadlist,
deadlist_enqueue_cb, &ds->ds_deadlist, tx);
ASSERT(!dmu_objset_is_dirty(os, txg));
dmu_buf_rele(ds->ds_dbuf, ds);
}
-
- start = gethrtime();
- while (dd = txg_list_remove(&dp->dp_dirty_dirs, txg))
+ while ((dd = txg_list_remove(&dp->dp_dirty_dirs, txg)) != NULL) {
dsl_dir_sync(dd, tx);
- write_time += gethrtime() - start;
+ }
/*
* The MOS's space is accounted for in the pool/$MOS
* (dp_mos_dir). We can't modify the mos while we're syncing
* it, so we remember the deltas and apply them here.
@@ -450,24 +542,14 @@
dp->dp_mos_used_delta = 0;
dp->dp_mos_compressed_delta = 0;
dp->dp_mos_uncompressed_delta = 0;
}
- start = gethrtime();
if (list_head(&mos->os_dirty_dnodes[txg & TXG_MASK]) != NULL ||
list_head(&mos->os_free_dnodes[txg & TXG_MASK]) != NULL) {
- zio = zio_root(dp->dp_spa, NULL, NULL, ZIO_FLAG_MUSTSUCCEED);
- dmu_objset_sync(mos, zio, tx);
- err = zio_wait(zio);
- ASSERT(err == 0);
- dprintf_bp(&dp->dp_meta_rootbp, "meta objset rootbp is %s", "");
- spa_set_rootblkptr(dp->dp_spa, &dp->dp_meta_rootbp);
+ dsl_pool_sync_mos(dp, tx);
}
- write_time += gethrtime() - start;
- DTRACE_PROBE2(pool_sync__4io, hrtime_t, write_time,
- hrtime_t, dp->dp_read_overhead);
- write_time -= dp->dp_read_overhead;
/*
* If we modify a dataset in the same txg that we want to destroy it,
* its dsl_dir's dd_dbuf will be dirty, and thus have a hold on it.
* dsl_dir_destroy_check() will fail if there are unexpected holds.
@@ -474,76 +556,33 @@
* Therefore, we want to sync the MOS (thus syncing the dd_dbuf
* and clearing the hold on it) before we process the sync_tasks.
* The MOS data dirtied by the sync_tasks will be synced on the next
* pass.
*/
- DTRACE_PROBE(pool_sync__3task);
if (!txg_list_empty(&dp->dp_sync_tasks, txg)) {
dsl_sync_task_t *dst;
/*
* No more sync tasks should have been added while we
* were syncing.
*/
- ASSERT(spa_sync_pass(dp->dp_spa) == 1);
- while (dst = txg_list_remove(&dp->dp_sync_tasks, txg))
+ ASSERT3U(spa_sync_pass(dp->dp_spa), ==, 1);
+ while ((dst = txg_list_remove(&dp->dp_sync_tasks, txg)) != NULL)
dsl_sync_task_sync(dst, tx);
}
dmu_tx_commit(tx);
- dp->dp_space_towrite[txg & TXG_MASK] = 0;
- ASSERT(dp->dp_tempreserved[txg & TXG_MASK] == 0);
-
- /*
- * If the write limit max has not been explicitly set, set it
- * to a fraction of available physical memory (default 1/8th).
- * Note that we must inflate the limit because the spa
- * inflates write sizes to account for data replication.
- * Check this each sync phase to catch changing memory size.
- */
- if (physmem != old_physmem && zfs_write_limit_shift) {
- mutex_enter(&zfs_write_limit_lock);
- old_physmem = physmem;
- zfs_write_limit_max = ptob(physmem) >> zfs_write_limit_shift;
- zfs_write_limit_inflated = MAX(zfs_write_limit_min,
- spa_get_asize(dp->dp_spa, zfs_write_limit_max));
- mutex_exit(&zfs_write_limit_lock);
- }
-
- /*
- * Attempt to keep the sync time consistent by adjusting the
- * amount of write traffic allowed into each transaction group.
- * Weight the throughput calculation towards the current value:
- * thru = 3/4 old_thru + 1/4 new_thru
- *
- * Note: write_time is in nanosecs while dp_throughput is expressed in
- * bytes per millisecond.
- */
- ASSERT(zfs_write_limit_min > 0);
- if (data_written > zfs_write_limit_min / 8 &&
- write_time > MSEC2NSEC(1)) {
- uint64_t throughput = data_written / NSEC2MSEC(write_time);
-
- if (dp->dp_throughput)
- dp->dp_throughput = throughput / 4 +
- 3 * dp->dp_throughput / 4;
- else
- dp->dp_throughput = throughput;
- dp->dp_write_limit = MIN(zfs_write_limit_inflated,
- MAX(zfs_write_limit_min,
- dp->dp_throughput * zfs_txg_synctime_ms));
- }
+ DTRACE_PROBE2(dsl_pool_sync__done, dsl_pool_t *dp, dp, uint64_t, txg);
}
void
dsl_pool_sync_done(dsl_pool_t *dp, uint64_t txg)
{
zilog_t *zilog;
- dsl_dataset_t *ds;
while (zilog = txg_list_remove(&dp->dp_dirty_zilogs, txg)) {
- ds = dmu_objset_ds(zilog->zl_os);
+ dsl_dataset_t *ds = dmu_objset_ds(zilog->zl_os);
zil_clean(zilog, txg);
ASSERT(!dmu_objset_is_dirty(zilog->zl_os, txg));
dmu_buf_rele(ds->ds_dbuf, zilog);
}
ASSERT(!dmu_objset_is_dirty(dp->dp_meta_objset, txg));
@@ -581,86 +620,51 @@
resv >>= 1;
return (space - resv);
}
-int
-dsl_pool_tempreserve_space(dsl_pool_t *dp, uint64_t space, dmu_tx_t *tx)
+boolean_t
+dsl_pool_need_dirty_delay(dsl_pool_t *dp)
{
- uint64_t reserved = 0;
- uint64_t write_limit = (zfs_write_limit_override ?
- zfs_write_limit_override : dp->dp_write_limit);
+ uint64_t delay_min_bytes =
+ zfs_dirty_data_max * zfs_delay_min_dirty_percent / 100;
+ boolean_t rv;
- if (zfs_no_write_throttle) {
- atomic_add_64(&dp->dp_tempreserved[tx->tx_txg & TXG_MASK],
- space);
- return (0);
- }
-
- /*
- * Check to see if we have exceeded the maximum allowed IO for
- * this transaction group. We can do this without locks since
- * a little slop here is ok. Note that we do the reserved check
- * with only half the requested reserve: this is because the
- * reserve requests are worst-case, and we really don't want to
- * throttle based off of worst-case estimates.
- */
- if (write_limit > 0) {
- reserved = dp->dp_space_towrite[tx->tx_txg & TXG_MASK]
- + dp->dp_tempreserved[tx->tx_txg & TXG_MASK] / 2;
-
- if (reserved && reserved > write_limit)
- return (SET_ERROR(ERESTART));
- }
-
- atomic_add_64(&dp->dp_tempreserved[tx->tx_txg & TXG_MASK], space);
-
- /*
- * If this transaction group is over 7/8ths capacity, delay
- * the caller 1 clock tick. This will slow down the "fill"
- * rate until the sync process can catch up with us.
- */
- if (reserved && reserved > (write_limit - (write_limit >> 3))) {
- txg_delay(dp, tx->tx_txg, zfs_throttle_delay,
- zfs_throttle_resolution);
- }
-
- return (0);
+ mutex_enter(&dp->dp_lock);
+ if (dp->dp_dirty_total > zfs_dirty_data_sync)
+ txg_kick(dp);
+ rv = (dp->dp_dirty_total > delay_min_bytes);
+ mutex_exit(&dp->dp_lock);
+ return (rv);
}
void
-dsl_pool_tempreserve_clear(dsl_pool_t *dp, int64_t space, dmu_tx_t *tx)
+dsl_pool_dirty_space(dsl_pool_t *dp, int64_t space, dmu_tx_t *tx)
{
- ASSERT(dp->dp_tempreserved[tx->tx_txg & TXG_MASK] >= space);
- atomic_add_64(&dp->dp_tempreserved[tx->tx_txg & TXG_MASK], -space);
-}
-
-void
-dsl_pool_memory_pressure(dsl_pool_t *dp)
-{
- uint64_t space_inuse = 0;
- int i;
-
- if (dp->dp_write_limit == zfs_write_limit_min)
- return;
-
- for (i = 0; i < TXG_SIZE; i++) {
- space_inuse += dp->dp_space_towrite[i];
- space_inuse += dp->dp_tempreserved[i];
+ if (space > 0) {
+ mutex_enter(&dp->dp_lock);
+ dp->dp_dirty_pertxg[tx->tx_txg & TXG_MASK] += space;
+ dsl_pool_dirty_delta(dp, space);
+ mutex_exit(&dp->dp_lock);
}
- dp->dp_write_limit = MAX(zfs_write_limit_min,
- MIN(dp->dp_write_limit, space_inuse / 4));
}
void
-dsl_pool_willuse_space(dsl_pool_t *dp, int64_t space, dmu_tx_t *tx)
-{
- if (space > 0) {
+dsl_pool_undirty_space(dsl_pool_t *dp, int64_t space, uint64_t txg) {
+ ASSERT3S(space, >=, 0);
+ if (space == 0)
+ return;
mutex_enter(&dp->dp_lock);
- dp->dp_space_towrite[tx->tx_txg & TXG_MASK] += space;
- mutex_exit(&dp->dp_lock);
+ if (dp->dp_dirty_pertxg[txg & TXG_MASK] < space) {
+ /* XXX writing something we didn't dirty? */
+ space = dp->dp_dirty_pertxg[txg & TXG_MASK];
}
+ ASSERT3U(dp->dp_dirty_pertxg[txg & TXG_MASK], >=, space);
+ dp->dp_dirty_pertxg[txg & TXG_MASK] -= space;
+ ASSERT3U(dp->dp_dirty_total, >=, space);
+ dsl_pool_dirty_delta(dp, -space);
+ mutex_exit(&dp->dp_lock);
}
/* ARGSUSED */
static int
upgrade_clones_cb(dsl_pool_t *dp, dsl_dataset_t *hds, void *arg)