ZIL: Improve next log block size prediction

Track history in context of bursts, not individual log blocks. It
allows to not blow away all the history by single large burst of
many block, and same time allows optimizations covering multiple
blocks in a burst and even predicted following burst.  For each
burst account its optimal block size and minimal first block size.
Use that statistics from the last 8 bursts to predict first block
size of the next burst.

Remove predefined set of block sizes. Allocate any size we see fit,
multiple of 4KB, as required by ZIL now.  With compression enabled
by default, ZFS already writes pretty random block sizes, so this
should not surprise space allocator any more.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #15635

include/sys/zil_impl.h

@@ -182,7 +182,6 @@ typedef struct zil_vdev_node {
 } zil_vdev_node_t;
 
 #define	ZIL_BURSTS 8
-#define	ZIL_PREV_BLKS 16
 
 /*
  * Stable storage intent log management structure.  One per dataset.
@@ -217,7 +216,9 @@ struct zilog {
 	uint64_t	zl_parse_lr_count; /* number of log records parsed */
 	itxg_t		zl_itxg[TXG_SIZE]; /* intent log txg chains */
 	list_t		zl_itx_commit_list; /* itx list to be committed */
-	uint64_t	zl_cur_used;	/* current commit log size used */
+	uint64_t	zl_cur_size;	/* current burst full size */
+	uint64_t	zl_cur_left;	/* current burst remaining size */
+	uint64_t	zl_cur_max;	/* biggest record in current burst */
 	list_t		zl_lwb_list;	/* in-flight log write list */
 	avl_tree_t	zl_bp_tree;	/* track bps during log parse */
 	clock_t		zl_replay_time;	/* lbolt of when replay started */
@@ -225,7 +226,8 @@ struct zilog {
 	zil_header_t	zl_old_header;	/* debugging aid */
 	uint_t		zl_parallel;	/* workload is multi-threaded */
 	uint_t		zl_prev_rotor;	/* rotor for zl_prev[] */
-	uint_t		zl_prev_blks[ZIL_PREV_BLKS]; /* size - sector rounded */
+	uint_t		zl_prev_opt[ZIL_BURSTS]; /* optimal block size */
+	uint_t		zl_prev_min[ZIL_BURSTS]; /* minimal first block size */
 	txg_node_t	zl_dirty_link;	/* protected by dp_dirty_zilogs list */
 	uint64_t	zl_dirty_max_txg; /* highest txg used to dirty zilog */
 

module/zfs/zil.c

@@ -144,6 +144,7 @@ static kmem_cache_t *zil_zcw_cache;
 
 static void zil_lwb_commit(zilog_t *zilog, lwb_t *lwb, itx_t *itx);
 static itx_t *zil_itx_clone(itx_t *oitx);
+static uint64_t zil_max_waste_space(zilog_t *zilog);
 
 static int
 zil_bp_compare(const void *x1, const void *x2)
@@ -1710,24 +1711,6 @@ zil_lwb_write_open(zilog_t *zilog, lwb_t *lwb)
 	mutex_exit(&zilog->zl_lock);
 }
 
-/*
- * Define a limited set of intent log block sizes.
- *
- * These must be a multiple of 4KB. Note only the amount used (again
- * aligned to 4KB) actually gets written. However, we can't always just
- * allocate SPA_OLD_MAXBLOCKSIZE as the slog space could be exhausted.
- */
-static const struct {
-	uint64_t	limit;
-	uint64_t	blksz;
-} zil_block_buckets[] = {
-	{ 4096,		4096 },			/* non TX_WRITE */
-	{ 8192 + 4096,	8192 + 4096 },		/* database */
-	{ 32768 + 4096,	32768 + 4096 },		/* NFS writes */
-	{ 65536 + 4096,	65536 + 4096 },		/* 64KB writes */
-	{ UINT64_MAX,	SPA_OLD_MAXBLOCKSIZE},	/* > 128KB writes */
-};
-
 /*
  * Maximum block size used by the ZIL.  This is picked up when the ZIL is
  * initialized.  Otherwise this should not be used directly; see
@@ -1735,6 +1718,91 @@ static const struct {
  */
 static uint_t zil_maxblocksize = SPA_OLD_MAXBLOCKSIZE;
 
+/*
+ * Plan splitting of the provided burst size between several blocks.
+ */
+static uint_t
+zil_lwb_plan(zilog_t *zilog, uint64_t size, uint_t *minsize)
+{
+	uint_t md = zilog->zl_max_block_size - sizeof (zil_chain_t);
+
+	if (size <= md) {
+		/*
+		 * Small bursts are written as-is in one block.
+		 */
+		*minsize = size;
+		return (size);
+	} else if (size > 8 * md) {
+		/*
+		 * Big bursts use maximum blocks.  The first block size
+		 * is hard to predict, but it does not really matter.
+		 */
+		*minsize = 0;
+		return (md);
+	}
+
+	/*
+	 * Medium bursts try to divide evenly to better utilize several SLOG
+	 * VDEVs.  The first block size we predict assuming the worst case of
+	 * maxing out others.  Fall back to using maximum blocks if due to
+	 * large records or wasted space we can not predict anything better.
+	 */
+	uint_t s = size;
+	uint_t n = DIV_ROUND_UP(s, md - sizeof (lr_write_t));
+	uint_t chunk = DIV_ROUND_UP(s, n);
+	uint_t waste = zil_max_waste_space(zilog);
+	waste = MAX(waste, zilog->zl_cur_max);
+	if (chunk <= md - waste) {
+		*minsize = MAX(s - (md - waste) * (n - 1), waste);
+		return (chunk);
+	} else {
+		*minsize = 0;
+		return (md);
+	}
+}
+
+/*
+ * Try to predict next block size based on previous history.  Make prediction
+ * sufficient for 7 of 8 previous bursts.  Don't try to save if the saving is
+ * less then 50%, extra writes may cost more, but we don't want single spike
+ * to badly affect our predictions.
+ */
+static uint_t
+zil_lwb_predict(zilog_t *zilog)
+{
+	uint_t m, o;
+
+	/* If we are in the middle of a burst, take it into account also. */
+	if (zilog->zl_cur_size > 0) {
+		o = zil_lwb_plan(zilog, zilog->zl_cur_size, &m);
+	} else {
+		o = UINT_MAX;
+		m = 0;
+	}
+
+	/* Find minimum optimal size.  We don't need to go below that. */
+	for (int i = 0; i < ZIL_BURSTS; i++)
+		o = MIN(o, zilog->zl_prev_opt[i]);
+
+	/* Find two biggest minimal first block sizes above the optimal. */
+	uint_t m1 = MAX(m, o), m2 = o;
+	for (int i = 0; i < ZIL_BURSTS; i++) {
+		m = zilog->zl_prev_min[i];
+		if (m >= m1) {
+			m2 = m1;
+			m1 = m;
+		} else if (m > m2) {
+			m2 = m;
+		}
+	}
+
+	/*
+	 * If second minimum size gives 50% saving -- use it.  It may cost us
+	 * one additional write later, but the space saving is just too big.
+	 */
+	return ((m1 < m2 * 2) ? m1 : m2);
+}
+
 /*
  * Close the log block for being issued and allocate the next one.
  * Has to be called under zl_issuer_lock to chain more lwbs.
@@ -1742,7 +1810,7 @@ static uint_t zil_maxblocksize = SPA_OLD_MAXBLOCKSIZE;
 static lwb_t *
 zil_lwb_write_close(zilog_t *zilog, lwb_t *lwb, lwb_state_t state)
 {
-	int i;
+	uint64_t blksz, plan, plan2;
 
 	ASSERT(MUTEX_HELD(&zilog->zl_issuer_lock));
 	ASSERT3S(lwb->lwb_state, ==, LWB_STATE_OPENED);
@@ -1758,33 +1826,39 @@ zil_lwb_write_close(zilog_t *zilog, lwb_t *lwb, lwb_state_t state)
 
 	/*
 	 * Log blocks are pre-allocated.  Here we select the size of the next
-	 * block, based on size used in the last block.
+	 * block, based on what's left of this burst and the previous history.
-	 * - first find the smallest bucket that will fit the block from a
+	 * While we try to only write used part of the block, we can't just
-	 *   limited set of block sizes. This is because it's faster to write
+	 * always allocate the maximum block size because we can exhaust all
-	 *   blocks allocated from the same metaslab as they are adjacent or
+	 * available pool log space, so we try to be reasonable.
-	 *   close.
-	 * - next find the maximum from the new suggested size and an array of
-	 *   previous sizes. This lessens a picket fence effect of wrongly
-	 *   guessing the size if we have a stream of say 2k, 64k, 2k, 64k
-	 *   requests.
-	 *
-	 * Note we only write what is used, but we can't just allocate
-	 * the maximum block size because we can exhaust the available
-	 * pool log space.
 	 */
-	uint64_t zil_blksz = zilog->zl_cur_used + sizeof (zil_chain_t);
+	if (zilog->zl_cur_left > 0) {
-	for (i = 0; zil_blksz > zil_block_buckets[i].limit; i++)
+		/*
-		continue;
+		 * We are in the middle of a burst and know how much is left.
-	zil_blksz = MIN(zil_block_buckets[i].blksz, zilog->zl_max_block_size);
+		 * But if workload is multi-threaded there may be more soon.
-	zilog->zl_prev_blks[zilog->zl_prev_rotor] = zil_blksz;
+		 * Try to predict what can it be and plan for the worst case.
-	for (i = 0; i < ZIL_PREV_BLKS; i++)
+		 */
-		zil_blksz = MAX(zil_blksz, zilog->zl_prev_blks[i]);
+		uint_t m;
-	DTRACE_PROBE3(zil__block__size, zilog_t *, zilog,
+		plan = zil_lwb_plan(zilog, zilog->zl_cur_left, &m);
-	    uint64_t, zil_blksz,
+		if (zilog->zl_parallel) {
-	    uint64_t, zilog->zl_prev_blks[zilog->zl_prev_rotor]);
+			plan2 = zil_lwb_plan(zilog, zilog->zl_cur_left +
-	zilog->zl_prev_rotor = (zilog->zl_prev_rotor + 1) & (ZIL_PREV_BLKS - 1);
+			    zil_lwb_predict(zilog), &m);
+			if (plan < plan2)
+				plan = plan2;
+		}
+	} else {
+		/*
+		 * The previous burst is done and we can only predict what
+		 * will come next.
+		 */
+		plan = zil_lwb_predict(zilog);
+	}
+	blksz = plan + sizeof (zil_chain_t);
+	blksz = P2ROUNDUP_TYPED(blksz, ZIL_MIN_BLKSZ, uint64_t);
+	blksz = MIN(blksz, zilog->zl_max_block_size);
+	DTRACE_PROBE3(zil__block__size, zilog_t *, zilog, uint64_t, blksz,
+	    uint64_t, plan);
 
-	return (zil_alloc_lwb(zilog, zil_blksz, NULL, 0, 0, state));
+	return (zil_alloc_lwb(zilog, blksz, NULL, 0, 0, state));
 }
 
 /*
@@ -1835,7 +1909,7 @@ next_lwb:
 	int wsz = lwb->lwb_sz;
 	if (lwb->lwb_error == 0) {
 		abd_t *lwb_abd = abd_get_from_buf(lwb->lwb_buf, lwb->lwb_sz);
-		if (!lwb->lwb_slog || zilog->zl_cur_used <= zil_slog_bulk)
+		if (!lwb->lwb_slog || zilog->zl_cur_size <= zil_slog_bulk)
 			prio = ZIO_PRIORITY_SYNC_WRITE;
 		else
 			prio = ZIO_PRIORITY_ASYNC_WRITE;
@@ -1996,6 +2070,42 @@ zil_max_copied_data(zilog_t *zilog)
 	return (MIN(max_data, zil_maxcopied));
 }
 
+static uint64_t
+zil_itx_record_size(itx_t *itx)
+{
+	lr_t *lr = &itx->itx_lr;
+
+	if (lr->lrc_txtype == TX_COMMIT)
+		return (0);
+	ASSERT3U(lr->lrc_reclen, >=, sizeof (lr_t));
+	return (lr->lrc_reclen);
+}
+
+static uint64_t
+zil_itx_data_size(itx_t *itx)
+{
+	lr_t *lr = &itx->itx_lr;
+	lr_write_t *lrw = (lr_write_t *)lr;
+
+	if (lr->lrc_txtype == TX_WRITE && itx->itx_wr_state == WR_NEED_COPY) {
+		ASSERT3U(lr->lrc_reclen, ==, sizeof (lr_write_t));
+		return (P2ROUNDUP_TYPED(lrw->lr_length, sizeof (uint64_t),
+		    uint64_t));
+	}
+	return (0);
+}
+
+static uint64_t
+zil_itx_full_size(itx_t *itx)
+{
+	lr_t *lr = &itx->itx_lr;
+
+	if (lr->lrc_txtype == TX_COMMIT)
+		return (0);
+	ASSERT3U(lr->lrc_reclen, >=, sizeof (lr_t));
+	return (lr->lrc_reclen + zil_itx_data_size(itx));
+}
+
 /*
  * Estimate space needed in the lwb for the itx.  Allocate more lwbs or
  * split the itx as needed, but don't touch the actual transaction data.
@@ -2038,16 +2148,9 @@ zil_lwb_assign(zilog_t *zilog, lwb_t *lwb, itx_t *itx, list_t *ilwbs)
 	}
 
 	reclen = lr->lrc_reclen;
-	if (lr->lrc_txtype == TX_WRITE && itx->itx_wr_state == WR_NEED_COPY) {
-		ASSERT3U(reclen, ==, sizeof (lr_write_t));
-		dlen = P2ROUNDUP_TYPED(
-		    lrw->lr_length, sizeof (uint64_t), uint64_t);
-	} else {
 	ASSERT3U(reclen, >=, sizeof (lr_t));
-		dlen = 0;
-	}
 	ASSERT3U(reclen, <=, zil_max_log_data(zilog, 0));
-	zilog->zl_cur_used += (reclen + dlen);
+	dlen = zil_itx_data_size(itx);
 
 cont:
 	/*
@@ -2088,6 +2191,7 @@ cont:
 		clrw->lr_length = dnow;
 		lrw->lr_offset += dnow;
 		lrw->lr_length -= dnow;
+		zilog->zl_cur_left -= dnow;
 	} else {
 		citx = itx;
 		clr = lr;
@@ -2109,10 +2213,8 @@ cont:
 	list_insert_tail(&lwb->lwb_itxs, citx);
 
 	dlen -= dnow;
-	if (dlen > 0) {
+	if (dlen > 0)
-		zilog->zl_cur_used += reclen;
 		goto cont;
-	}
 
 	if (lr->lrc_txtype == TX_WRITE &&
 	    lr->lrc_txg > spa_freeze_txg(zilog->zl_spa))
@@ -2139,13 +2241,8 @@ zil_lwb_commit(zilog_t *zilog, lwb_t *lwb, itx_t *itx)
 	if (lr->lrc_txtype == TX_COMMIT)
 		return;
 
-	if (lr->lrc_txtype == TX_WRITE && itx->itx_wr_state == WR_NEED_COPY) {
-		dlen = P2ROUNDUP_TYPED(
-		    lrw->lr_length, sizeof (uint64_t), uint64_t);
-	} else {
-		dlen = 0;
-	}
 	reclen = lr->lrc_reclen;
+	dlen = zil_itx_data_size(itx);
 	ASSERT3U(reclen + dlen, <=, lwb->lwb_nused - lwb->lwb_nfilled);
 
 	lr_buf = lwb->lwb_buf + lwb->lwb_nfilled;
@@ -2576,6 +2673,7 @@ zil_get_commit_list(zilog_t *zilog)
 		ASSERT(zilog_is_dirty_in_txg(zilog, txg) ||
 		    spa_freeze_txg(zilog->zl_spa) != UINT64_MAX);
 		list_t *sync_list = &itxg->itxg_itxs->i_sync_list;
+		itx_t *itx = NULL;
 		if (unlikely(zilog->zl_suspend > 0)) {
 			/*
 			 * ZIL was just suspended, but we lost the race.
@@ -2585,10 +2683,20 @@ zil_get_commit_list(zilog_t *zilog)
 			if (!list_is_empty(sync_list))
 				wtxg = MAX(wtxg, txg);
 		} else {
+			itx = list_head(sync_list);
 			list_move_tail(commit_list, sync_list);
 		}
 
 		mutex_exit(&itxg->itxg_lock);
+
+		while (itx != NULL) {
+			uint64_t s = zil_itx_full_size(itx);
+			zilog->zl_cur_size += s;
+			zilog->zl_cur_left += s;
+			s = zil_itx_record_size(itx);
+			zilog->zl_cur_max = MAX(zilog->zl_cur_max, s);
+			itx = list_next(commit_list, itx);
+		}
 	}
 	return (wtxg);
 }
@@ -2728,13 +2836,20 @@ static void
 zil_burst_done(zilog_t *zilog)
 {
 	if (!list_is_empty(&zilog->zl_itx_commit_list) ||
-	    zilog->zl_cur_used == 0)
+	    zilog->zl_cur_size == 0)
 		return;
 
 	if (zilog->zl_parallel)
 		zilog->zl_parallel--;
 
-	zilog->zl_cur_used = 0;
+	uint_t r = (zilog->zl_prev_rotor + 1) & (ZIL_BURSTS - 1);
+	zilog->zl_prev_rotor = r;
+	zilog->zl_prev_opt[r] = zil_lwb_plan(zilog, zilog->zl_cur_size,
+	    &zilog->zl_prev_min[r]);
+
+	zilog->zl_cur_size = 0;
+	zilog->zl_cur_max = 0;
+	zilog->zl_cur_left = 0;
 }
 
 /*
@@ -2867,6 +2982,8 @@ zil_process_commit_list(zilog_t *zilog, zil_commit_waiter_t *zcw, list_t *ilwbs)
 					 * itx list to somebody else who care.
 					 */
 					zilog->zl_parallel = ZIL_BURSTS;
+					zilog->zl_cur_left -=
+					    zil_itx_full_size(itx);
 					break;
 				}
 			} else {
@@ -2876,8 +2993,10 @@ zil_process_commit_list(zilog_t *zilog, zil_commit_waiter_t *zcw, list_t *ilwbs)
 				}
 				list_insert_tail(&nolwb_itxs, itx);
 			}
+			zilog->zl_cur_left -= zil_itx_full_size(itx);
 		} else {
 			ASSERT3S(lrc->lrc_txtype, !=, TX_COMMIT);
+			zilog->zl_cur_left -= zil_itx_full_size(itx);
 			zil_itx_destroy(itx);
 		}
 	}
@@ -2960,9 +3079,9 @@ zil_process_commit_list(zilog_t *zilog, zil_commit_waiter_t *zcw, list_t *ilwbs)
 		 * of each individual itx.
 		 */
 		if (lwb->lwb_state == LWB_STATE_OPENED && !zilog->zl_parallel) {
+			zil_burst_done(zilog);
 			list_insert_tail(ilwbs, lwb);
 			lwb = zil_lwb_write_close(zilog, lwb, LWB_STATE_NEW);
-			zil_burst_done(zilog);
 			if (lwb == NULL) {
 				while ((lwb = list_remove_head(ilwbs)) != NULL)
 					zil_lwb_write_issue(zilog, lwb);
@@ -3120,12 +3239,11 @@ zil_commit_waiter_timeout(zilog_t *zilog, zil_commit_waiter_t *zcw)
 	 * since we've reached the commit waiter's timeout and it still
 	 * hasn't been issued.
 	 */
+	zil_burst_done(zilog);
 	lwb_t *nlwb = zil_lwb_write_close(zilog, lwb, LWB_STATE_NEW);
 
 	ASSERT3S(lwb->lwb_state, ==, LWB_STATE_CLOSED);
 
-	zil_burst_done(zilog);
-
 	if (nlwb == NULL) {
 		/*
 		 * When zil_lwb_write_close() returns NULL, this
@@ -3720,7 +3838,9 @@ zil_alloc(objset_t *os, zil_header_t *zh_phys)
 	zilog->zl_dirty_max_txg = 0;
 	zilog->zl_last_lwb_opened = NULL;
 	zilog->zl_last_lwb_latency = 0;
-	zilog->zl_max_block_size = zil_maxblocksize;
+	zilog->zl_max_block_size = MIN(MAX(P2ALIGN_TYPED(zil_maxblocksize,
+	    ZIL_MIN_BLKSZ, uint64_t), ZIL_MIN_BLKSZ),
+	    spa_maxblocksize(dmu_objset_spa(os)));
 
 	mutex_init(&zilog->zl_lock, NULL, MUTEX_DEFAULT, NULL);
 	mutex_init(&zilog->zl_issuer_lock, NULL, MUTEX_DEFAULT, NULL);
@@ -3740,6 +3860,11 @@ zil_alloc(objset_t *os, zil_header_t *zh_phys)
 	cv_init(&zilog->zl_cv_suspend, NULL, CV_DEFAULT, NULL);
 	cv_init(&zilog->zl_lwb_io_cv, NULL, CV_DEFAULT, NULL);
 
+	for (int i = 0; i < ZIL_BURSTS; i++) {
+		zilog->zl_prev_opt[i] = zilog->zl_max_block_size -
+		    sizeof (zil_chain_t);
+	}
+
 	return (zilog);
 }