diff --git a/cddl/contrib/opensolaris/cmd/ztest/ztest.c b/cddl/contrib/opensolaris/cmd/ztest/ztest.c
index be2f2cdd029f..65a4858b95d9 100644
--- a/cddl/contrib/opensolaris/cmd/ztest/ztest.c
+++ b/cddl/contrib/opensolaris/cmd/ztest/ztest.c
@@ -1544,7 +1544,7 @@ ztest_log_write(ztest_ds_t *zd, dmu_tx_t *tx, lr_write_t *lr)
 	if (zil_replaying(zd->zd_zilog, tx))
 		return;
 
-	if (lr->lr_length > ZIL_MAX_LOG_DATA)
+	if (lr->lr_length > zil_max_log_data(zd->zd_zilog))
 		write_state = WR_INDIRECT;
 
 	itx = zil_itx_create(TX_WRITE,
diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zil.h b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zil.h
index 040dfaa29a94..a27a9547ac43 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zil.h
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zil.h
@@ -20,7 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2012, 2017 by Delphix. All rights reserved.
+ * Copyright (c) 2012, 2018 by Delphix. All rights reserved.
  * Copyright (c) 2014 Integros [integros.com]
  */
 
@@ -452,6 +452,9 @@ extern void	zil_set_sync(zilog_t *zilog, uint64_t syncval);
 
 extern void	zil_set_logbias(zilog_t *zilog, uint64_t slogval);
 
+extern uint64_t	zil_max_copied_data(zilog_t *zilog);
+extern uint64_t	zil_max_log_data(zilog_t *zilog);
+
 extern int zil_replay_disable;
 
 #ifdef	__cplusplus
diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zil_impl.h b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zil_impl.h
index 5717c94a0c0c..a19ba970574f 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zil_impl.h
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zil_impl.h
@@ -20,7 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2012, 2017 by Delphix. All rights reserved.
+ * Copyright (c) 2012, 2018 by Delphix. All rights reserved.
  * Copyright (c) 2014 Integros [integros.com]
  */
 
@@ -208,6 +208,13 @@ struct zilog {
 	uint_t		zl_prev_rotor;	/* rotor for zl_prev[] */
 	txg_node_t	zl_dirty_link;	/* protected by dp_dirty_zilogs list */
 	uint64_t	zl_dirty_max_txg; /* highest txg used to dirty zilog */
+	/*
+	 * Max block size for this ZIL.  Note that this can not be changed
+	 * while the ZIL is in use because consumers (ZPL/zvol) need to take
+	 * this into account when deciding between WR_COPIED and WR_NEED_COPY
+	 * (see zil_max_copied_data()).
+	 */
+	uint64_t	zl_max_block_size;
 };
 
 typedef struct zil_bp_node {
@@ -215,28 +222,6 @@ typedef struct zil_bp_node {
 	avl_node_t	zn_node;
 } zil_bp_node_t;
 
-/*
- * Maximum amount of write data that can be put into single log block.
- */
-#define	ZIL_MAX_LOG_DATA (SPA_OLD_MAXBLOCKSIZE - sizeof (zil_chain_t) - \
-    sizeof (lr_write_t))
-#define	ZIL_MAX_COPIED_DATA \
-    ((SPA_OLD_MAXBLOCKSIZE - sizeof (zil_chain_t)) / 2 - sizeof (lr_write_t))
-
-/*
- * Maximum amount of log space we agree to waste to reduce number of
- * WR_NEED_COPY chunks to reduce zl_get_data() overhead (~12%).
- */
-#define	ZIL_MAX_WASTE_SPACE (ZIL_MAX_LOG_DATA / 8)
-
-/*
- * Maximum amount of write data for WR_COPIED.  Fall back to WR_NEED_COPY
- * as more space efficient if we can't fit at least two log records into
- * maximum sized log block.
- */
-#define	ZIL_MAX_COPIED_DATA ((SPA_OLD_MAXBLOCKSIZE - \
-    sizeof (zil_chain_t)) / 2 - sizeof (lr_write_t))
-
 #ifdef	__cplusplus
 }
 #endif
diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_log.c b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_log.c
index b1f98071a42a..c00c60a25ebb 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_log.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_log.c
@@ -20,7 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2015 by Delphix. All rights reserved.
+ * Copyright (c) 2015, 2018 by Delphix. All rights reserved.
  * Copyright (c) 2014 Integros [integros.com]
  */
 
@@ -493,7 +493,14 @@ zfs_log_write(zilog_t *zilog, dmu_tx_t *tx, int txtype,
 		itx_wr_state_t wr_state = write_state;
 		ssize_t len = resid;
 
-		if (wr_state == WR_COPIED && resid > ZIL_MAX_COPIED_DATA)
+		/*
+		 * A WR_COPIED record must fit entirely in one log block.
+		 * Large writes can use WR_NEED_COPY, which the ZIL will
+		 * split into multiple records across several log blocks
+		 * if necessary.
+		 */
+		if (wr_state == WR_COPIED &&
+		    resid > zil_max_copied_data(zilog))
 			wr_state = WR_NEED_COPY;
 		else if (wr_state == WR_INDIRECT)
 			len = MIN(blocksize - P2PHASE(off, blocksize), resid);
diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zil.c b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zil.c
index d8b7498a71a8..a2b9f9bbeaa0 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zil.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zil.c
@@ -1364,6 +1364,15 @@ struct {
     { 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
+ * zl_max_block_size instead.
+ */
+int zil_maxblocksize = SPA_OLD_MAXBLOCKSIZE;
+SYSCTL_INT(_vfs_zfs, OID_AUTO, zil_maxblocksize, CTLFLAG_RWTUN,
+    &zil_maxblocksize, 0, "Limit in bytes of ZIL log block size");
+
 /*
  * Start a log block write and advance to the next log block.
  * Calls are serialized.
@@ -1440,7 +1449,7 @@ zil_lwb_write_issue(zilog_t *zilog, lwb_t *lwb)
 	zil_blksz = zilog->zl_cur_used + sizeof (zil_chain_t);
 	for (i = 0; zil_blksz > zil_block_buckets[i].limit; i++)
 		continue;
-	zil_blksz = zil_block_buckets[i].blksz;
+	zil_blksz = MIN(zil_block_buckets[i].blksz, zilog->zl_max_block_size);
 	zilog->zl_prev_blks[zilog->zl_prev_rotor] = zil_blksz;
 	for (i = 0; i < ZIL_PREV_BLKS; i++)
 		zil_blksz = MAX(zil_blksz, zilog->zl_prev_blks[i]);
@@ -1497,13 +1506,47 @@ zil_lwb_write_issue(zilog_t *zilog, lwb_t *lwb)
 	return (nlwb);
 }
 
+/*
+ * Maximum amount of write data that can be put into single log block.
+ */
+uint64_t
+zil_max_log_data(zilog_t *zilog)
+{
+	return (zilog->zl_max_block_size -
+	    sizeof (zil_chain_t) - sizeof (lr_write_t));
+}
+
+/*
+ * Maximum amount of log space we agree to waste to reduce number of
+ * WR_NEED_COPY chunks to reduce zl_get_data() overhead (~12%).
+ */
+static inline uint64_t
+zil_max_waste_space(zilog_t *zilog)
+{
+	return (zil_max_log_data(zilog) / 8);
+}
+
+/*
+ * Maximum amount of write data for WR_COPIED.  For correctness, consumers
+ * must fall back to WR_NEED_COPY if we can't fit the entire record into one
+ * maximum sized log block, because each WR_COPIED record must fit in a
+ * single log block.  For space efficiency, we want to fit two records into a
+ * max-sized log block.
+ */
+uint64_t
+zil_max_copied_data(zilog_t *zilog)
+{
+	return ((zilog->zl_max_block_size - sizeof (zil_chain_t)) / 2 -
+	    sizeof (lr_write_t));
+}
+
 static lwb_t *
 zil_lwb_commit(zilog_t *zilog, itx_t *itx, lwb_t *lwb)
 {
 	lr_t *lrcb, *lrc;
 	lr_write_t *lrwb, *lrw;
 	char *lr_buf;
-	uint64_t dlen, dnow, lwb_sp, reclen, txg;
+	uint64_t dlen, dnow, lwb_sp, reclen, txg, max_log_data;
 
 	ASSERT(MUTEX_HELD(&zilog->zl_issuer_lock));
 	ASSERT3P(lwb, !=, NULL);
@@ -1552,15 +1595,27 @@ zil_lwb_commit(zilog_t *zilog, itx_t *itx, lwb_t *lwb)
 	 * For WR_NEED_COPY optimize layout for minimal number of chunks.
 	 */
 	lwb_sp = lwb->lwb_sz - lwb->lwb_nused;
+	max_log_data = zil_max_log_data(zilog);
 	if (reclen > lwb_sp || (reclen + dlen > lwb_sp &&
-	    lwb_sp < ZIL_MAX_WASTE_SPACE && (dlen % ZIL_MAX_LOG_DATA == 0 ||
-	    lwb_sp < reclen + dlen % ZIL_MAX_LOG_DATA))) {
+	    lwb_sp < zil_max_waste_space(zilog) &&
+	    (dlen % max_log_data == 0 ||
+	    lwb_sp < reclen + dlen % max_log_data))) {
 		lwb = zil_lwb_write_issue(zilog, lwb);
 		if (lwb == NULL)
 			return (NULL);
 		zil_lwb_write_open(zilog, lwb);
 		ASSERT(LWB_EMPTY(lwb));
 		lwb_sp = lwb->lwb_sz - lwb->lwb_nused;
+
+		/*
+		 * There must be enough space in the new, empty log block to
+		 * hold reclen.  For WR_COPIED, we need to fit the whole
+		 * record in one block, and reclen is the header size + the
+		 * data size. For WR_NEED_COPY, we can create multiple
+		 * records, splitting the data into multiple blocks, so we
+		 * only need to fit one word of data per block; in this case
+		 * reclen is just the header size (no data).
+		 */
 		ASSERT3U(reclen + MIN(dlen, sizeof (uint64_t)), <=, lwb_sp);
 	}
 
@@ -2992,6 +3047,7 @@ 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;
 
 	mutex_init(&zilog->zl_lock, NULL, MUTEX_DEFAULT, NULL);
 	mutex_init(&zilog->zl_issuer_lock, NULL, MUTEX_DEFAULT, NULL);
diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zvol.c b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zvol.c
index 822e795720e4..2397247a2244 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zvol.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zvol.c
@@ -1438,7 +1438,7 @@ zvol_log_write(zvol_state_t *zv, dmu_tx_t *tx, offset_t off, ssize_t resid,
 		itx_wr_state_t wr_state = write_state;
 		ssize_t len = resid;
 
-		if (wr_state == WR_COPIED && resid > ZIL_MAX_COPIED_DATA)
+		if (wr_state == WR_COPIED && resid > zil_max_copied_data(zilog))
 			wr_state = WR_NEED_COPY;
 		else if (wr_state == WR_INDIRECT)
 			len = MIN(blocksize - P2PHASE(off, blocksize), resid);