diff --git a/cmd/ztest/ztest.c b/cmd/ztest/ztest.c
index 3bf840d88..49833a423 100644
--- a/cmd/ztest/ztest.c
+++ b/cmd/ztest/ztest.c
@@ -1692,7 +1692,7 @@ 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/include/sys/zil.h b/include/sys/zil.h
index fb7b38a06..cfa5e3995 100644
--- a/include/sys/zil.h
+++ b/include/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.
  */
 
 /* Portions Copyright 2010 Robert Milkowski */
@@ -515,6 +515,9 @@ 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/include/sys/zil_impl.h b/include/sys/zil_impl.h
index 174fef334..d2f401865 100644
--- a/include/sys/zil_impl.h
+++ b/include/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.
  */
 
 /* Portions Copyright 2010 Robert Milkowski */
@@ -209,6 +209,13 @@ 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 {
@@ -216,26 +223,6 @@ 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))
-
-/*
- * 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/man/man5/zfs-module-parameters.5 b/man/man5/zfs-module-parameters.5
index 8ad3ce466..fa83b44bd 100644
--- a/man/man5/zfs-module-parameters.5
+++ b/man/man5/zfs-module-parameters.5
@@ -2890,6 +2890,18 @@ value of 100% will create a maximum of one thread per cpu.
 Default value: \fB100\fR%.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzil_maxblocksize\fR (int)
+.ad
+.RS 12n
+This sets the maximum block size used by the ZIL.  On very fragmented pools,
+lowering this (typically to 36KB) can improve performance.
+.sp
+Default value: \fB131072\fR (128KB).
+.RE
+
 .sp
 .ne 2
 .na
diff --git a/module/zfs/zfs_log.c b/module/zfs/zfs_log.c
index 5966b7612..622ce08ac 100644
--- a/module/zfs/zfs_log.c
+++ b/module/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.
  */
 
 
@@ -541,7 +541,14 @@ 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/module/zfs/zil.c b/module/zfs/zil.c
index 5249a0e93..98678aa44 100644
--- a/module/zfs/zil.c
+++ b/module/zfs/zil.c
@@ -1423,6 +1423,13 @@ uint64_t zil_block_buckets[] = {
     UINT64_MAX
 };
 
+/*
+ * 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;
+
 /*
  * Start a log block write and advance to the next log block.
  * Calls are serialized.
@@ -1499,9 +1506,7 @@ 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]; i++)
 		continue;
-	zil_blksz = zil_block_buckets[i];
-	if (zil_blksz == UINT64_MAX)
-		zil_blksz = SPA_OLD_MAXBLOCKSIZE;
+	zil_blksz = MIN(zil_block_buckets[i], 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]);
@@ -1562,13 +1567,47 @@ 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);
@@ -1617,15 +1656,27 @@ cont:
 	 * 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);
 	}
 
@@ -3114,6 +3165,7 @@ 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);
@@ -3627,5 +3679,8 @@ MODULE_PARM_DESC(zil_nocacheflush, "Disable ZIL cache flushes");
 
 module_param(zil_slog_bulk, ulong, 0644);
 MODULE_PARM_DESC(zil_slog_bulk, "Limit in bytes slog sync writes per commit");
+
+module_param(zil_maxblocksize, int, 0644);
+MODULE_PARM_DESC(zil_maxblocksize, "Limit in bytes of ZIL log block size");
 /* END CSTYLED */
 #endif
diff --git a/module/zfs/zvol.c b/module/zfs/zvol.c
index 7c7500dba..f74eb28ae 100644
--- a/module/zfs/zvol.c
+++ b/module/zfs/zvol.c
@@ -684,7 +684,7 @@ zvol_log_write(zvol_state_t *zv, dmu_tx_t *tx, uint64_t offset,
 		itx_wr_state_t wr_state = write_state;
 		ssize_t len = size;
 
-		if (wr_state == WR_COPIED && size > ZIL_MAX_COPIED_DATA)
+		if (wr_state == WR_COPIED && size > zil_max_copied_data(zilog))
 			wr_state = WR_NEED_COPY;
 		else if (wr_state == WR_INDIRECT)
 			len = MIN(blocksize - P2PHASE(offset, blocksize), size);