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.
@@ -217,15 +216,18 @@ 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 */
 	uint64_t	zl_replay_blks;	/* number of log blocks replayed */
 	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 */