Refactor Log Size Limit

Original Log Size Limit implementation blocked all writes in case of
limit reached until the TXG is committed and the log is freed.  It
caused huge delays and following speed spikes in application writes.

This implementation instead smoothly throttles writes, using exactly
the same mechanism as used for dirty data.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: jxdking <lostking2008@hotmail.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Issue openzfs#12284
Closes openzfs#13476
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
# Conflicts:
#	man/man4/zfs.4

include/sys/dmu_tx.h

@@ -124,8 +124,8 @@ typedef struct dmu_tx_stats {
 	kstat_named_t dmu_tx_dirty_throttle;
 	kstat_named_t dmu_tx_dirty_delay;
 	kstat_named_t dmu_tx_dirty_over_max;
-	kstat_named_t dmu_tx_wrlog_over_max;
 	kstat_named_t dmu_tx_dirty_frees_delay;
+	kstat_named_t dmu_tx_wrlog_delay;
 	kstat_named_t dmu_tx_quota;
 } dmu_tx_stats_t;
 

include/sys/dsl_pool.h

@@ -165,7 +165,7 @@ uint64_t dsl_pool_unreserved_space(dsl_pool_t *dp,
     zfs_space_check_t slop_policy);
 uint64_t dsl_pool_deferred_space(dsl_pool_t *dp);
 void dsl_pool_wrlog_count(dsl_pool_t *dp, int64_t size, uint64_t txg);
-boolean_t dsl_pool_wrlog_over_max(dsl_pool_t *dp);
+boolean_t dsl_pool_need_wrlog_delay(dsl_pool_t *dp);
 void dsl_pool_dirty_space(dsl_pool_t *dp, int64_t space, dmu_tx_t *tx);
 void dsl_pool_undirty_space(dsl_pool_t *dp, int64_t space, uint64_t txg);
 void dsl_free(dsl_pool_t *dp, uint64_t txg, const blkptr_t *bpp);