Illumos 5138 - add tunable for maximum number of blocks freed in one txg

Reviewed by: Adam Leventhal <[email protected]>
Reviewed by: Mattew Ahrens <[email protected]>
Reviewed by: Josef 'Jeff' Sipek <[email protected]>
Reviewed by: Richard Elling <[email protected]>
Reviewed by: George Wilson <[email protected]>
Approved by: Dan McDonald <[email protected]>

References:
  https://www.illumos.org/issues/5138
  illumos/illumos-gate@af3465d

Porting notes:

Because support for exposing a uint64_t parameter wasn't added
until v3.17-rc1 the zfs_free_max_blocks variable has been declared
as a unsigned long.  This is already far larger than required and
it allows us to avoid additional autoconf compatibility code.

The default value has been set to 100,000 on Linux instead of
ULONG_MAX which is used on Illumos.  This was done to limit the
number of outstanding IOs in the system when snapshots are destroyed.
This helps ensure individual TXG sync times are kept reasonable and
memory isn't wasted managing a huge backlog of outstanding IOs.

Ported by: Turbo Fredriksson <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #2675
Closes #2581

man/man5/zfs-module-parameters.5

@@ -620,6 +620,17 @@ Start syncing out a transaction group if there is at least this much dirty data.
 Default value: \fB67,108,864\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_free_max_blocks\fR (ulong)
+.ad
+.RS 12n
+Maximum number of blocks freed in a single txg.
+.sp
+Default value: \fB100,000\fR.
+.RE
+
 .sp
 .ne 2
 .na

module/zfs/dsl_scan.c

@@ -69,6 +69,8 @@ int zfs_no_scrub_io = B_FALSE; /* set to disable scrub i/o */
 int zfs_no_scrub_prefetch = B_FALSE; /* set to disable scrub prefetch */
 enum ddt_class zfs_scrub_ddt_class_max = DDT_CLASS_DUPLICATE;
 int dsl_scan_delay_completion = B_FALSE; /* set to delay scan completion */
+/* max number of blocks to free in a single TXG */
+ulong zfs_free_max_blocks = 100000;
 
 #define	DSL_SCAN_IS_SCRUB_RESILVER(scn) \
 	((scn)->scn_phys.scn_func == POOL_SCAN_SCRUB || \
@@ -1370,6 +1372,9 @@ dsl_scan_free_should_pause(dsl_scan_t *scn)
 	if (zfs_recover)
 		return (B_FALSE);
 
+	if (scn->scn_visited_this_txg >= zfs_free_max_blocks)
+		return (B_TRUE);
+
 	elapsed_nanosecs = gethrtime() - scn->scn_sync_start_time;
 	return (elapsed_nanosecs / NANOSEC > zfs_txg_timeout ||
 	    (NSEC2MSEC(elapsed_nanosecs) > zfs_free_min_time_ms &&
@@ -1868,4 +1873,7 @@ MODULE_PARM_DESC(zfs_no_scrub_io, "Set to disable scrub I/O");
 
 module_param(zfs_no_scrub_prefetch, int, 0644);
 MODULE_PARM_DESC(zfs_no_scrub_prefetch, "Set to disable scrub prefetching");
+
+module_param(zfs_free_max_blocks, ulong, 0644);
+MODULE_PARM_DESC(zfs_free_max_blocks, "Max number of blocks freed in one txg");
 #endif