Allow opt-in of zvol blocks in special class

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Signed-off-by: Don Brady <dev.fs.zfs@gmail.com>
Closes #14876

man/man7/zfsprops.7

@@ -1281,12 +1281,12 @@ This feature must be enabled to be used
 .Pc .
 .It Sy special_small_blocks Ns = Ns Ar size
 This value represents the threshold block size for including small file
-blocks into the special allocation class.
+or zvol blocks into the special allocation class.
 Blocks smaller than or equal to this
 value will be assigned to the special allocation class while greater blocks
 will be assigned to the regular class.
 Valid values are zero or a power of two from 512 up to 1048576 (1 MiB).
-The default size is 0 which means no small file blocks
+The default size is 0 which means no small file or zvol blocks
 will be allocated in the special class.
 .Pp
 Before setting this property, a special class vdev must be added to the

man/man7/zpoolconcepts.7

@@ -488,7 +488,8 @@ current state of the pool won't be scanned during a scrub.
 Allocations in the special class are dedicated to specific block types.
 By default, this includes all metadata, the indirect blocks of user data, and
 any deduplication tables.
-The class can also be provisioned to accept small file blocks.
+The class can also be provisioned to accept small file blocks or zvol blocks
+on a per dataset granularity.
 .Pp
 A pool must always have at least one normal
 .Pq non- Ns Sy dedup Ns /- Ns Sy special
@@ -503,7 +504,7 @@ Deduplication tables can be excluded from the special class by unsetting the
 .Sy zfs_ddt_data_is_special
 ZFS module parameter.
 .Pp
-Inclusion of small file blocks in the special class is opt-in.
+Inclusion of small file or zvol blocks in the special class is opt-in.
 Each dataset can control the size of small file blocks allowed
 in the special class by setting the
 .Sy special_small_blocks