Workload Tuning: actualize volblocksize

It was a little bit obsolete and obscure.

Signed-off-by: George Melikov <mail@gmelikov.ru>

docs/Performance and Tuning/Workload Tuning.rst

@@ -203,7 +203,7 @@ ZFS datasets use an internal recordsize of 128KB by default. The dataset
 recordsize is the basic unit of data used for internal copy-on-write on
 files. Partial record writes require that data be read from either ARC
 (cheap) or disk (expensive). recordsize can be set to any power of 2
-from 512 bytes to 128 kilobytes. Software that writes in fixed record
+from 512 bytes to 1 megabyte. Software that writes in fixed record
 sizes (e.g. databases) will benefit from the use of a matching
 recordsize.
 
@@ -233,10 +233,32 @@ must support the large_blocks feature.
 zvol volblocksize
 ~~~~~~~~~~~~~~~~~
 
-Zvols have a volblocksize property that is analogous to record size. The
-default size is 8KB, which is the size of a page on the SPARC
-architecture. Workloads that use smaller sized IOs (such as swap on x86
-which use 4096-byte pages) will benefit from a smaller volblocksize.
+Zvols have a ``volblocksize`` property that is analogous to ``recordsize``.
+Current default (16KB since v2.2) balances the metadata overhead, compression
+opportunities and decent space efficiency on majority of pool configurations
+due to 4KB disk physical block rounding (especially on RAIDZ and DRAID),
+while incurring some write amplification on guest FSes that run with smaller
+block sizes [#VOLBLOCKSIZE]_.
+
+Users are advised to test their scenarios and see whether the ``volblocksize``
+needs to be changed to favor one or the other:
+
+- sector alignment of guest FS is crucial
+- most of guest FSes use default block size of 4-8KB, so:
+
+  - Larger ``volblocksize`` can help with mostly sequential workloads and
+    will gain a compression efficiency
+
+  - Smaller ``volblocksize`` can help with random workloads and minimize
+    IO amplification, but will use more metadata
+    (e.g. more small IOs will be generated by ZFS) and may have worse
+    space efficiency (especially on RAIDZ and DRAID)
+
+  - It's meaningless to set ``volblocksize`` less than guest FS's block size
+    or :ref:`ashift <alignment_shift_ashift>`
+
+  - See :ref:`Dataset recordsize <dataset_recordsize>`
+    for additional information
 
 Deduplication
 ~~~~~~~~~~~~~
@@ -760,3 +782,4 @@ AIO should be used to maximize IOPS when using files for guest storage.
 .. [#sqlite_ps_change] <https://www.sqlite.org/pgszchng2016.html>
 .. [#FS_CASEFOLD_FL] <https://github.com/openzfs/zfs/pull/13790>
 .. [#init_on_alloc] <https://patchwork.kernel.org/project/linux-security-module/patch/20190626121943.131390-2-glider@google.com/#22731857>
+.. [#VOLBLOCKSIZE] <https://github.com/openzfs/zfs/pull/12406>