HBASE-23697 Document new RegionProcedureStore operation and migration

src/main/asciidoc/_chapters/architecture.adoc

@@ -693,9 +693,12 @@ See <<arch.catalog.meta>> for more information on the meta table.
 [[master.wal]]
 === MasterProcWAL
 
+_MasterProcWAL is replaced in hbase-2.3.0 by an alternate Procedure Store implementation; see
+<<in-master-procedure-store-region>>. This section pertains to hbase-2.0.0 through hbase-2.2.x_
+
 HMaster records administrative operations and their running states, such as the handling of a crashed server,
-table creation, and other DDLs, into its own WAL file. The WALs are stored under the MasterProcWALs
-directory. The Master WALs are not like RegionServer WALs. Keeping up the Master WAL allows
+table creation, and other DDLs, into a Procedure Store. The Procedure Store WALs are stored under the
+MasterProcWALs directory. The Master WALs are not like RegionServer WALs. Keeping up the Master WAL allows
 us run a state machine that is resilient across Master failures. For example, if a HMaster was in the
 middle of creating a table encounters an issue and fails, the next active HMaster can take up where
 the previous left off and carry the operation to completion. Since hbase-2.0.0, a

src/main/asciidoc/_chapters/upgrading.adoc

@@ -319,6 +319,26 @@ Quitting...
 The RegionServer Grouping feature has been reimplemented. See section
 <<migrating.rsgroup>> in <<ops_mgt>> for more details.
 
+[[upgrade2.3]]
+=== Upgrade from 2.0.x-2.2.x to 2.3+
+There is no special consideration upgrading to hbase-2.3.x from earlier versions. From 2.2.x, it should be
+rolling upgradeable. From 2.1.x or 2.0.x, you will need to clear the <<upgrade2.2>> hurdle first.
+
+[[in-master-procedure-store-region]]
+==== New In-Master Procedure Store
+
+Of note, HBase 2.3.0 changes the in-Master Procedure Store implementation. It was a dedicated custom store
+(see <<master.wal>>) to instead use a standard HBase Region (link:https://issues.apache.org/jira/browse/HBASE-23326).
+The migration from the old to new format is automatic run by the new 2.3.0 Master on startup. The old _MasterProcWALs_
+dir which hosted the old custom implementation files in _${hbase.rootdir}_ is deleted on successful
+migration. A new _MasterProc_ sub-directory replaces it to host the Store files and WALs for the new
+Procedure Store in-Master Region. The in-Master Region is unusual in that it writes to an
+alternate location at _${hbase.rootdir}/MasterProc_ rather than under _${hbase.rootdir}/data_ in the
+filesystem and the special Procedure Store in-Master Region is hidden from all clients other than the active
+Master itself. Otherwise, it is like any other with the Master process running flushes and compactions,
+archiving WALs when over-flushed, and so on. Its files are readable by standard Region and Store file
+tooling for triage and analysis as long as they are pointed to the appropriate location in the filesystem.
+
 [[upgrade2.2]]
 === Upgrade from 2.0 or 2.1 to 2.2+