Doc - 3.3 to 3.4 notes about upgrading (#7513)

Documentation/Books/Manual/Upgrading/VersionSpecific/Upgrading34.md

@@ -1,6 +1,32 @@
 Upgrading to ArangoDB 3.4
 =========================
 
-Please be sure that you have checked the list of [incompatible changes in 3.4](../../ReleaseNotes/UpgradingChanges34.md)
+Please be sure that you have checked the list of
+[incompatible changes in 3.4](../../ReleaseNotes/UpgradingChanges34.md)
 before upgrading.
 
+Upon upgrading from 3.3 to 3.4, the following storage engine-specific data conversion tasks
+will be executed:
+
+- **MMFiles storage engine:**
+
+  All collection datafiles will be rewritten into a
+  new data format. This data format is required in order to support using the collections
+  in [ArangoSearch Views](../../Views/ArangoSearch/README.md) introduced in ArangoDB 3.4. 
+
+  The conversion will read each datafile sequentially and write out a new datafile in the
+  new format sequentially. This means the disk will be involved, but both reading and
+  writing are done in a sequential fashion. Preliminary tests have shown that it will need 
+  at most 2-3 times as long as it takes to copy the database directory.
+
+- **RocksDB storage engine:**
+
+  All existing geo indexes will be rewritten into a new 
+  data format. This data format is required for using the indexes with the improved
+  [geo index feature](../../Indexing/Geo.md) in ArangoDB 3.4. 
+  
+  Preliminary tests have shown that the conversion can process about 500K to 1M geo index 
+  entries per second on commodity hardware.
+
+If you upgrade without any existing data (new blank data folder), then none of these tasks
+needs to be run because the datafiles will be created using the new format already.