Does Data Guard read Redo from memory Log Buffer when using sync AND async?
New to Data Guard and reading a book on it and MOS notes. I'm confused over when redo is read from the log buffer rather than the ORL. It's unclear to me whether LNS in 11g always, regardless of whether transport is SYNC or ASYNC reads from the log buffer first then goes looking in the ORL.
MOS note "View X$LOGBUF_READHIST and In-Memory Log Buffer Hit Rate Histogram [ID 951152.1]" says: Starting with 11gR1, Oracle Data Guard asynchronous redo transport will read redo directly from the in-memory log buffer, provided that the requested redo blocks still reside in the log buffer
Confusingly it also says: Oracle Data Guard asynchronous redo transport will only read in-memory redo blocks that have already been written to the ORL
This note doesn't exclude SYNC transport also reading from the log buffer, but it does imply by stating async that sync isn't covered by the statment.
The Oracle Press Data Guard 11g Handbook in its first chapter says that both SYNC and ASYNC transports read from the log buffer, but then later in another chapter only discusses this in relation to ASYNC.
Can anyone give a definitive answer?