Compaq AA-Q88CE-TE manual Shadow Site Failure and Journaling

Server Shadowing and Recovery

B.3 The RTR Journal System

The number of blocks specified by the /BLOCKS qualifier specifies the size of the journal that RTR attempts to keep to. (The actual number of blocks used may vary, depending upon the load on RTR.)

The command MODIFY JOURNAL also accepts the /BLOCKS and /MAXIMUM_ BLOCKS qualifiers.

Journal file extension occurs on demand when RTR detects that a ``write to journal'' would otherwise fail due to lack of space. Journal file truncation takes place automatically when blocks are freed.

Refer to MODIFY JOURNAL for the syntax description of the MODIFY JOURNAL command.

RTR> show journal/files/full
RTR journal:-
Disk:	/dev/rz3a	Blocks:	2500 Allocated: 1253 Maximum: 3500
File:	//rtrjnl/anders/BRONZE.J00

RTR>

B.4 Shadow Site Failure and Journaling

If a shadow-site fails, RTR allows transactions to continue to be processed on the remaining site. The intermediate transactions processed by the remaining server or servers are retained by RTR; when the failed site restarts these transactions are sent to this site as part of a shadow-recovery operation, thus bringing the failed site back up-to-date.

Since the transactions are stored in the RTR journal, it must be created with enough disk space in reserve to store data for the longest expected outage. It can be calculated using:

( Nr. transaction messages per second multiplied by ( transaction message length + 70 ) multiplied by seconds of outage

) + 5% file overhead.

The overhead required when calculating journal size comes from internal journal data (block stamping) of approximately 3%. In addition, there is internal transaction data per (client to server) transactional message, and some further data per transaction (concerning voting and transaction completion).

Also, note that RTR prevents further transactional data being written to the journal when it is nearly full, but continues to allow deletes from the journal (deletes also cause data to be written to the journal). Ten segments are held in reserve for storing information about deleted transactions even when RTR cannot accept further transactions because the journal is full.

Warning

If the journal disk becomes full, transactions are aborted until the shadow partner re-starts and empties the journal of transactions to be replayed.

Server Shadowing and Recovery B–3