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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