Preventing DDRBLOCK Mode

If the database server comes close to overwriting a logical log that Enterprise Replication has not yet processed (log wrap), Enterprise Replication enters DDRBLOCK mode. In DDRBLOCK mode, user transactions are blocked and you see the following error in the message log:

DDR Log Snooping - DDRBLOCK phase started, userthreads blocked 

Although user transactions are blocked while the server is in DDRBLOCK mode, Enterprise Replication activity is allowed to continue. During this time, Enterprise Replication attempts process transactions to advance the replay position and remove the risk of a log overrun. Enterprise Replication can usually resolve the cause of the DDRBLOCK state. However, in more extreme cases, the replay position can be overrun. If the replay position is overrun, the following message appears in the message log file:

WARNING:  The replay position was overrun, data may not be replicated.

If this occurs, you must resynchronize the source and target servers. For more information, see Resynchronizing Data Among Replication Servers.

DDRBLOCK mode is usually caused by the logical logs being misconfigured for the current transaction activity or by the Enterprise Replication system having to spool more than usual. More-than-usual spooling could be caused by one of the following situations:

• A one-time job might be larger than normal and thus require more log space.
• If one of the target servers is currently unavailable, more spooling of replicated transactions can be required.
• The spool file or paging space could be full and needs to be expanded. For more information, see Preventing Memory Queues from Overflowing.

If Enterprise Replication detects that the log files are configured too small for the current database activity, then the following message might appear in the message log file:

Warning - The log space appears to be configured too small for use 

with Enterprise Replication (ER). ER may require additional logical

logs to avoid a DDRBLOCK state and/or replay position log wrap.

It is recommended that the logical log configuration be expanded.

Enterprise Replication can be configured to dynamically add a logical log file instead of placing the database server in DDRBLOCK mode. Set the CDR_MAX_DYNAMIC_LOGS configuration parameter to enable Enterprise Replication to dynamically add a logical log file when the system is in danger of a log wrap. This allows the system to better adjust to unusual activity. Set CDR_MAX_DYNAMIC_LOGS to one of the following values:

3You can execute cdr list commands, 3such as cdr list repl, while the replication server 3is in DDRBLOCK mode. You must create a temporary dbspace with the onspaces 3utility and set the DBSPACETEMP configuration parameter before executing the cdr list command.