Sybase DC38133-01-0902-01 manual Protecting against media failure on the transaction log

Adaptive Server Enterprise transaction log and backup management

You must protect against losing transactions that have been replicated to remote databases. If transactions are lost that have already been replicated to remote databases, the remote databases will be inconsistent with the consolidated database. In this situation, you may have to re-extract all remote databases.

Protecting against media failure on the transaction log

	Media failure on the transaction log can cause committed transactions to be
	lost. If the transaction log has been scanned and these transactions have
	already been sent to subscriber databases, then the subscribing databases
	contain transactions that are lost from the publishing database, and the
	databases are in an inconsistent state.
Why the transaction log	The transaction log is needed, even after the entries have been scanned into
is needed	the stable queue, to guard against media failure on the database file. If the
	database is lost, it must be recovered to a point that includes every
	transaction that may have been sent to remote databases.
	This recovery is done by restoring a database dump and loading transaction
	dumps to bring the database up to date. The last transaction dump restored is
	the dump of the active transaction log at the time of the failure.
Protecting against	There are two ways of protecting against inconsistency arising from media
transaction log loss	failure on the transaction log:
	♦ Mirror the transaction log When a device is mirrored, all writes to the
	device are copied to a separate physical device.
	♦ Only replicate backed-up transactions There is a command-line
	option for the Message Agent that prevents it from sending transactions
	until they are backed up.
Mirroring the transaction	The only way to protect against media failure on the transaction log is by
log	mirroring the transaction log.

Disk mirroring can provide nonstop recovery in the event of media failure. The disk mirror command causes an Adaptive Server Enterprise database device to be duplicated—that is, all writes to the device are copied to a separate physical device. If one of the devices fails, the other contains an up-to-date copy of all transactions.

☞For information on disk mirroring in Adaptive Server Enterprise, see the chapter “Mirroring Database Devices”, in the Adaptive Server Enterprise

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