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For performance and recoverability, the Exchange database uses transaction logs to accept, track, and maintain data. All transactions are first written to transaction logs and memory, and then committed to their respective databases. Transaction logs can be used to recover Information Store databases in the event that a failure corrupted the database. The Exchange 2010 Information Store can have multiple separate databases, each of which has its own set of transaction logs. For Exchange 2007 and earlier, the databases in each storage group share a common set of transaction logs.
Transactions are first written to the log file and then later written to the database. The effective database is a combination of the uncommitted transactions in the transaction log file and the actual database file. When the log file is filled with transaction data, it is renamed and a new log file is created. When the log file is renamed, the other renamed log files are stored in the same subdirectory. The renamed log files are named in a sequential numbering order, in hexadecimal.
For Exchange 2007 and
later the database transaction log for the Information Store is
named EXXYYYYYYYY.log. XX is the database number or storage group number
(in hex). YYYYYYYY is the log file
number (in hex). The size of the transaction logs is 1 MB.
For Exchange 2003, the
database transaction log for the Information Store is named
E0yXXXXX.log. Y
is a number from 0 to 3 representing the storage group. XXXXX is a five-digit hexadecimal number that is
incremented each time an E0y.log file
is renamed. For example, E010029E.log, E010029F.log, E010029A.log.The size of the transaction logs is
5 MB.
After every 1 MB or 5 MB of transaction log data is written, a new log is created. The log is created even though the transaction data may not be committed to the database. There may be several transaction logs that contain uncommitted data, therefore they cannot be purged.
Exchange 2010 transaction logs get committed to their database over time or when the services are brought down. Any transactions that existed in log files and not in the database file are committed to the database.
Exchange 2007 and earlier transactions in log files are committed to the respective database over time, or when the service shuts down normally. For example, consider when the Information Store service experiences a normal shutdown (service shuts down with no errors). Any transactions that existed in log files and not in the database file are committed to the database.
Do not manually purge log files. Instead, purge logs through the backup process. For backups of a replicated copy (LCR, CCR or DAG), the log truncation is scheduled. It starts with the active copy when Exchange has the resources to start truncation. It does not happen instantly after a backup as with non-replicated copies.
For information on how transaction logs are truncated, see the following topics: