Identifying Required Transaction Logs
Identifying Required Transaction Logs
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If Microsoft® Exchange Server 2003 database is backed up while is it mounted, at least one transaction log file will always be backed up with it. After restoration of a mounted backup, information from the transaction logs must be applied to the database before the database will be mountable again. The Log Required field of each database header records the sequence (generation) numbers of the range of the transaction log files that must be replayed into the database.
If the Log Required field reads 0-0, the database is mountable without having to replay any additional transaction log data. The only time the Log Required value will be 0-0 is after a database is brought into a clean-shutdown state. While a database is running, the Log Required field always records the range of transaction logs that have not yet been applied to the database.
A database backed up while it is mounted will always have a non-zero Log Required range and these logs will be backed up along with the database. If, after restoration, these logs are not available, the database will not be mountable. You can repair the database if the necessary transaction log files cannot be found, but there can be no guarantee the repair will be successful and will almost always result in some level of data loss, even if the only data missing is the data from the missing transaction logs.
The Volume Shadow Copy Service automatically backs up the transaction log files required to remount a database. If only the Log Required fields are replayed, the database will be restored to the point in time at which the backup finished. If you want to roll the database forward past that point, you must also play the log files generated after the backup was done.
To completely roll the database forward from any particular backup, you must preserve all the log files in unbroken sequence starting from the lowest log in the Log Required range up to the most recently generated log file in the database's storage group. If any log in this series is missing or damaged, the database can be recovered only up to the point of the last good transaction log before the missing or damaged file.
Therefore, if you want to recover from a backup with no data loss, it is essential that you maintain good copies of all transaction log files going forward from your last verified good database backup.
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This topic last updated: December 2004
Build: June 2007 (2007.618.1)
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