What are MS Exchange Transaction log files

Microsoft Exchange Server transaction logging is a robust disaster recovery mechanism that is designed to reliably restore an Exchange database to a consistent state after any sudden stop of the database.

Before changes are actually made to an Exchange database file, Exchange writes the changes to a transaction log file. After a change has been safely logged, it can then be written to the database file.

If after a start the database is found in an inconsistent state or has not been shut down properly, Exchange scans the log files, and reconstructs and applies any changes not yet written to the database file. This process is called replaying log files. The database is structured so that Exchange can determine whether any operation in any log file has already been applied to the database, needs to be applied to the database, or does not belong to the database.

How Exchange Server Maintains Transaction log files

Before committing a transaction to a database file, Exchange logs it to a transaction log file. To track which of the logged transactions have been committed to the database, Exchange uses checkpoint files. Once the transactions are committed to the database and tracked by the checkpoint files, the log files are no longer needed by the database.

If the log files are not deleted, they will eventually consume all the available disk space and the Exchange databases will be taken offline until the log files are purged from the disk.

Exchange automatically removes unnecessary log files by using one of the following methods:

• If circular logging is enabled, Exchange Server removes transaction logs soon after they have been written to the database file.
• If circular logging is disabled, Exchange Server removes excess logs after a full or incremental online backup of all the databases in a storage group is performed.

Using circular logging is not the best practice for a production environment. When circular logging is enabled, Exchange overwrites the first log file after its data has been committed to the database, and you can recover the data only up until the last backup.

One can use Acronis Backup in several ways to protect their MS Exchange server. Below you can find detailed description how each backup type affects Exchange transaction logs.

You can also jump to Summary Table section if you do not want to read through all the details.

1. 1. Use Acronis Agent for Exchange to run database-level backups of Exchange Information Store.
       
      Such backup operation will copy only databases of the Exchange server itself, it does not protect OS files or any data files on the server. Data can be recovered to a live Exchange Server or to a PST file.

      (!) This type of backup truncates transaction logs files automatically with no user interaction:

      • After a successful full backup of the corresponding Exchange information store, storage group or database
        (this applies to all types of backups: general backup, CDP Full backup and Express backup, except for the copy-only backup).
      • After a successful transaction log backup, but excepting CDP Transaction log backup.

      In either case, Exchange deletes only those files that are older than the checkpoint. This means that transactions from these files have already been committed and written to databases.

      However, there is a couple of exclusions:

      • Transaction logs are not truncated after copy-only full backup and CDP Transaction log backups
      • Transaction logs backups cannot run if circular logging is enabled, see Acronis Backup Advanced 11.7/11.5 and Recovery for Microsoft Exchange Cannot Create an Incremental Backup in Circular Logging Mode
   2. Use Acronis Agent for Exchange to run mailbox-level backups of individual Exchange Server mailboxes, folders or emails.

      Such backup operation will copy only specific items of the Exchange server itself (mailboxes, folders, emails, etc), it does not protect OS files or any data files on the server. Data can be recovered to a live Exchange Server or to a PST file.

      (!) This type of backup does not truncate transaction logs automatically.

      Unless you have circular logging enabled, transaction logs will sooner or later fill up the whole disk. They will be also be backed up together with the rest of the system causing backups to become larger.

      To avoid this you can either enable circular logging to make Exchange Server delete old transaction logs automatically (this may cause some decrease in Exchange performance) or delete transaction logs manually as described in the user guide, see Transaction log truncation for Exchange Server.

      This is why we do not recommend mailbox-level backup as the primary Exchange backup means. The recommendation is to protect only critical mailboxes in addition to the full Exchange database backup. For the details and best practices please refer to Acronis Backup Advanced 11.7/11.5 for Exchange: Usage Scenarios.
       

   3. Run disk-level backups of his Exchange server (or only disks holding OS and Exchange Server).

      Such backup is useful if you plan to recover Exchange server as a whole in case of disaster recovery (OS and Exchange Server files will be recovered). Also, you can use this backup to easily move the whole OS and installed Exchange Server to another machines.

      (!) This type of backup does not truncate transaction logs automatically.

      Unless you have circular logging enabled, transaction logs will sooner or later fill up the whole disk. They will be also backed up all together with the rest of the system causing backups to become larger.

      To avoid this you can:

      1. Enable circular logging to make Exchange Server delete old transaction logs automatically (this may cause some decrease in Exchange performance).
      2. Delete the transaction logs manually as described in the user guide, see Transaction log truncation for Exchange Server.
      3. Starting from Acronis Backup Advanced (Acronis Backup & Recovery 11.5) Update 1, you can use the Enable VSS Full backup option for the Acronis Agent installed on the machine (it is turned off by default; it does not work for agent-less of virtual machines on hypervisor level). If enabled, logs of Microsoft Exchange Server and of other VSS-aware applications (except for Microsoft SQL Server) will be truncated after each successful full, incremental or differential backup.

      Leave this option disabled in the following cases:

      • If you use Acronis Agent for Exchange or third-party software for backing up the Exchange Server data. This is because the log truncation will interfere with the consecutive transaction log backups.
      • If you use third-party software for backing up the SQL Server data. The reason for this is that the third-party software will take the resulting disk-level backup for its "own" full backup. As a result, the next differential backup of the SQL Server data will fail. The backups will continue failing until the third-party software creates the next "own" full backup.
      • If other VSS-aware applications are running on the machine and you need to keep their logs for any reason.

      Enabling this option does not result in the truncation of Microsoft SQL Server logs. To truncate the SQL Server log after a single-pass backup, enable the Log truncation setting in the Single-pass disk and application backup section of the Create backup plan or Back up now page.

Below is a summary table of the above described backup types and the way Acronis Backup handles transaction logs:

 

Backup method	Transaction logs will be truncated automatically
	Circular logging disabled	Circular logging enabled *
Database-level backup using Acronis Agent for Exchange
Full backup	Yes	Yes
Transaction logs backup	Yes	It is not possible to run transaction logs backup with circular logs enabled, see Acronis Backup Advanced 11.7/11.5 and Recovery for Microsoft Exchange Cannot Create an Incremental Backup in Circular Logging Mode
CDP Full backup	Yes	Yes
CDP transaction logs backup	No	It is not possible to run transaction logs backup with circular logs enabled, see Acronis Backup Advanced 11.7/11.5 and Recovery for Microsoft Exchange Cannot Create an Incremental Backup in Circular Logging Mode
Express backup	Yes	Yes
Copy-only backup	No	Yes
Mailbox-level backup using Acronis Agent for Exchange
Full backup	No	Yes
Incremental backup	No	Yes
Common disk/system backup of Exchange server using Acronis Backup
Full disk/system backup of Exchange server	No, but starting from Acronis Backup 11.5 Update 1, you can use Enable VSS Full backup option to truncate logs automatically	Yes
Incremental backup		Yes

* (!) Circular logging can handle the problem with transaction logs filling up the disk, but using circular logging is not the best practice for a production environment. When circular logging is enabled, Exchange overwrites the first log file after its data has been committed to the database, and you can recover data only up until the last backup.