Calculating LUN size for transaction logs and SnapInfo directory

This topic describes how to calculate the required size for the LUNs to hold your transaction logs and SnapInfo directory or directories. A SnapInfo directory can reside on the same LUN as the transaction logs, or it can be placed on a separate LUN. Use the following calculations as they apply to your planned configuration of SnapManager for Exchange:

Transaction log and SnapInfo directory LUN size calculation
SnapInfo directory LUN size calculation
Transaction logs LUN size calculation
Note
For information about using multiple SnapInfo directories, see About transaction log archiving using NTFS hard links and About multiple SnapInfo directories.

Transaction log and SnapInfo directory LUN size calculation

The size of the LUN containing the transaction logs and SnapInfo directory depends on how much message traffic your Exchange server gets and your backup usage. The size calculation is performed per storage group.

Estimated transaction log values

The following values (or estimates) are required for each storage group whose transaction logs will be stored on this LUN:

How many transaction log files are generated between backups
How many backups will be retained online
Note
You can estimate how many transaction log files are generated by noting the maximum number of transaction logs that accumulate in the transaction log directory immediately before a backup over a period of several days.

Calculation formula

The following formula calculates the LUN size required for a LUN containing transaction logs and the SnapInfo directory:

transaction log & SnapInfo LUN size = ((number of transaction logs 
generated * 5 MB) * (number of backups to keep online + 1 for the 
active file system))

Example:

Given the following parameters, at least 6 GB of LUN drive is required to accommodate the transaction logs and SnapInfo directory for the example Exchange databases. This is expressed by the formula following the parameters.

The server generates, at most, 200 transaction logs between backups.
Each transaction log is 5 MB.
The plan is to keep five SnapManager backups online at the same time.

transaction log & SnapInfo LUN size = ((200 * 5 MB)*(5+1)) = 6,000 
MB (~6 GB)

SnapInfo directory LUN size calculation

The SnapInfo directory contains the logs for all backups currently retained online. Therefore, if your SnapInfo directory is on a separate LUN from your transaction logs, the LUN size calculation is similar to the calculation for a combined transaction log and SnapInfo LUN, except that you do not have to include space for the current logs.

Calculation formula

The following formula calculates the LUN size required for a LUN containing only the SnapInfo directory:

SnapInfo LUN size = ((number of transaction logs generated * 5 MB) 
* (number of backups to keep online))

Example:

Given the following parameters, at least 5 GB of LUN drive is required to accommodate the SnapInfo directory for the example Exchange databases. This is expressed by the formula following the parameters.

The server generates, at most, 200 transaction logs between backups.
Each transaction log is 5 MB.
The plan is to keep five SnapManager backups online at the same time.

transaction log & SnapInfo LUN size = ((200 * 5 MB)*5) = 5,000 MB 
(~5 GB)

Transaction logs LUN size calculation

If the SnapInfo directory is on a separate LUN from your transaction logs, the LUN size calculation for the transaction log LUN has to take into account only how many logs accumulate between backups.

Calculation formula

The following formula calculates the LUN size required for a LUN containing only the transaction logs:

transaction log LUN size = number of transaction logs generated * 5 
MB

Example:

Given the following parameters, at least 1 GB of LUN drive is required to accommodate the transaction logs for the example Exchange databases. This is expressed by the formula following the parameters.

The server generates, at most, 200 transaction logs between backups.
Each transaction log is 5 MB.

transaction log LUN size = 200 * 5 MB = 1,000 MB (~1 GB)