LUN size calculation

Database LUN size calculation

The size of the LUN to hold Exchange databases must be at least as big as the sum of the expected maximum sizes of all databases on that LUN.

Estimated database values

To calculate the expected maximum size for an Exchange database LUN, you need the following values or estimates for each database on the LUN:

Number of mailboxes in the database
Mailbox quota
Size of the deleted item cache (expressed as a percentage of the overall size of the data)
Single instance sharing ratio
For more information about calculating the single instance sharing ratio, see Single instance sharing ratio calculation.

For more information about calculating the deleted item cache size, see Calculating the deleted item cache size.

Calculation formula

The following formula calculates the expected Exchange database size:

database size = ((number of mailboxes * mailbox quota) / single 
instance sharing ratio) + deleted item cache

The sum of the maximum expected database size for each database in your LUN provides the maximum expected database LUN size.

Example

The following facts and assumptions represent a sample Exchange installation:

There is one storage group, with one database.
There are 1,000 mailboxes with 100-MB quotas in the database.
The single instance sharing ratio is equal to 1.5.
The deleted item cache is 20% of the database.
In the following calculation, this is expressed by multiplying the estimated storage group size by 1.2 (or 120%).

For this example, the database size requirement calculation would be as follows:

database size = (((1,000 * 100 MB)/1.5) * 1.2) = 80,000 MB (~80 GB)

The maximum LUN size for this database would be approximately 80 GB. For this example, a LUN size of 90 GB is used to include 10 GB of free space.

Transaction log and SnapInfo directory LUN size calculation

The size of the LUN containing the transaction logs and SnapInfo directory depends on how much 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 your SnapInfo directory is on a separate LUN from your transaction logs, the LUN size calculation for the transaction log LUN only has to take into account 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)