Calculating LUN size for Exchange databases

This topic describes how to calculate the required size for the LUNs to hold your Exchange databases. 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
Single-instance sharing ratio
For more information about calculating the single-instance sharing ratio, see Calculating single-instance sharing ratio for a database LUN.
Size of the deleted item cache (expressed as a percentage of the overall size of the data)
For more information about calculating the deleted-item cache size, see Calculating deleted-item cache size for a database LUN.

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.

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)

Note
In this calculation, the addition of 20% of the database size (to allow for the size of the deleted item cache in this example) is expressed by multiplying the estimated storage group size by a factor of 1.2 (or 120%).

The calculated maximum LUN size for this database is approximately 80 GB. If you want to allow for 10 GB of free space, increase the LUN size from 80 GB to 90 GB.