Calculating volume size

This topic describes how to calculate the required size for the volumes to contain your LUNs:

• Considerations for converting to individual database restoration
• Database volume size calculation
• Transaction log volume size calculation
• Volume size calculation for multiple Exchange LUNs

  Note
  For a summary description of Data ONTAP fractional space reservation and a detailed description of SnapManager fractional space reservation policies, see Managing fractional space reservation.

Considerations for converting to individual database restoration  

If you are upgrading an existing SnapManager implementation and you are creating new LUNs to use individual database restore, your space requirements increase. This is because the database must be copied from its current location to the new location; in effect, it exists in both places until the move is complete. Depending on the size of the volume containing both LUNs, you might need to increase the volume size.

For more information about sizing your volumes, see Calculating volume size. Remember to include both the old and new LUNs when calculating the volume size.

Database volume size calculation  

The calculation for the Exchange database volume size depends on the size of the databases in the volume, the rate of change of your data, and your backup usage.

Estimated database volume values

To calculate the volume size requirement for a LUN holding Exchange databases, you need the following values or estimates for your installation:

• The size of the LUNs (as calculated using the database LUN formula)
• The approximate percentage of the databases that you expect to change between backups
• How many backups you plan to retain online

Calculation formula

The following formula calculates the minimum volume size requirement for the Exchange database LUNs:

minDatabaseVolumeSize = (2 * LUN size) + (number of online backups 
* percentage data change * max database size)

Example

For the example, assume the following facts:

• Approximately 10% of the database changes between backups.
• Five backups will be retained online.

The minimum volume size for the database LUN is calculated as follows:

(2 * 90 GB) + (5 * 10% * 80 GB) = 220 GB

Transaction log volume size calculation  

The size of the transaction log volume depends on how much traffic your Exchange server gets, and your backup usage.

Estimated transaction log values

The following values or estimates are required for the entire volume where the transaction logs are stored:

• How many transaction log files are generated between backups
• How many backups will be retained online

Calculation formula

The following formula calculates the transaction log volume size:

transaction log volume size = (transaction log LUN size in MB * 2) 
+ (2 * number of transaction logs generated * 5 MB * number of 
backups to keep online)

This is always less than four times the size of the transaction log LUN. Therefore, if disk capacity is not constrained, you can use this simplified formula:

transaction log volume size = 4 * transaction log LUN size

Example

For the example, the transaction log volume size could be calculated using the full formula, as follows:

transaction log volume size = (6,000 MB * 2) + (2 * 200 * 5 MB * 5) 
= 12,000 + 10,000 MB = 22,000 MB (~22 GB)

Alternatively, you could calculate the volume using the simplified formula, as follows:

transaction log volume size = (6,000 MB * 4) = 24,000 MB (~24 GB)

Volume size calculation for multiple Exchange LUNs  

If you plan to use a single storage system volume for storing multiple LUNs to hold Exchange or SnapManager data, the volume sizing requirements of all the components must be summed to calculate the size requirement for the volume.

Note
Microsoft recommends that Exchange databases and transaction logs be kept on separate volumes. This example is for illustration only.

Example

Assume now that you want to create one volume for an Exchange installation with the following characteristics:

• Four storage groups, each with 1,000 mailboxes and 100-MB quotas
• Single instance sharing ratio of 1.5
• Deleted item cache is 20% of the data
• A combined maximum of 800 transaction logs generated between backups
• 10% database change between backups
• Five online backups retained

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Last updated: 28-Mar-2006 11:30:56