Effect of new features on volume sizing

Several new features in SnapManager for Exchange 3.2 are designed to reduce the amount of disk space required to run SnapManager:

Transaction log archiving using NTFS hard links
For more information, see About transaction log archiving using NTFS hard links.
Fractional space reservation
For more information, see Managing fractional space reservation.
Option to backup transaction logs that Exchange will truncate
For more information, see page Ways to manage the backup and deletion of transaction logs.

These new features will have an affect on some of the sizing formulas in this chapter. Following are the updated formulas:

Transaction log archiving using NTFS hard links

Configurations in which the transaction logs reside on the same NTFS volume and LUN can take advantage of the new NTFS hard link support in SnapManager. During a backup, SnapManager copies the transaction log files to the SnapInfo directory for safe keeping. However, in supported configurations that can take advantage of NTFS hard links, a link is created to the file rather than copying the file itself. Since there are fewer blocks changed on the storage system volume as a result of this feature, the formula to calculate the size of the storage system volume for the Exchange transaction logs needs to be slightly modified. The difference in this case is that the number of transaction logs generated does not need to be multiplied by a factor of two as with the original formula. In the following updated formula, the number of transaction logs generated is only accounted for once, so the "2 *" is removed from the original formula.

The following formula calculates the transaction log volume size:

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

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

TransactionLogVolumeSize = (6,000 MB * 2) + (200 * 5 MB * 5) = 12,000 + 5,000 MB = 17,000 MB (~17 GB)

Fractional space reservation

Prior to the support for fractional space reservation in SnapManager, there was a requirement that the volume size on the storage system be at least twice the size of the LUN that it contains plus allow extra volume space to store SnapManager backups as well as accommodate changes in the Exchange data. With the new fractional space reservation support in SnapManager enabled, this factor of 2 can been relaxed to a factor that is greater than 1 and less than or equal to 2.

This change affects two storage system volume size calculation formulas:

Volume size for Exchange databases
Volume size for Exchange transaction logs and SnapManager SnapInfo directory

Only the transaction log LUN size in MB * 2 portion of the formulas are updated from the original formulas. The updated portion of the formula can be expressed as transaction log LUN size in MB * (1+n) , where n is a number greater than 0 and less than or equal to 1.

Volume size for Exchange databases

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

DatabaseVolumeSize = ((1+n) * LUN size) + (number of online backups * percentage data change * max database size)

As an example, assume the following facts:

Approximately 10% of the database changes between backups.
Five backups will be retained online.
Fractional space reservations are in use with a setting of 65 (or enough space reservation to accommodate 65% of the size of the LUN)

In this case, the minimum volume size for the database LUN is calculated as follows:

((1 + 0.65) * 90 GB) + (5 * 10% * 80 GB) = 188 GB

Volume size for transaction logs and SnapInfo directory

The following formula calculates the volume size for the transaction log and SnapInfo LUNs:

TransactionLogVolumeSize = (transaction log LUN size in MB * (1 + n)) + (2 * number of transaction logs generated * 5 MB * number of backups to keep online)

As an example, assume the following facts:

Approximately 10% of the database changes between backups.
Five backups will be retained online.
Fractional space reservations are in use with a setting of 65 (or enough space reservation to accommodate 65% of the size of the LUN)

In this case, the minimum volume size for the transaction log and SnapInfo LUNs is calcualted as follows:

TransactionLogVolumeSize = (6,000 MB * 1.65) + (2 * 200 * 5 MB * 5) = 9,900 + 10,000 MB = 19,900 MB (~20 GB)

Option to back up transaction logs that Exchange will truncate

This is another feature that can be used to save on amount storage system volume space consumed. However, since you cannot predict the number of transaction logs Exchange will truncate or when it will truncate them, you cannot accurately calculate the amount of disk space that can be saved by using this configuration.