The following values and variables are used in the formulas:
N is the number of emails.
m is the average number of identical copies of attachments across user mailboxes.
The compression factor for attachments is estimated as 60%.
If the attachments are mostly Office 2007 files, the compression factor to use is 90%.
The average number of emails that have attachments is estimated at 20%.
The average size of an email attachment is estimated at 250 KB.
Table: Calculations for disk space requirements for the Exchange Mailbox Archiving Option
Vault store partitions
The size of a vault store partition depends on the following items:
The size of the emails.
The type of attachments.
The number and size of the attachments.
The number of emails with attachments.
If single instance storage is enabled, items are shared within and across vault stores and vault store partitions. Shareable parts of a message that exceed the single-instance threshold of 20 KB are shared. These parts include attachments and message bodies. User information and shareable parts under the single instance threshold are not shared.
You can use the following calculations to approximate the disk space requirements of a vault store partition:
Approximate vault store partition size for which single instance storage is not enabled:
(N x 16) + (N x 0.2 x 0.6 x 250) kilobytes
Approximate vault store partition size for which single instance storage is enabled:
(N x 16) + ((1/m) x (N x 0.2 x 0.6 x 250) kilobytes
For example, you want to know the disk space requirements for a vault store partition for 100,000 emails. You estimate that each email attachment is shared across three people on average.
If single instance storage of archived items is not enabled, the calculation for the disk space requirements is as follows:
(100000 x 16) + (100000 x 0.2 x 0.6 x 250) kilobytes = 4.6 GB approximately
If single instance storage is enabled, the calculation for the disk space requirements is as follows:
(100000 x 16) + ((1/3) x 100000 x 0.2 x 0.6 x 250) kilobytes = 2.6 GB approximately
The size of an index is approximately 8% of the total size of the items that are archived. The percentage may be less if there is less content to index. For example, there is less content to index when there are large attachments such as MP3 or .jpeg files.
For example, you have 100,000 emails that each have a body size of 8 KB. About 20% of the emails have attachments, each with an average total size of 250 KB. The index size is approximately 450 MB.
The Directory database only grows when a new mailbox or share is archived for the first time.
The recommended disk space is 500 MB.
Vault store database
The size of a vault store database is approximately N x 500 bytes. The vault store database grows with every item that is archived. Temporary space is used to hold information on the items that have not been backed up or indexed.
The fingerprint database is created only if you enable single instance storage of archived items. Backup Exec initially allocates 212 MB for the fingerprint database. The fingerprint database grows with every item that is archived.
If the database grows to more than 212 MB, use the following calculation to estimate the disk space that it requires: