Space requirements and Recovery Point retention policy

Problem

Users typically want to set the option Limit the number of recovery point sets saved for this backup to maximize the number of Recovery Point (RP) sets retained within the space allotted to a given BESR client on the Backup Destination.  To know what value to set, it is important to understand how the retention policy is executed.

Solution

A RP set is a base RP plus all its associated incremental RPs.  When RPs are deleted to comply with the retention policy, an entire set (the oldest one) is deleted at once.  The deletion is done after a new base RP is successfully created.  This ensures that the number of RP sets available for recovery is never less than the number specified in the policy.

For example, you configure a monthly Backup Job that yields 35GB of data (after compression) in the base RP and an average of 1GB in each daily incremental RP.  Therefore each RP set will be ~65GB.  You have 200GB alotted in the Backup Destination and therefore reason that you can retain 3 RP sets for this job (3 x 65 < 200).  The job runs successfully for 3 months, creating RPs as follows:

WEBSERVER02_C_Drive001.v2i
WEBSERVER02_C_Drive001_i001.iv2i
WEBSERVER02_C_Drive001_i002.iv2i
...
WEBSERVER02_C_Drive001_i030.iv2i
WEBSERVER02_C_Drive002.v2i
WEBSERVER02_C_Drive002_i001.iv2i
WEBSERVER02_C_Drive002_i002.iv2i
...
WEBSERVER02_C_Drive002_i030.iv2i
WEBSERVER02_C_Drive003.v2i
WEBSERVER02_C_Drive003_i001.iv2i
WEBSERVER02_C_Drive003_i002.iv2i
...
WEBSERVER02_C_Drive003_i030.iv2i

Then it fails with an error like these:

Error EBAB03F1: Not enough storage is available to complete this operation.
Error EC8F17E5: Your recovery point location of x:\ is running out of space. Please run the Cleanup Recovery Points task to free up some space.
Error ED800016: The target disk is full.

Why?  On the first day of month 4, the process is:

1.  Create WEBSERVER02_C_Drive004.v2i
2.  Check retention policy
3.  Delete WEBSERVER02_C_Drive001.v2i and WEBSERVER02_C_Drive001_i001.iv2i through WEBSERVER02_C_Drive001_i030.iv2i
 

So when setting Limit the number of recovery point sets to n you must have free space for n+1 RP sets.  More precisely, you need space for n+1 base RPs plus n x [the number of incrementals per week/month/quarter/year].  So in this example you would need space for 4 base RPs and 3x30 incrementals:  4x35GB + 3x30x1GB == 230GB.