In Exchange 2010, passive database copies have a very low checkpoint depth, which is required for fast failover. In addition, the passive copy performs aggressive pre-reading of data to keep up with a 5-megabyte (MB) checkpoint depth. As a result of using a low checkpoint depth and performing these aggressive pre-read operations, IOPS for a passive database copy was equal to IOPS for an active copy in Exchange 2010.
In Exchange 2013, the system is able to provide fast failover while using a high checkpoint depth on the passive copy (100 MB). Because passive copies have 100-MB checkpoint depth, they’ve been de-tuned to no longer be so aggressive. As a result of increasing the checkpoint depth and de-tuning the aggressive pre-reads, IOPS for a passive copy is about 50 percent of the active copy IOPS in Exchange 2013.
Having a higher checkpoint depth on the passive copy also results in other changes. On failover in Exchange 2010, the database cache is flushed as the database is converted from a passive copy to an active copy.
In Exchange 2013, ESE logging was rewritten so that the cache is persisted through the transition from passive to active. Because ESE doesn’t need to flush the cache, you get fast failover.
One other change is background database maintenance (BDM). BDM is now throttled back from 5 MB per sec per copy to 1 MB per sec per copy.
As a result of these changes, Exchange 2013 provides a 50 percent reduction in IOPS over Exchange 2010.