Factors that Affect RTO

During a disaster, an administrator must first determine that the link is down between the primary datacenter and the secondary datacenter. Next, the administrator begins the process of switching applications that were running on the primary datacenter over to the secondary datacenter. For write applications, the administrator begins converting mirror volumes to read-write volumes, starting with volumes that contain the most critical data. Note that read applications can run on read-only mirrors, but write applications can only run on read-write volumes.

To gauge how long it will take to switch applications from the primary datacenter to the secondary datacenter (and to set the RTO accordingly), consider these factors:

  • Detection time (how long it takes to determine that the link is down between the two datacenters)
  • Switching time (how long it takes to switch applications from one datacenter to the other)
  • Promotion time (how long it takes to change read-only mirror volumes to read-write volumes that can run write applications). Promotion time is a function of:
    • the size of each volume (larger volumes take longer to change than smaller ones)
    • the number of files in each volume (the smaller the number of files to change, the faster the conversion)
    • the size of each file (file size determines number of files - the larger each file is, the fewer files you need to store data; fewer files take less time to change)
    • the number of containers in each volume
    • the number of volumes
  • Whether mirror throttling is enabled (the default) or disabled (which speeds up the mirroring process)

Various factors affect how much data can be recovered through the use of mirror volumes. To specify a realistic recovery point objective in your disaster recovery plan, take these factors into account:

  • Mirror schedule (how often the mirror is synchronized with its source volume) Note that the first mirror operation is a full synchronization between source and mirror volumes. Subsequent mirror operations are incremental - only the changes that occurred since the last mirror event need to be copied in order to synchronize the contents between the two volumes.
  • Network link between the source volume and the mirror volume (consider the stability and quality of the link, as well as latency, throughput, and other activities across the link)