While preparing for my VCAP5-DTA, I made the following notes about the replica disk.
- The replica disk is the master image that Composer will use for provisioning linked clone desktops. To reduce space consumption, replica disks are thin provisioned.
- To arrive at the recommendations for the OS disks (keeping calculations simple and assuming the entire provisioned space will be used) View Manager will ensure there’s at least enough space for two replica disks. The first one is always there, the second one is created when the pool is being recomposed. The second one is the one to which the linked-clones are then anchored. The first one is then deleted.
- It’s a good idea to keep replica and OS disks on the same datastore if a separate datastore for replica disk doesn’t offer any performance increases. If linked clone datastores are local, the replica disk should be stored on the same disks.
- If the replica is not stored on a separate datastore, View Composer creates a replica on each datastore on which linked clones are created. If the replica is located on a separate datastore, one replica is created for the entire pool, even when linked clones are created on multiple datastores.
- You are unable to delete the replica disk until all desktops have been deleted or recomposed (no more linked desktops).
- The process of creation of the desktop pools gives the option of choosing a different datastore for the replica disks from the OS disks of the linked clones. This is a good idea if the array has different tiers of storage performance, with say RAID 1 being Tier 1 and RAID 5 being Tier 2. Replica disks can be placed on Tier 1 disk. This separation is only a good idea if the array has tiers, otherwise it’s just overhead.
I was of the opinion that replica disks should be placed on your high performance disk and linked clones on slower disks to strike a balance between performance and cost – until I read this article by the VDI Guru, Andre Leibovici. To summarize his findings, if you don’t recompose often or refresh at logoff, you are much better off with placing your linked clones on fast disk instead. Basically, it depends. One size does not fit all!