In my previous article EMC FAST Cache effectiveness with VDI I focused on FAST Cache benefits for VDI deployments. The tests I completed also generated a huge quantity of data that is tremendously valuable for better sizing of VDI environments.
I previously have discussed (here) how IOs suffer severe split between replica, linked clone and persistent disks. Depending on the storage tier serving the data it is possible to have different latencies for different parts of the Guest OS (Windows XP or Windows 7). However, in a linked clones environment the disks are interdependent at GuestOS layer, and a VMDK suffering from latency will also affect a different VMDK that is not suffering. Example: Replica disks that are backed by Flash Drive drives with Linked Clones that are backed by SATA drives.
In my validation with 200 VMs during Steady State each VM has an average of 8 IOPs. The 8 IOPS per virtual desktop is split into 0.29 IOPs for the Replica and 7 IOPs for the Linked Clone.
This represents 4% read-only operations for the replica and 96% read/write operations for the Linked Clone.
These numbers clearly identity that during Steady State the Replica disks are barely used and most of the IOs are absorbed by the linked clones. The two graphs below demonstrate the IO split between replicas and linked clones.
With knowledge of the disk behavior it is possible to size storage for the exact performance required in each tier.
VMware recommend placing Replicas disks in a Flash Drive (SSD) tier. However, you know now that Flash drives would only really be effectively used out of the Steady State period.
On the right picture (above) two spikes are clearly observed. The first is the logon storm; the second is the VMware View Refresh operation storm. Those are the periods when the Replica disks are utilized more aggressively. However, Linked Clones are equally utilized.
In fact, Linked Clones disks are more comprehensively utilized than the Replicas. This actually makes me evidently think that if there is a limited number of Flash Drives to be utilized they should be supporting Linked Clones, not Replicas. Most recent intelligent storage arrays will cache most utilized blocks in L1 DRAM cache, and if the Replica disk is being heavily utilized those blocks will end up in cache anyway.
It all comes down to how the VDI environment is utilized by the users. If boot storms, logon storms and VM refreshes are often requested, such in a Non-Persistent Pool serving school classes, then it is important to support Replica disks as much as possible. That means Replica disks should be on the fastest storage tier.
If there are no constant VM refresh or extreme login storms, then it is probably it is better to support Linked Clones as much as possible. That means Linked Clones should be on the fastest storage tier.
I know, I know… that goes against industry recommendation. Well, I guess there is always someone to go 1st against the flow. I have done my tests and math and I recommend you to do yours.
Of course, if your organization have budget to employ Flash Drives for everything, go for it! There are technologies on the market that help in reducing the number of disks required to serve a larger number of IOPs without having to utilize an all SSD based array. Find out more here.