In part 1 of this series, I talked about replica disk optimizations that FVP provides for your linked clone environment. In part 2 the focus will be on the different use cases for persistent and non-persistent disks and how it relates to the acceleration that FVP can provide to your VDI environment.
I often hear confusing remarks about what some may call a persistent desktop and a non-persistent desktop. I have found that at times this terminology is based on confusion between a linked clone and a full clone. It also makes a difference what criteria one bases their understanding of a non-persistent or persistent desktop. For example, if you just look at linked clones, you will notice that several disks are non-persistent and persistent, depending on your design decisions. If one looks only at a dedicated linked clone with windows profile persistence then some may articulate this linked clone as a persistent desktop.
The interesting thing is that Horizon View doesn’t refer to a linked clone in this context. The only time Horizon View refers to a persistent or non-persistent desktop is under the context of refreshing a cloned desktop. In other words, it doesn’t mean that just having a linked clone makes you a non-persistent or even persistent VDI environment.
I also think some of the confusion revolves around the use of dedicated vs. floating assignment of linked clones. The dedicated configuration assigns each user a dedicated desktop, so, if the user has multiple sessions, they will always reconnect to the same desktop by default. In a floating configuration the user is assigned to a pool of desktops. This means they could login to a different desktop with each new session. The only way to keep windows profile persistence in the floating configuration scenario is use persona management solution outside the default configuration of view composer.
So, when an admin decides to use a dedicated linked clone, view composer gives the option to redirect the windows profile to a persistent disk. This will provide user personalization persistence during refresh, recompose, and rebalance operations. This is an optional setting as seen in the screenshot. The default disk size is 2GB.
When one chooses a floating assignment for linked clones, view composer does not provide an option for a persistent disk, this means that no user personalization will be retained after a refresh, recompose or rebalance operation. If you chose not to redirect the windows profile, then the data would be stored on the non-persistent delta disk. In either case, both read and write I/O will be accelerated with FVP. However, there will be a longer warm up time for read acceleration when using the non-persistent delta disk for user profiles, as this will depend on how frequent the refresh, recompose and rebalance cycles are.
Whether you select floating or dedicated assignments and choose some level of windows profile persistence or not, FVP will automatically accelerate reads and writes for all disks that are part of the desktop VM. In the past, the choice on when to schedule a recompose, rebalance operation came with varied importance. Now with FVP offloading IO from the storage array, a refresh, recompose or rebalance operation can provide some breathing room for these tasks to finish without impact to the production environment.
The delta disk is probably where most desktop I/O will be seen from a linked clone. The delta disk becomes active as soon as the desktop is booted from the replica disk. Any desktop changes are stored on the delta disk and so depending on the user and the desktop use case, the I/O profile could vary drastically. This will not impact FVP negatively, as FVP will keep context on which disk is more active and thus provide the resource intelligence for acceleration no matter the use case.
A default configuration will have a separate non-persistent disposable disk 4GB in size. Having this as separate disk is recommended since it slows the growth of the delta disk between refresh, rebalance, and powered off tasks. This disk contains temp files, and the paging file, so FVP can help normalize OS operations by accelerating reads and writes associated with the disposable disk. If you choose not to redirect, then this data will reside on the delta disk. There is no negative impact to FVP on either option chosen. However it’s a best practice to help control the growth of the delta disk between refreshes, and so separating the non-persistent disk will help alleviate bloated delta disks.
There is an Internal Disk that is created with each cloned desktop. This disk is Thick Provision Lazy Zeroed, with a default size of 20mb. This disk stores Sysprep, QuickPrep and AD account information, so very little IO will be realized from this disk. Keep in mind that this disk is not visible in windows, but it still has a SCSI address, so FVP will still recognize the disk and accelerate any I/O that comes from this disk. This is another advantage of being a kernel module, as FVP will recognize disks not mounted to the windows OS and yet FVP will still do its magic of acceleration.
As you can see no matter the configuration, FVP will automatically capture all I/O from all disks that are part of a given desktop clone. Depending on the configuration, a desktop clone can have several disks and knowing when or which disks are active or are need of given resources at any given point, is not an easy task to determine. This is exactly why PernixData decided to develop FVP, a solution to take the guesswork out of each disks IO profile. This means the only item you are tasked with, is whether you accelerate the desktop or not! Talk about seamless and transparent, it doesn’t get any better than that!!