Citrix XenDesktop hits the VDI high notes
Citrix XenDesktop 2.0 leverages streaming applications, server virtualization, and swift tools for a scalable and manageable virtual desktop infrastructure solutionFollow @pvenezia
The Provisioning Server is also the key to managing all the write caching that occurs during user sessions. Write caching is an important aspect of Citrix's VDI infrastructure. When a user logs into a session on a VM, changes made to the OS itself are not written to the VM, but to a write cache that lives on a shared LUN or other shared storage medium. This allows the user to make changes that are not retained when the VM is rebooted. This maintains the integrity of the VM and is also helpful in reducing the chances of malware infiltrating the infrastructure. If something is amiss, simply reboot the VM to a known-good state. The Provisioning Server is relatively smart, and is capable of not only provisioning new desktop VMs but also adding them to the AD domain on the fly.
Citrix estimates that a single physical Provisioning Server can handle between 350 and 500 simultaneous XenDesktop users.
The Desktop Delivery Controller is just like it sounds – it manages user access to desktop VMs. Pools of desktops can be defined and linked to specific Active Directory groups. In this way, you could give all HR users a desktop VM with 512MB of RAM and a specific CPU share, while all Engineering users get a desktop with 1,024MB of RAM and a more powerful CPU. Obviously, you could also deliver Windows XP to one group and Vista to another.
The DDC is also where time-based resource management is handled. It's possible to create rules that will keep a minimum number of desktop VMs ready and waiting for a login between 8:30 a.m. and 9:30 a.m., and then reduce the number of idle VMs throughout the rest of the day, finally keeping a small number active after business hours. This reduces the load on the VM infrastructure and helps handle the morning ramp-up. It's also possible to assign desktop VMs to specific users, rather than pooling them for a group of users.
The transient nature of VDI requires some method of delivering per-user profiles to the desktops, which is generally handled via roaming profiles, much like in the traditional terminal server world. Windows administrators have typically weathered bristly relationships with roaming profiles, but the reality is that they're not going away, and their benefits outweigh their detriments, at least for now.
However, Citrix is heading toward a better world after licensing code from Sepago to address profile management issues. Citrix will be leveraging sepagoPROFILE to hopefully ease this particular burden in the future.
XenDesktop's management methods also allow for quick updating of VMs, as it's possible to modify the baseline VM image that will then be used to boot all new VMs. You can update that image in the middle of the day, and every VM will pick up those changes the next time it reboots.