Virtual desktops may make management easier, but proper planning is needed to reduce storage bottlenecks, ensure performance and accommodate growth. Storage subsystems can ease VDI deployments, whose costs can balloon if you don't follow best practices for supervision. In this third segment of a four-part e-book, we'll help you understand storage allocation for virtual desktop environments.
Disks and RAID for VDI
Choosing storage for VDI deployments involves a variety of variables. A main consideration is the choice of disk and disk subsystem, and administrators can select from high-end, midrange and low-end disks. Fibre Channel disks are available at the high end, providing exceptional performance and significant IOPS (I/O operations per second), but the disks are relatively small and expensive. Serial-Attached SCSI (SAS) disks occupy the midrange and provide a good balance among performance, capacity and expense. Serial Advanced Technology Attachment (SATA) disks usually fill the low end with large inexpensive disks but are lacking when performance demands are high.
If RAID is also implemented in the disk subsystem, administrators will need to consider the tradeoffs involved with RAID 1+0 versus RAID 4, RAID 5 and RAID 6 (dual parity), as well as vendor-specific RAID versions such as RAID-DP for NetApp arrays, MetaRAID in EMC Clariion or vRAID in HP EVA arrays. Ultimately, desktop virtualization will work with any disk type or RAID class as long as the storage system can provide the necessary storage capacity, IOPS performance and resilience for the size of VDI deployment.
Larger deployments demand more capacity, performance and resilience. However, disk choices are not as critical for early or limited deployments. "A lot of desktops that are being deployed today don't have the performance characteristics and the workload that requires such a demand on high-performance, high-capacity storage," said Enterprise Strategy Group's Bowker. "With the initial deployments we see, I think capacity and cost per gigabyte are going to be a lot more important than the performance metric."
Thin provisioning and VDI
There are also several storage technologies that should be considered when planning VDI deployments. Thin provisioning is a means of creating a logical disk space that is actually larger than the amount of physical storage assigned to it. The idea stems from the notion that an application does not use all of the space assigned to it right away, but once space is allocated, no other applications can use that space. The result is that "unused" space is essentially paid for and wasted until it's actually used (if ever). With thin provisioning, it’s possible to create a LUN, but only assign a fraction of that actual disk space to start, and then add more physical space to the LUN as needed.
For example, it's possible to create a 100 GB LUN, but only provide 10 GB of storage upfront. As that initial 10 GB fills, an administrator can add another 10 GB or 20 GB (up to the 100 GB size limit). "If your standard user is allocated 20 GB of storage, that's going to cost an awful lot over a thousand users -- it's 20 TB," said Silverton Consulting's Lucchesi. "With thin provisioning, you might get by with half of that."
The challenge with thin provisioning is the need for careful storage management. Applications have no way to know the difference between the logical limit and the actual disk space available, so it's possible to run short of space and encounter serious write errors for virtual desktops relying on that space. Regular storage monitoring and alerts will help prevent thin-provisioning problems.
Deduplication and VDI
Data deduplication can also save enormous amounts of storage space. The technology works by identifying and removing redundant data blocks and replacing them with simple placeholders. As a simple example, consider a virtual desktop environment where 50 users have the same 10 MB report in their user data stores. That's 500 MB.
"Deduplication is not going to be as impactful with an operating system image…but from a user data perspective, online data deduplication of user data would be very important to me," Bowker said. "I want to know how that storage system can actually deduplicate that data and save from a capacity perspective."
By removing redundant copies of that data and pointing to one working copy of that data left on disk, the amount of storage needed to hold redundant information is slashed. The same principle can be used to remove redundancy in other storage like snapshots, golden images and all elements of the organization's storage, not just VDI storage.
Deduplication is handled within the storage array itself -- it's not a virtualization feature, and desktop virtualization software like VMware View or Citrix XenDesktop won't know that deduplication is even in place. Deduplication, however, can affect storage performance. For deduplication to help VDI storage requirements, the process has to take place in real time on data that is actually being used. Deduplication that takes place on archival or backup data is not helpful for VDI environments.
Snapshots and VDI
Snapshots take a point-in-time copy of a LUN. They're a proven technology that has become invaluable in server virtualization as a means of protecting virtual machines in operation. The snapshot can then be used to restore corrupted or non-operational virtual machines (VMs) or to create clones of VMs for new servers. Desktop virtualization can also take advantage of the snapshot features included with many storage arrays by cloning virtual desktops and providing those snapshots for new VDI users. For example, the snapshot would be a read-only file, and any changes to that desktop would be written elsewhere in storage for that user. It affords administrators a means of quickly and conveniently deploying new desktops without having to create images from scratch.
In the final segment of a four-part e-book, we'll help you understand how VDI storage affects your bottom line.
This was first published in July 2011