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LUN sizes and RAID
When sizing LUNs/volumes for VDI, don't focus on performance rather than capacity to ensure that your LUNS can provide the required IOPS. There truly is no magic number for LUN sizes as many factors come into play. Generally, the more spindles you have in the RAID group that makes up your LUN the better. You also shouldn't size your LUNs too small for the number of virtual desktops you'll have on them. Whether or not you're using full virtual disks or linked clones will also influence sizing as the latter requires much less disk space.
|Use linked clones to save storage|
Linked clones can be an invaluable feature
to use in a virtual desktop infrastructure (VDI) environment. Linked clones work by having a single master virtual machine (VM) that holds an image of the base operating system the desktops will use. All virtual desktops read from this image with any writes captured in a separate delta file created for each VM. Delta files are typically small, although they can grow if every disk block was written to -- but that's unlikely to happen. Linked clones can be periodically refreshed to include patches and operating system and application updates. Linked clones offer clear advantages, but they can be more complicated to maintain than full disk images.
You have a range of RAID options to achieve either better protection or better performance. A key factor that will influence your RAID choice is the read/write ratio of your virtual desktops. When reading data from a RAID group there's no I/O penalty associated with the RAID overhead, but there's an I/O penalty when writing. The more protection you want, the more it will cost you in I/O penalties. For example, RAID 1 has an I/O penalty of two as writes have to be written to both drives; with RAID 5 it increases to four and for RAID 6 it's six. If your I/O workloads will involve more writing than reading, you want to use a RAID level that has less of a penalty when writing. Having a larger write cache in your array controller or using a custom RAID level like NetApp's RAID-DP can also help.
SAS drives offer better performance but SATA drives can lower storage costs. Fast 15K drives can speed things up but at an increased cost compared to 10K drives. Solid-state drives (SSDs) offer blazing performance but have a hefty price tag. Choosing drives to handle virtual desktop infrastructure workloads usually comes down to buying the best drives you can afford. Slower performing SATA drives typically aren't desirable for most VDI workloads, so SAS drives are a better choice.
The platters of a 15K drive read and write data faster, and overall latency is reduced, but the head actuator that moves across the drive to access data doesn't. So even if the drive is spinning 50% faster, overall performance increases by approximately 30%, which results in higher IOPS.
You can mix and match drive types to provide faster storage where needed and use cheaper, slower storage for less demanding workloads. You might store the master disks for linked clones on fast SSD storage and the delta disks on SAS storage. You could take this a step further and use an automated tiering application to automatically balance workloads based on demand.
This was first published in March 2011