VDI + Windows PageFile Done Right!

This is an interesting article about the use of page files in Windows based VDI environments.

Windows pagefile configuration for VDI environments has been a long standing discussion ground. I first wrote about it in 2010 (Pagefiles and VDI. Not so simple). However, because of few recent discussions I decided to review the subject and run some validation tests against well-known best practices. “The problem with best practices is that they around stay for longer than the technology they were based on.”

This validation is rather simple; one Windows 7 virtual desktop configured with pagefile, and one Windows 7 virtual desktop configured without pagefile. Besides pagefile, the virtual desktops are configured with exact same setting. In fact, both desktops originate from the same Parent VM. (Windows 7, 2GB RAM, 1GB Pagefile)

Additionally, for this test I have enabled Linked Clones. The reason for that is that with Linked Clones I am able to analyze individually the base image and the delta utilization.

Myth Number One – The Windows boot

The first myth is that during boot time a Windows virtual desktop with pagefile will produce more write IOs against virtual disks than a virtual desktop without the pagefile. The graphs below demonstrate that during boot storms read IOs are very similar. However during the boot without pagefile there was 27% less write IOs. On average number of write IOs is smaller in virtual desktops with pagefile.

Virtual Desktop with Pagefile during Boot


Virtual Desktop without Pagefile during Boot


Myth Number Two – The IDLE time

Many administrators think that the existence of pagefile automatically means that there will be more write activity, even when Windows is idle. The truth is that there is no such thing as idle time, since Windows is always running processes in the background and those processes are in most part write IO intensive. This is the reason why Windows operating systems can be as much as 80% writes during an average workload.

The graphs below demonstrate that virtual desktops during idle time with or without pagefile behave in a very similar fashion in regards to disk IO. For write IOs both virtual desktops had a maximum of 3 IOPs and an average of 0.23 IOPs.

For read IOs there was a little spike in the virtual desktop without pagefile, but that could have been a process that kicked off during the analysis period. It didn’t happen again.

What the graph below proves, is that when there is no memory pressure both virtual desktops will behave very similarly in regards to disk IO during the idle period.

Virtual Desktop Performance with pagefile during IDLE


Virtual Desktop Performance without pagefile during IDLE


Myth Number Three– The User Login

User login process generates different type of activity then boot and idle periods. Logins are as write intensive as read intensive, and that is clearly demonstrated on the average numbers when we compare read and write IOs.

For user login I ran tests multiple times. Below you will find one of the tests that represent the resultant set of all tests. The virtual desktop with pagefile enabled required less read IOs than the desktop without pagefile; on average 20% less read IOs.

On the other hand, the virtual desktop without pagefile required approximately 4% less write IOs during user login process.

The results look very similar, but the read IO difference at peak time could represent almost 16,000 IOPs in an environment with 1,000 virtual desktops. From an infrastructure perspective that could represent the requirement to add more spindles of even an additional storage array.

Most storage arrays nowadays have some type of cache to serve read IOs; some have features to serve write IOs from cache. However, write IOs cost more than read IOs due to the RAID striping mechanism that needs to occur for every write IO. The 3 IO difference at peak time would represent 3,000 IO for 1,000 virtual desktops. After RAID penalty is applied we would have 12,000 write IOs when using RAID5; or 18,000 write IOs if using RAID6.

Virtual Desktop with Pagefile during Login


Virtual Desktop without Pagefile during Login


If my workload was purely made up of Logins, than I probably would choose not to have pagefile, just to alleviate write IO pressure on storage. Please, bear in mind that this is applicable to any type of shared storage, SSD or PCIe appliance or DAS implementation.

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