Eric's Technical Outlet

Learning the hard way so you don't have to

Monthly Archives: November 2011

Page File Disappears After a Reboot on a Hyper-V Virtual Machine

Symptom: You add a page (swap) file on a VM’s disk besides its C: drive, but at each reboot, it disappears.

Fix: Shut the VM down and move the VHD with the page file from the virtual SCSI chain to the virtual IDE chain. Start the VM. If the page file wasn’t created that time, double-check the settings. Another reboot of the VM may be required.

Cause: Hyper-V guests try to initialize the page file ahead of loading the virtual SCSI driver.

Performance impact: Effectively none. The IDE driver is emulated but highly optimized. The SCSI driver is synthetic, which is why it loads when it does. The two types have been extensively stress-tested and the performance differences can only be seen in the most extreme of edge cases. If a VM’s page file is getting hit that hard, you desperately need to increase the RAM allotment of the VM. For a very minor performance gain, you can put the page file’s VHD on the secondary chain so that the VM doesn’t alternate I/Os between it and the emulated C: drive.