Hyper-V and slow guest OS installation

re: Hyper-V and slow guest OS installation

Anthony — Thu, 06 Mar 2008 03:37:45 GMT

From what I've seen, it seems that the biggest bottleneck is the CD/DVD access speed. It's been one of my few serious gripes about hyper-V so far - if I'm pointing at an ISO it should be quite fast (OS installs via an ISO are very fast in VMWare). Even in a running guest OS, reading ISOs is unusually slow, and incurs significant guest CPU time.

re: Hyper-V and slow guest OS installation

Chris.Knight — Fri, 29 Feb 2008 03:38:50 GMT

So the upshot is to not bother with Hyper-V for OSes that don't have Integration Components?

Best to stick with Virtual Server / VMWare Server for these guests?

re: Hyper-V and slow guest OS installation

sj — Wed, 27 Feb 2008 21:30:40 GMT

You really only need to do an OS install once. After that you can simply copy the installed VM and run sysprep or newSID. Not even an issue.

Anyway what sys admin has time to sit and watch an install. The VM install is always waiting for me to come back as I'm doing other things

re: Hyper-V and slow guest OS installation

Ranjith Purush — Wed, 27 Feb 2008 17:54:20 GMT

If the speed of guest OS installation from an optical media is a concern, another option is to configure your VM [with emulated NIC] to boot off the network and then do an install via WDS (Windows Deployment Services). That is my preferred primary guest OS install mechanism.

re: Hyper-V and slow guest OS installation

Mike Dimmick — Wed, 27 Feb 2008 16:51:05 GMT

As to why the OS installation is slow, it's usually the case that the OS has to install using only the drivers available on its boot disc. There isn't much opportunity to load drivers for any devices that weren't known when the OS boot disc was built, and that certainly applies to Hyper-V's synthetic devices.

The one place that Windows allows drivers to be added is to load new storage bus drivers, by pressing F6 when Windows 2000/XP/2003 is loading or clicking Load Driver at the 'select volume for installation' prompt in Windows Vista or 2008. For Virtual Server 2005, if the guest OS hard drive is attached to the emulated SCSI controller, there is a virtual floppy you can attach to load the Additions SCSI driver by pressing F6. I would have expected this to be the case for Hyper-V too (note that Additions are now called Integration Components) but I haven't yet installed it.

re: Hyper-V and slow guest OS installation

Mike Dimmick — Wed, 27 Feb 2008 16:11:46 GMT

@MAJawed: because the guest's use of the hardware has to be shared with other guests and the host itself. The native driver for the device would expect to have full control of the hardware. The only way that Virtual PC/Virtual Server/Hyper-V can share the device is by intercepting the commands to the emulated or synthetic device and redirecting them to the host operating system, either in user- or kernel-mode APIs.

However, the drivers for the emulated devices are expecting to talk to real hardware. That means they're using instructions and physical memory areas that are banned from use by user-mode programs. Without a hypervisor, the processor raises a hardware exception which Windows turns into a software exception. Virtual PC or Virtual Server can then emulate the requested operation and dismiss the exception, allowing the guest driver to perform the next step. With hardware virtualization and a hypervisor, the processor instead calls the hypervisor directly, a much faster operation than the exception handling.

Installing the 'additions' drivers allows the communication between guest and host/hypervisor to be improved, but I believe the device is still emulated.

The new 'synthetic' devices have a much closer match to the Windows API so they effectively turn a guest I/O request directly into a host I/O request. This cuts out many of the steps where a high-level request is turned into lower-level requests by the guest, which then has to send many more requests through the exception/hypervisor channel.

You can get a 'cleaner' experience by using SCSI devices on the guest, as the SCSI interface is a better match to the OS file system API. I believe you can get an even better experience if the controller for your hard disks implements the SCSI protocol itself (RAID controllers for SATA drives tend to appear as SCSI adapters to Windows). I believe this is the function of the 'storage bus' driver, to inject I/O requests into the OS at a lower level.

If you want to improve your disk performance, you can avoid the file system overhead by using a raw physical disk. In Virtual Server 2005, this is done by creating a Linked Virtual Hard Disk. This does mean you need a separate physical hard disk per VM. If you do have a RAID controller it might be able to carve out a separate volume to present to the host OS from one or more physical disks.

However, if you're looking at doing this anyway, you should be aware of the physical characteristics of hard disks and how they behave when handling random and sequential I/Os. Basically the observed speed of a hard disk is governed by the disk head seek time, which is the reason that sequential I/O is far faster than random I/O. If you require sequential I/O speeds but you share the physical disk with something else doing random I/O, your sequential I/O performance is destroyed.

re: Hyper-V and slow guest OS installation

Keith Ward — Wed, 27 Feb 2008 15:59:40 GMT

Xepol, could you contact me about your experience with OS installs in Hyper-V? I'd like to talk to you about it. I'm at kward@1105media.com.

re: Hyper-V and slow guest OS installation

Xepol — Wed, 27 Feb 2008 15:38:23 GMT

Slow as molasses OS installs are a definite barrier to adoption.

Might want to seriously consider that before calling any decission "final".

re: Hyper-V and slow guest OS installation

Kjetil — Wed, 27 Feb 2008 10:35:39 GMT

Been testing windows 2003 and windows 2008 as hosts with the synthetic devices and it works great :-) Fun to experiment with in my (IT Pro) environment and considering moveing it into the test environment for software developers soon.

Tested to run Windows XP as a host os and it was horrible! :-( Simply useless so back to vmware server for the client os. Is there any plans to make xp work as a client?

re: Hyper-V and slow guest OS installation

MAJawed — Wed, 27 Feb 2008 09:41:20 GMT

what i would really like to see is a GUEST that is comparable in performance with the actual HOST.

Why does the VM has to use a pre-defined/fixed set of hardware drivers ??

why can't the VM use New/Actual device drivers of the hardware available ?