Paravirtualized and Hardware Emulation - differences

[Dmitry@Rivermine]

Hi, I was wondering if you have a quick writeup on the differences between those two. Are there any benefits on using one mode over the other? As I understand this class configuration parameter was introduced in 2.3.

Thank you.

[Dmitry@Rivermine]

Please? ..........

[PeterNic]

Dimtry, sorry for the delay.

Paravirtualized mode works only for OS that know they are being virtualized and instead of trying to access the hardware directly, they make calls to the hypervisor. The pros are that they are usually faster than hardware emulation and work even on CPUs that don't provide hardware assisted emulation (all new Intel and AMD server CPUs do). The cons are that the OS has to support paravirtualization -- not all OS do. Linux (any flavor with 2.6 kernel) and OpenSolaris are the two OS that support paravirtualization under AppLogic.

Hardware emulation mode is for OS that don't know they are being virtualized. The advantage of this mode is that it doesn't require a special version of the OS -- you can run the same system that runs on a regular physical server. The disadvantages are (a) works only on CPU with hardware-assisted virtualization, (b) may have a performance penalty (the jury is out on this; modern hardware-assisted virtualization is pretty good); and (c) for good I/O performance, the OS still requires special paravirtualization drivers.

Currently, the only HVM-mode OS AppLogic 2.3.9 supports is Solaris 10 (Sun offers paravirtualization drivers for the I/O and, with them, the I/O gives quite a decent performance). Other OS should work as well. The 2.3.9 beta does not provide a self-service way to install an OS, so using your own distro requires some assistance from technical support -- this will be eliminated in an upcoming release.

The main point is, don't tweak this setting -- the setting documents whether the disk image contains a paravirtualized kernel or not; you should not change that setting on an existing appliance -- it will most likely not work if the setting doesn't match the disk image (we may add a warning message in the UI if we don't have one already).

Best regards,
-- Peter

[Dmitry@Rivermine]

Thanks! This explains a lot. I wish you'd just add a big red note in the documentation saying 'DO NOT TOUCH' :)