Linux Virtulization Options (or to KVM or not to KVM?).

[kentborg-KwkGvOEf1og at public.gmane.org (Kent Borg)]

Mick T wrote :
> And if anyone has done any (or come across any) performance
> comparisons between KVM and VMWare and any other virtualization
> technologies? 

For raw compute power, KVM is damn near as fast as running on raw hardware.

Last I looked IO performance is less for virtual machines. From what I
understand the Linux kernel and libraries are very well optimized for
speed, specifically: great effort has been made to not do any extra data
buffer copies. You know, pass pointers. Unfortunately virtual machines
need an extra data copy, both coming and going, and disk IO is fast
enough that a buffer copy is noticeable--depending on how you measure
it, disk access is half as fast as raw hardware. I think there are
efforts to avoid this, but it seems a tricky issue (I think the approach
is give the actual hardware to a guest--which means not being quite so
virtual anymore), and I don't think fixes for this are primetime yet.

I think lots of RAM is again your friend, disk caching always helps.
There also might be situations where a RAM disk makes sense and in that
case a RAM disk could be nearly as fast as raw hardware.

Obviously running virtual machines also wants to have lots of fast disk
space.


In a previous job I was planning on setting up two matched 1U machines
in a KVM-based configuration that would have been completely redundant
(doubly redundant in that each machine would have had mirrored SW raid 1
disks, redundant power supplies in each box, ECC RAM, etc.). The only
single-points-of-failure being: (1) a crossover cable between the two
boxes and the associated NICs, (2) the cabinet and larger facility where
the two machines would live, and (3) the common software and
administration. The design is capable of doing live migration of VMs
between machines, and therefore everything is effectively hot replaceable.

Unfortunately I never got the go-ahead to buy the ~$15K of hardware and
now I hear they have laid off everyone but a skeleton staff, I don't
know whether anyone is currently managing the decaying old hardware for
which they had no spares...


-kb