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XEN and OVZ and KVM, Burst, Swap, oh my!
Hello All,
Frequent lurker here, really appreciate all of the hosting company participation.
I'm an experienced linux admin (~30 servers virtualized), but we do VMWare, and as such I'm unfamiliar with some of the terms.
I'm in the process of transitioning my side clients to VPS and having trouble comparing plans between providers where the virtualization technologies and terms are different.
For the benefit of others, can someone go over some of the pro/cons of the different virtualization technologies and what terms are equivalent?
Things I'm trying to figure out:
Are burst and swap the same? Should I care at all about anything but guaranteed RAM?
Some providers list the # of CPUs allocated your VM, some say nothing.
Which virutalization technologies discourage/prevent over-subscription?
Does 1GB XEN=1GB KVM=1GB OVZ?
I've googled around, but everything seems "salesy", written like a research paper, or a religious war.
Is there a cliff note lying around that my google-fu hasn't picked up?
PLEASE, let's not create a religious war here.
Thanks to all, it's always educational.
Cam
Comments
we have balloon for Xen, KSM for KVM, and bla2pages for OVZ..
i personally use KVM just because they use native kernel not like the others.
No. OpenVZ VPSs limit memory based on how much is allocated (what stuff thinks it might need). This is usually 1.5-3x the real usage, but may be way higher in some cases. while KVM and Xen memory management is just like a real server's. If you give a KVM or Xen VPS 128mb RAM, it's just like putting a 128mb ram module in a dedicated server.
its better to try each and feel the difference
Burst is (usually) real ram alocated for a short time when your machine needs it. Swap is usually virtual ram on the disk. First one is much faster. Both should not be considered as extra ram you can use, providers will tax you as abuser if you use those for extended periods of time, burst because others may need it and swap because it slows down io on the node. However, in swap's case, it matters the number of real read/write operations, if it just sits there offloaded from ram because is not used, then it is ok.
What kind of virtualization, well, it depends.
OVZ is cheaper as it uses less resources but can have some limitations. Recently can do swap as well, but there are problems with some networking features used mostly for VPNs which have to be enabled in the host kernel and some providers dont do that.
Xen with HVM and KVM can emulate a whole machine and as such you can install any OS you do not depend on host for that. It uses more resources and is more expensive, but gives you more freedom.
For fast machines and hosting choose openvz, for custom builds and peculiar OSes use full virtualization. KVM is the hype now, personally i preffer xen for full virtualization, for everyday stuff openvz does the job.
M
That really depends on the provider. Some say it's only meant to be used for small bursts, others say that you can use it always.
where to find this info? tos?
That is not my usual experience and is also counterintuitive. Why would they not include that ram in the main ram when they make the offer if you can use it all the time ?
I had ocasional problems with machines out of control taking all the burst ram for extended periods of time and didnt get the dear jane letter, but maybe because it happens rarely.
I would say, to be on the safe side, take an offer with the ram you need and keep burst as a reserve for emergencies.
Some specify their policy in ToS. If it isnt there and you plan using the burst all the time, ask before, but I am pretty sure the answer will be, yes, you can use it for a short time, but dont make a habit out of it.
M