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Smallest Usable KVM VPS?
I came across a ServerFault post comparing OpenVZ to KVM, and the respondent said something very insightful:
It's extremely important to note that KVM requires a running kernel inside the VPS, whereas OpenVZ runs containers using a shared kernel. This means that a 256MB KVM VPS does not actually have the same amount of memory available to userspace that OpenVZ does.
I was always under the impression that you don't see tiny (128M, 256M, etc) KVM VPS because the overhead for the provider was too great compared to LxC or OpenVZ, but apparently it's the user who ends up with less "bang for their buck" so to speak.
Do you find this to be true in practice? Do users not prefer a tiny KVM VPS because you have a higher overhead leading to less usable resources? I spun up a quick 2G KVM server with Debian 10, and this is the output of free -m:
# free -m
total used free shared buff/cache available
Mem: 1997 70 1661 4 265 1784
Swap: 0 0 0
Compare this to a similar 2G OpenVZ server running the Debian 10 template:
# free -m
total used free shared buff/cache available
Mem: 2048 20 1966 9 60 2017
Swap: 512 0 512
While I can't say for certain, I would assume this slight disparity would render a 128M KVM server worthless as far as doing anything, and a 256M KVM server as nearly worthless along those same lines. Thoughts?
Comments
RAM cost is negligible this days compared to other expenses. The biggest cost for a small VM is IPv4, and that is the reason why we do not see those small VMs often, cost difference from 128MB to 512MB is only a few cents.
In a technical sense, KVM being a virtual machine indeed has a bit more resource overhead for the provider compared to OpenVZ which is a container in a shared system, but all things considered it is not very relevant except for some special cases like heavy overselling of very small containers.
128MB is "doable" with a Debian minimal install and some severe constraints. 256MB is small, but there is way more leeway, I could run my current personal server within 256MB of RAM + 512MB of SWAP as a safeguard if I wanted to. 512MB of RAM would be enough to run Debian + MariaD + database comfortably.
If you want to use a RHEL family distro, 52MB would be the lowest somewhat reasonable minimum while 1GB is very much required for doing anything practical.
2GB DDR4 ECC is the new lowend period.
A couple of Linux distributions claim to work fine even with basic X apps in 128Mb (https://en.wikipedia.org/wiki/Trisquel#Trisquel_Mini and https://en.wikipedia.org/wiki/Tiny_Core_Linux for instance) so without needing the overhead of a GUI environment running simple server processes should OK in 128 or 256. Running in KVM should be the same as on bare metal in that regard, that environment naturally has a swap partition but you can define a swap file in a KVM guest too. You will likely find things faster unless the host is badly oversold as it'll have a better CPU and better IO subsystem then an old tin box.
Just don't expect to run anything large, or anything quickly. A DNS server, a small MTA, a simple VPN endpoint or other relay - there are several things that don't need much RAM. On something that small you might end up being NAT or IPv6 only these days given the cost of IPv4 addresses, unless you are paying enough to get a bigger VM anyway.
I have a 128MB kvm from virmach, could only allow to run some proxy service and nginx.
X86 kernel could be much more better for this situation.
It really doesn't make much practical sense to have less than 512M RAM on a KVM server these days (or let's say 384M RAM as a bare minimum)
There are those who like to make a sport out of trying to run (e.g.) Debian on a server with as little RAM as possible, but the great majority of us just don't see the practical point of this exercise
I used to have 512mb from time4vps for backups and I have installed vestacp on it
FWIW: I just looked at my 128 MB Alpine 3.x VM and it has about 80 MB of free memory which could be driven up to about 100 MB. That's easily good enough to run a name or mail server or even a (somewhat modest) web server.
The problem isn't 128 MB, the problem is that it's increasingly hard and at the same time decreasingly rewarding to find a 128 MB KVM VPS which in part is due to the fact that RAM cost (in that league) is a very small factor vs. e.g. and in particular IP cost. So you might be able to save yet another 50 cents (or €1 at most I guess) if, supposed you could choose you took a 128 MB VPS vs. a 256 MB one at about €10/yr. Or, looking from the other side, would it really be smart to save (highly likely even less that) €1/yr instead of getting double the memory for €1/yr "more"? I don't think so (and I still run some 32 MB systems (and even less) and love them).
Side note: with many low end VPS running on "stone-age" processors, memory is what allows you to get some performance out of them, so 'more memory' usually also translates to "much better performance'.