Redis under virtualization
Adrián Navarro
Until now I've been running redis under a vmware esx system. The
memory allocation provided by esx swaps some memory to ssd disks and
as redis is allocating a lot of memory there is a bottleneck that
causes redis to be slow and use a lot of cpu.
Now I'm moving to a dedicated server but I'd like to keep using
virtualization (easier on backups plus I can use different
environments on the same machine). I have to choose between Xen
(Citrus) or OpenVZ (Proxmox).
Does anyone have any experiences in this topic?
Thank you,
A.
Javier Guerra Giraldez
> I have to choose between Xen
> (Citrus) or OpenVZ (Proxmox).
While I have only used KVM and LXC on production so far, I've tested
and analyzed Xen (back in 3.1 days) and OpenVZ, so i think i can clear
up a little bits:
About performance:
- Xen is the most 'hard' hypervisor of all these, meaning that what
you allocate for a VM, you will get. the performance is very solid
and constant once you get it right. Apart from disk and network
bottlenecks in extreme cases, load on a VM won't affect another one.
- OpenVZ (and LXC) have the lowest overhead and most flexibility in
terms of resource allocation. it's usual to give lower and upper
bounds for CPU, RAM, I/O, etc. That means you can have higher
utilization ratios; but also that in peak times you'll get less than
what you'd like.
about swapping:
- Xen doesn't swap, the hosted OS will. it's common to provide two
virtual block devices for each VM, one for the filesystem and one for
swapping. If IO bandwith is well controlled, you won't get surprises.
- OpenVZ runs all VMs on a single kernel, and swapping is done at that
level. You lose fine control from within a single VM of which
processes will be swapped, but since there's no other 'lower' level
swapping, you won't find crucial kernel structures suddenly being
paged out. (like is so common on VMWare settings where you allow it
to swap VM's memory). Also, since the single kernel has a total
picture of memory usage, it can better optimize the whole set of
resources.
in the end, i think you'll be far better served with either than with
VMWare. the choice between them is more of personal taste: if you
want to set it up and be sure that it will stay without hiccups, go
with Xen and sleep peacefully. if you want to intelligently adapt
resource allocation to a wide range of changing scenarios, try OpenVZ
and keep a close eye on how it reacts.
--
Javier
Adrián Navarro
I've chosen xen, mostly because I've already used it before…
Right now redis is taking 25-30% of the cpu (a lot less than over 100%
with esx):
552 service 20 0 483M 374M 812 R 25.0 36.7 4h13:34
/usr/local/bin/redis-server /etc/redis.conf
The vm has 2 vcpus and the load avg inside the VM ranges between 0.30
and 0.50. The redis server is very responsive (and that's awesome!)
but I'm still wondering if it is normal to have such cpu load.
Other (almost idling) vms are okay (0.00) and *surprisingly* the dom0
(host) is really really calm:
03:02:01 up 13:11, 1 user, load average: 0.01, 0.00, 0.00
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Adrián Navarro
outside of the htop preview… nice!
I've also remarked that memory has jumped from 200mb to 470mb, but I
think I still can deal with that.