Re: [Xen-users] Cheap IOMMU hardware and ECC support importance

[Adam Goryachev <mailinglists@xxxxxxxxxxxxxxxxxxxxxx>]

On 02/07/14 05:37, lee wrote:
> Adam Goryachev <mailinglists@xxxxxxxxxxxxxxxxxxxxxx> writes:
>
> > PS, this is my personal advantage of using virtualisation... If I need
> > n servers to run all my VM's, then with n+1 I can survive a failure of
> > a dom0 (albeit with a reboot of the VM's being run there at the
> > time). I still get to save on overall hardware requirements, as well
> > as being able to live migrate for scheduled hardware maintenance/etc.
> For how many servers do you need one standby?
>
> Wouldn't you be better off not using a standby but moving each of the
> VMs from a failed server to another one that still works?  One more or
> less VM on a number of servers probably won't hurt?  You could even plan
> for it and spare resources on existing servers for this, or enjoy things
> running faster than required until you need the additional resources for
> a migrated VM.
>
> You'd need one server less per so many servers.  The less servers, the
> less can fail.
Correct, mostly.

If I decide I need 4 dom0's to allow my VM's to work properly, then I 
will use 5 dom0's, but the 5th is not idle, I will assign some VM's 
there all the time. On failure, I re-balance the VM's from the failed 
dom0 to the remaining dom0's.
Essentially, as long as total RAM requirements of all VM's is less than 
the amount of RAM available on the remaining dom0's, then "service" can 
continue. This is one limitation of xen, (can't overcommit RAM), which 
limits the number of survivable dom0 failures.
Of course, performance may become degraded due to the over-provisioning 
of CPU (or RAM if it was possible).
Using this guideline, the minimum number of dom0 should be 2, but for 
me, I prefer a minimum of 3, assuming at least 3 VM's.
As such, I tend to buy "desktop" class machines for dom0, with single 
CPU, and not so much RAM, but just buy more of them. Instead of one 
server for $3000, I'll get 3 servers for $1000 each. I'm convinced the 
chance of all three desktop machines failing at the same time is much 
lower than the single server class machine failing (assuming dual power 
supply, dual CPU, etc...).
Regards,
Adam

--
Adam Goryachev Website Managers www.websitemanagers.com.au

_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxx
http://lists.xen.org/xen-users