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

  • To: xen-users@xxxxxxxxxxxxx
  • From: Gordan Bobic <gordan@xxxxxxxxxx>
  • Date: Wed, 02 Jul 2014 07:27:26 +0100
  • Delivery-date: Wed, 02 Jul 2014 06:27:36 +0000
  • List-id: Xen user discussion <xen-users.lists.xen.org>

On 07/02/2014 12:29 AM, Adam Goryachev wrote:
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...).

Of course not all failures are the same. Data corruption due to lack of ECC RAM or just everyday disk sector failures is an issue.


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