Re: [Xen-users] Memory overcommitment in Xen.

[Carl Schneider <carl.schneider+mailinglists@xxxxxx>]

> How Memory overcommitment work in Xen? It is automatically or must be enabled?

Overcommitment is not a feature, but a state of the system. Basically,
things like Ballooning allow you to take away Memory from a domain,
while it "thinks" that it still has it available (From within the
domain, it looks like the balloon driver allocates a big chunk of
memory, as far as I know).
So, if you have e.g. a system with 8GB of memory, and start up 4 domains
(including dom0) that have an maxmem of 2GB, you are not overcommiting,
but use up all your available memory.

Let us assume that the domains now just needs 1GB, so the ballooning
driver gives some memory "back" to the hypervisor. Now it is possible to
start another domain, but the sum of all maxmems is now bigger than 8GB.
This state is called overcommitment.

Note that this is not only possible with Memory, but also with disk
storage (LVM thin provisioning, e.g.).

My opinion about this: If you do not know the workload of your system
and do not understand the problems of this exactly, do not over-commit
too much and keep an eye on your system. I do not have any experience
with the problems of over-committing, so I can not estimate the risks of it.



Note that I do not have any noteworthy experience with ballooning or
overcommitting, so if I made a mistake, please correct me.
It may also be possible that I am just plain wrong, too. (Take this as a
disclamer)

Cheers
CRT

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