IrNico
Contributor
Contributor

Memory reservation on a large scale

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Does anyone use ESX memory reservations for almost every VM?

If yes, why and how many VMs?

Thanks

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AntonVZhbankov
Immortal
Immortal

Actually VMware ESX is not AIX, and as already was said, memory reservation should be used only if host memory is oversubscribed (som of VM memory > host physical memory).

Memory reservation affects HA also.

The only reason to use memory reservation is to guarantee certain critical services that VM won't go into swap and they will have physical memory anytime, but this is not really common situation. I have only 2 such VMs out of 150.


---

MCSA, MCTS, VCP, VMware vExpert '2009

http://blog.vadmin.ru

EMCCAe, MCITP: SA+VA, VCP 3/4/5, VMware vExpert http://blog.vadmin.ru

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FredPeterson
Expert
Expert

Setting reservations per VM should really only be done if you are over subscribing host memory as the purpose of memory reservations is to guarantee physical memory.

If you are doing this regularly and on a large scale, you may want to consider using resource pools instead of trying to manage resources per VM.

if you do not plan to over-subscribe memory, there is no reason to use reservations. If you're concerned about resource contention otherwise, then using share allocations may be more appropriate.

jimmvm
Enthusiast
Enthusiast

NO, NO, NO

And, YES, YES, FredPeterson is correct.

Memory reservations should only be used when applications will benefit.

If you are NOT aware of true application requirements and choose to reserve memory

as a convenience measure, you will potentially may create adverse memory management

problems in a cluster - I have seen this in a large customer configuration.

Example:

Clusters contain 10 ESX hosts with large memory foot print (64G each)

Resource pools contain VMs with reserved memory (over 20G per resource pool).

As VM workloads change, conditions change in the cluster and periodically

cause degradation in VMs. This will prompt you to want to adjust VMs. In adjusting VM

configurations regularly, the problem is exasperated.The result is an impact on DRS and/or memory management with the,

continuos change in memory requirements in the cluster and the reserved memory in multiple VMs, Resource pools and clusters.

The reserved memory has to be regularly factored in.

In VMware corporate architectures the prevailing practice is to NOT reserve resources arbitrarily.

Jimm

Message was edited by: jimmvm

IrNico
Contributor
Contributor

Thanks for the answer.

If I understand it correctly, is the use of memory for every VM not a VMWare Best practice.

When you look at other environments, for example AIX on Power, the IBM best practice there is to guarantee every lpar (=VM) a certain amount of memory and when they need (temporary) more they get memory from a shared pool.

Why the VMware best practice differs from the IBM best practice? Has this to do with Power technology, or ....?

Thanks

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AntonVZhbankov
Immortal
Immortal

Actually VMware ESX is not AIX, and as already was said, memory reservation should be used only if host memory is oversubscribed (som of VM memory > host physical memory).

Memory reservation affects HA also.

The only reason to use memory reservation is to guarantee certain critical services that VM won't go into swap and they will have physical memory anytime, but this is not really common situation. I have only 2 such VMs out of 150.


---

MCSA, MCTS, VCP, VMware vExpert '2009

http://blog.vadmin.ru

EMCCAe, MCITP: SA+VA, VCP 3/4/5, VMware vExpert http://blog.vadmin.ru
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