Yellow Bricks

by Duncan Epping

BC-DR

VMTN Podcast number 71, vStorage API

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This weeks topic of the VMware Communities Roundtable was the vStorage API. It was most likely one of the most technical roundtables in months. Especially the part about the Virtual Disk Development Kit (aka VDDK) was educational! If you missed the episode you can download it here or subscribe to the podcast with iTunes here!

Here are the links that were dropped in the chat window for those who are interested:

SRM 4.0, Howto’s by Cormac Hogan

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One of my colleagues, Cormac Hogan, posted some excellent guides for SRM 4.0:

  1. Steps to setup VMware vCenter Site Recovery Manager 4.0 with IBM SVC
    VMware vCenter Site Recovery Manager 4.0 is a disater recovery product which uses array replication technologies to failover from one site to another. This particular document looks at one particular vendor (IBM) and one particular array model (SVC). The document will take you through the replication and snapshot setup steps.
  2. Steps to setup VMware vCenter Site Recovery Manager 4.0 with EMC Celerra NAS Replications
    VMware vCenter Site Recovery Manager 4.0 is a disater recovery product which uses array replication technologies to failover from one site to another. This particular document looks at one particular vendor (EMC) and one particular array model (Celerra). This document also focuses on configuring a new feature introduced in SRM 4.0, namely NAS (NFS) replictions. The document will take you through the replication and snapshot setup steps using only the command line interface (CLI).
  3. Steps to setup VMware vCenter Site Recovery Manager 4.0 with NetApp NAS Replications
    VMware vCenter Site Recovery Manager 4.0 is a disater recovery product which uses array replication technologies to failover from one site to another. This particular document looks at one particular vendor (NetApp) and uses their FAS simulator. However the steps are also applicable to their standard arrays. This document also focuses on configuring a new feature introduced in SRM 4.0, namely NAS (NFS) replictions. The document will take you through the replication and snapshot setup steps using only the command line interface (CLI).

Make sure to read them if you are implementing SRM in combination with one of the mentioned arrays!

SRM 4.0 Evaluator Guide

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For those starting out with SRM 4.0 the following documents/links might be very useful:

  • Site Recovery Manager 4.0 Evaluator’s Guide
    The Site Recovery Manager 4.0 Evaluator’s Guide is intended to provide Site Recovery Manager customers and evaluators a guide that walks them through the Site Recovery Manager workflow that has to be completed to allow for the successful and automated service failover from the designated Site Recovery Manager protected site to the designated Site Recovery Manager recovery site. It also provides an overview that includes the considerations and guidance to execute a failback of services from the recovery site back to the site that was originally designated as the Site Recovery Manager protected site. In addition, this guide covers the new Site Recovery Manager features – VMware vSphere support, NFS support, and shared recovery site. Evaluators can work through the exercises provided in this guide to gain a first-hand experience on operating the core and new features.
  • A Quick tour of SRM 4.0
    Now that SRM 4.0 is here it seemed like a good time to take quick tour around a few of the new features and also highlight some less obvious elements of the vSphere platform that make working with SRM 4.0 simpler

Slot sizes

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I’ve been receiving a lot of questions around slot sizes lately. Although I point everyone to my HA Deepdive post not everyone seems to understand what I am trying to explain. The foremost reason is that most people need to be able to visualize it; which is tough with slot sizes. Just to freshen up an outtake from the article:

HA uses the highest CPU reservation of any given VM and the highest memory reservation of any given VM. If there is no reservation a default of 256Mhz will be used for the CPU slot and the memory overhead will be used for the memory slot!

If VM1 has 2GHZ and 1024GB reserved and VM2 has 1GHZ and 2048GB reserved the slot size for memory will be 2048MB+memory overhead and the slot size for CPU will be 2GHZ.

Now how does HA calculate how many slots are available per host?

Of course we need to know what the slot size for memory and CPU is first. Then we divide the total available CPU resources of a host by the CPU slot size and the total available Memory Resources of a host by the memory slot size. This leaves us with a slot size for both memory and CPU. The most restrictive number is the amount of slots for this host. If you have 25 CPU slots but only 5 memory slots the amount of available slots for this host will be 5.

The first question I got was around unbalanced clusters. Unbalanced would for instance be a cluster with 5 hosts of which one contains substantially more memory than the others. What would happen to the total amount of slots in a cluster of the following specs:

Five hosts, each host has 16GB of memory except for one host(esx5) which has recently been added and has 32GB of memory. One of the VMs in this cluster has 4CPUs and  4GB of memory, because there are no reservations set the memory overhead of 325MB is being used to calculate the memory slot sizes. (It’s more restrictive than the CPU slot size.)

This results in 50 slots for esx01, esx02, esx03 and esx04. However, esx05 will have 100 slots available. Although this sounds great admission control rules the host out with the most slots as it takes the worst case scenario into account. In other words; end result: 200 slot cluster.

With 5 hosts of 16GB, (5 x 50) – (1 x 50), the result would have been exactly the same. To make a long story short: balance your clusters when using admission control!

The second question I received this week was around limiting the slotsizes with the advanced options das.slotCpuInMHz and/or das.slotMemInMB. If you need to use a high reservation for either CPU or Memory these options could definitely be useful, there is however something that you need to know. Check this diagram and see if you spot the problem, the das.slotMemInMB has been set to 1024MB.

Notice that the memory slotsize has been set to 1024MB. VM24 has a 4GB reservation set. Because of this VM24 spans 4 slots. As you might have noticed none of the hosts has 4 slots left. Although in total there are enough slots available; they are scattered and HA might not be able to actually boot VM24. Keep in mind that admission control does not take scattering of slots into account. It does count 4 slots for VM24, but it will not verify the amount of available slots per host.

To make sure you will always have enough slots and know what your current situation is Alan Renouf wrote an excellent script. This script reports the following:

Example Output:

Cluster        : Production
TotalSlots     : 32
UsedSlots      : 10
AvailableSlots : 22
SlotNumvCPUs   : 1
SlotCPUMHz     : 256
SlotMemoryMB   : 118

My article was a collaboration with Alan and I hope you find both article valuable. We’ve put a lot of time into making things as straight forward and simplistic as we possibly can.

SRM 4.0 KB Articles

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For those looking into implementing SRM 4.0 four KB articles have been released related to SRM 4.0: