VCDX or Bust

Explain DRS / storage DRS affinity and anti-affinity rules

There are two types of DRS rules:

• VM to Host rules
  • Allows you to specify rules for whether a particular VM will or won’t run on a particular host or set of hosts
  • Alows for Must rules and Should rules
    • Must rules will never be violated by DRS, DPM or HA – if the VM needs to power on and there’s no host available allowed by the rule then the VM will not power on.
    • Should rules will be respected unless there is no option – if the VM needs to power on and there’s no host available allowed by the rule then the VM will power on and DRS will seek to move the VM to an acceptable host as soon as one is available.
• VM to VM rules
  • Used to keep VMs either on the same host or on separate hosts, canonical use cases for these are either to respect software licensing arrangements or to provide for High Availability respectively.

There are three different types of Storage DRS rules:

• Inter-VM Anti-Affinity (Also known as VM Anti-Affinity)
  • Prevents virtual machines from residing on the same datastore within a datastore cluster
  • Maximises availability of a set of redundant VMs
• Intra-VM Anti-Affinity (Also known as VMDK Anti-Affinity)
  • Prevents specific virtual disks associated with a VM from residing on the same datastore within a datastore cluster
  • Canonical use case: Useful for separating log and database disks of database VMs
• Intra-VM Affinity
  • (The Default) – Keeps a particular virtual machine’s virtual disks together on the same datastore
  • Maximises VM availability when all disks needed in order to run

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Tune Virtual Machine memory configurations

• Use shares, limits and reservations to apply prioritisation policies to your VMware environment that reflect your requirements for what happens if memory contention occurs on an ESXi host
• Ensure VMware tools is installed and up-to-date on all guest VMs, this will help ensure that VMware is able to properly manage memory usage from within the guest VM.
• Use an appropriate location for the Host cache (SSD is the canonical example)

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Compare and contrast virtual and physical hardware resources

• Physical hardware resources provide the necessary platform on which to build virtual hardware resources
• Physical hardware resource are extremely specific, virtual hardware resources are abstract
  • Contrast a physical Intel X520-T2 10GB NIC or a Broadcom BCM57710 NIC vs a virtual VMXNET3 NIC – the two physical hardware adapters need very specific drivers, configuration etc. The VMXNET3 NIC is the abstracted virtual representation of any underlying physical NIC.
• Physical hardware resources are essentially finite and they are bounded by whatever hardware is installed and available to the hypervisor.
• Virtual hardware resources are less finite and can be overcommitted (at the risk of contention and performance problems) far in excess of the actual hardware available to the hypervisor.
  • An ESXi host with 64GB of RAM might easily be able to support multiple guest VMs which in aggregate have a multiple of 64GB RAM assigned to them
  • An ESXi host with just two 10GB NICS can be configured to support dozens of guest VMs all with multiple virtual NICs with all network traffic fairly sharing the available physical resources

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Tune ESXi host memory configuration

VMware 5.x has five memory management mechanisms:

1. Page Sharing
2. Ballooning
3. Memory Compression
4. Swap to Host Cache
5. Regular Swapping

If an ESXi host is experiencing Regular Swapping that is indication that the VMs running on the host will be experiencing memory related performance problems. To confirm if a host is actively swapping memory follow these steps:

1. Navigate to the Hosts and Clusters view in vSphere client
2. Select the host and click the performance tab
3. Select the advanced view and click chart options
4. Under chart options, select Memory / Real-time
5. Select the Swap in rate and Swap out rate counters
6. Click Apply, OK

A non zero value for either of these counters indicates that the host is actively swapping.

An ESXi host that is actively swapping can be examined using esxtop to discover which (if any) VM guests are experiencing performance problems related to this swapping. The %SWPWT counter indicates the percentage of time that the guest is waiting for swapped pages to be read back from disk.

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Identify appropriate BIOS and firmware setting requirements for optimal ESXi host performance

The VMware vSphere 5.0 Performance best practices guide http://www.vmware.com/pdf/Perf_Best_Practices_vSphere5.0.pdf has some things to say about BIOS and firmware settings:

• Make sure you are running the latest version of the BIOS available for your system
• Enable “Turbo Boost” in the BIOS if your processors support it
• Enable hyper-threading support in the BIOS for processors that support it
• Some NUMA capable systems provide an option in the BIOS to disable NUMA by enabling node interleaving, in most cases you will get best performance by disabling node interleaving.
• Make sure any hardware assisted virtualisation features (VT-x, AMD-V, EPT, RVI etc.) are enabled in the BIOS.
• Disable from the BIOS any devices you won’t be using.
• Cache prefetching mechanisms (sometimes called DPL prefetch, Hardware prefetched, L2 streaming prefetch or Adjacent Cache Line Prefetch) usually help performance, especially when memory access patterns are regular. When running applications that access memory randomly, however, disabling these mechanisms might result in improved performance.
• If the BIOS allows the memory scrubbing rate to be configured, VMware recommends leaving it at the manufacturers default setting
• In order to allow ESXi to control CPU power saving features set power management in the BIOS to “OS Controlled Mode” or equivalent.

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