Friday, May 9, 2014

Storage DRS and Drmdisk

Storage DRS and Drmdisk

Storage DRS leverages a special kind of disks for facilitating a more granular control over initial placement and migration recommendations. It also plays a major role in I/O load balancing by using these deeper details.

Let us read about it in detail:


vSphere Storage DRS uses the DrmDisk construct as the smallest entity it can migrate. A DrmDisk represents a consumer of datastore resources. This means that vSphere Storage DRS creates a DrmDisk for each VMDK file belonging to the virtual machine. A soft DrmDisk is created for the working directory containing the configuration files such as the .VMX file and the swap file.

  • A separate DrmDisk for each VMDK file
  • A soft DrmDisk for system files (VMX, swap, logs, and so on)
  • If a snapshot is created, both the VMDK file and the snapshot are contained in a single DrmDisk.

VMDK Anti-affinity Rule

When the datastore cluster or the virtual machine is configured with a VMDK-level anti-affinity rule, vSphere Storage DRS must keep the DrmDisk containing the virtual machine disk files on separate datastores.

Impact of VMDK Anti-Affinity Rule on Initial Placement

Initial placement immensely benefits from this increased granularity. Instead of searching a suitable datastore that can fit the virtual machine as a whole, vSphere Storage DRS can seek appropriate datastores for each DrmDisk file separately. Due to the increased granularity, datastore cluster fragmentation—described in the “Initial Placement” section—is less likely to occur; if prerequisite migrations are required, far fewer are expected.

Impact of VMDK Anti-Affinity Rule on Load Balancing

Similar to initial placement, I/O load balancing also benefits from the deeper level of detail. vSphere Storage DRS can find a better fit for each workload generated by each VMDK file. vSphere Storage DRS analyzes the workload and generates a workload model for each DrmDisk. It then determines in which datastore it must place the DrmDisk to keep the load balanced within the datastore cluster while offering sufficient performance for each DrmDisk. This becomes considerably more difficult when vSphere Storage DRS must keep all the VMDK files together. Usually in that scenario, the datastore chosen is the one that provides the best performance for the most demanding workload and is able to store all the VMDK files
and system files.

By enabling vSphere Storage DRS to load-balance on a more granular level, each DrmDisk of a virtual machine is placed to suit its I/O latency needs as well as its space requirements.

Virtual Machine–to–Virtual Machine Anti-Affinity Rule

An inter–virtual machine (virtual machine–to–virtual machine) anti-affinity rule forces vSphere Storage DRS to keep the virtual machines on separate datastores. This rule effectively extends the availability requirements from hosts to datastores. In vSphere DRS, an anti-affinity rule is created to force two virtual machines.

For example,

Microsoft Active Directory servers—to run on separate hosts; in vSphere Storage DRS, a virtual machine–to–virtual machine anti-affinity rule guarantees that the Active Directory virtual machines are not stored on the same datastore. There is a requirement that both virtual machines participating in the virtual machine–to–virtual machine anti-affinity rule be configured with an intra–virtual machine VMDK affinity rule.

Thoughts ?

vSphere Storage DRS is one cool piece of code and is continuously improving how we use traditional storage systems for better.

Always good to dig and understand the VMware vSphere features in detail.  Until next time :)

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