Making LUN Decisions for VMFS datastore

A proper planning is required to set up storage for ESXi system before formatting the LUNs with VMFS datastores. Keep in mind the following considerations when deciding the LUN:

• Each LUN must contain only one VMFS datastore
• If multiple VMs access the same VMFS, use disk shares to prioritize VMs.
• Each LUN should have the correct RAID level and storage characteristic for the applications running in VMs that use the LUN.

Large LUNs can be preferred for the following regions:

• Fewer VMFS datastores to manage.
• More flexible for resizing virtual disks, doing snapshots, and so on.
• More flexible to create more VMs

Smaller LUNs can be preferred for the following region:

• Less wasted storage space.
• Different applications might need different RAID characteristics.
• More flexible as multipathing policy and disk shares are set per LUN.
• Use of Microsoft Cluster Service requires that each cluster disk resource is in its own LUN.
• Better performance because there is less contention for a single volume.

It’s not a simple method to determine the number and size of LUNs to provision, when the storage characterization for a VM is not available. You should experiment using a Predictive or Adaptive scheme.

Predictive Scheme is that where you provision several small LUNs with different storage characteristics and then you create a VMFS datastore on each LUN. Create virtual disks on appropriate (RAID) VMFS datastore according to application recommendation.

Adaptive Scheme is that when you provision less Large LUNs (RAID 1+0 or RAID 5), with write caching enabled, create VMFS datastore on that LUN, create multiple virtual disks on the VMFS datastore, run your applications if performance is acceptable you can use this scheme.

Thanks to VMware, Information is from the white paper provided by VMware.