Monthly Archives: June 2015

VSAN not listed as a matching device when creating a SBP

Symptoms
When creating a virtual machine VSAN Storage Based Policy, you may experience one or more of these symptoms:

  • No matching Virtual SAN (VSAN) datastores are listed in the matching resources page of the Storage Based Policy creation wizard.
  • You are unable to create the storage based policy.

Cause
This issue may occur due to inconsistent data between the Storage Management Service (SMS) and the Storage Based Policy Manager (SPBM).

Resolution

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VSAN does not detect SSD with existing partitions

VSAN cannot use disks with existing partitions on them, this post describes how to delete partitions on a disk (in this case an SSD).

To obtain a list of disk device names that can be managed by partedUtil, run this command:

ls /vmfs/devices/disks/

Example of output under ESXi/ESX 4.0 and later:

mpx.vmhba0:C0:T0:L0 <– disk device
mpx.vmhba0:C0:T0:L0:1 <– partition 1
mpx.vmhba0:C0:T0:L0:2 <– partition 2
mpx.vmhba0:C0:T0:L0:3 <– partition 3
mpx.vmhba0:C0:T0:L0:5 <– partition 5
naa.60060160205010004265efd36125df11 <– disk device
naa.60060160205010004265efd36125df11:1 <– partition 1

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Disk Stripes vs. Failures to Tolerate

Number of Disk Stripes Per Object

This defines the number of physical disks across which each replica of a storage object is striped. This impacts read and write operations as below:

Since all writes go to SSD (write buffer), the value of an increased stripe width may or may not improve performance. This is because there is no guarantee that the new stripe will use a different SSD; the new stripe may be placed on a HDD in the same disk group and thus the new strip will use the same SSD. The only occasion where an increased stripe width could add value is when there are many writes to destage from SSD to HDD. In this case, having a stripe could improve destage performance.

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Objects and Componets in VSAN

One of the most important concepts to understand in VSAN is the notion of storage objects and components. Virtual Machines deployed on a vsanDatastore may have 4 different kinds of storage objects associated with it:

  • The Virtual Machine home or “namespace directory”
  • A swap object (if the virtual machine is powered on)
  • Virtual disks/VMDKs
  • Delta-disks created for snapshots. Each delta-disk is an object.

Of these four objects, the virtual machine namespace may need a little further explanation. All virtual machines files, excluding virtual machine disks, deltas (snapshots) and swap, reside in an area called the virtual machine namespace on VSAN. The sorts of files in here can be the .vmx, the .log files, .vmdk & snapshot deltas descriptors files and everything else one would expect to find in a VM home directory.

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