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6 Dell PS Series Arrays: Advanced Storage Features in VMware vSphere | TR1066
creating the fault-tolerant virtual machine while the array completes the zeroing task in the background.
By offloading the block zeroing to the PS Series array, fault-tolerant virtual machines can be created much
faster.
2.3 Hardware-assisted locking
To protect Virtual Machine File System (VMFS) metadata, the hardware-assisted locking primitive provides
a more granular method than SCSI reservations. Previously, whenever a virtual machine powered on,
powered off, grew a thin provisioned virtual disk, or was moved with vMotion to another host, a SCSI
reservation lock would be issued by the ESXi host to the underlying volume of the datastore. This
prevented other hosts from also issuing a SCSI reservation to service a similar request. While SCSI
reservations are short-lived, the impact can be noticed when powering on a large number of virtual
machines simultaneously, as typically observed in a virtual desktop infrastructure (VDI) environment.
The hardware-assisted locking primitive resolves this by working with the PS Series SAN to lock only the
necessary blocks rather than the entire volume. This enables other hosts to perform similar operations
against that same volume at the same time.
2.4 Thin provision stun
The thin provision stun primitive is relevant for environments leveraging thin provisioning both on the
virtual machine VMDK as well as on the datastore volume on the array. When both forms of thin
provisioning are used together, the likelihood of an out-of-space condition, in which the thin provisioned
volume has insufficient free space to support the growth required by a thin provisioned VMDK, is greatly
increased.
When this occurs without the thin provision stun primitive, the volume is taken offline to preserve existing
data and prevent further write attempts. This is disruptive and can be destructive to the virtual machines
using the datastore. Because the underlying storage is unexpectedly inaccessible, eventually these virtual
machines will crash.
When this shortage of space occurs on a volume configured with the thin provision stun primitive, the
virtual machine that is attempting to grow its VMDK is stunned (but not taken offline), and the VMware
vCenter™ administrator is notified. By enabling the array to communicate through the SCSI storage stack
with the initiator, the array can inform vSphere when the utilization of volume space passes an in-use
warning limit threshold, set in the PS Series interface. This generates a datastore usage on disk alert within
vCenter in addition to the traditional warning on the array. Should the volume reach the second warning
mark within the PS Series interface (the maximum in-use space limit), or if there is insufficient free space
to permit the volume to grow, the array informs the initiator of the out-of-space condition rather than
take the volume offline.
vCenter then stuns the individual virtual machine that is attempting to grow its thin provisioned VMDK, and
display a warning message to the vCenter administrator. Only the virtual machines that are attempting to
grow their thin provisioned VMDKs are stunned; the other virtual machines on the datastore are
unaffected and continue to operate. This process requires the administrator to allocate additional space to