Administrator Guide

Hardware acceleration with VAAI
VMware API for Array Integration (VAAI) allows you to:
● Offload storage operations from compute side to storage hardware.
● Shift I/O intensive operations of provisioning and creating a snapshot from hypervisor to metro node.
● Dedicate hypervisor memory and processing resources to other functions.
● UNMAP unused storage blocks from thin provisioned volumes. Thin support in metro node on page 23 Provides more
information on thin provisioning.
VAAI is implemented in metro node using four SCSI commands:
● “Compare and Write” offloads coordination of powering virtual machines (VMs) on/off, and moving them between
hypervisors.
● “WriteSame (16)” offloads writing same pattern of data, such as zeroing out blocks for disk initialization.
● XCOPY offloads copying data to and from the array through the hypervisor.
Enabling and disabling XCOPY using CLI provides more information on enabling and disabling XCOPY.
● UNMAP allows the hypervisor to reclaim deleted storage on thin-provisioned metro node virtual storage. See "Understanding
Thin Provisioning" for more information on thin-provisioned volume and UNMAP functionality.
Compare and Write
The CompareAndWrite (CAW) SCSI command is used to coordinate VMware operations such as powering-on/off VMs, moving
VMs from one ESX to another without halting applications (VMotion), and Distributed Resource Scheduler (DRS) operations.
CAW is used by VMware ESX servers to relieve storage contention, which may be caused by SCSI RESERVATION in distributed
virtual machine environments. CAW assists storage hardware acceleration by allowing ESX servers to lock a region of disk
instead of an entire disk.
ESX 5.0 servers use this strategy to increase the number of virtual machines that an ESX servers can host, and to increase the
performance of those virtual machines.
Support for CAW is enabled by default.
Enabling/disabling CAW
CAUTION: CAW can be enabled/disabled on metro node only by an Dell EMC Customer Support Representative.
VMware servers discover whether the CAW SCSI command is supported:
● During initial storage scanning
● When the VMFS3.HardwareAcceleratedLocking value on the ESX host is enabled (or toggled if it is enabled)
NOTE:
To toggle the value: In the vSphere client, toggle host > Configuration > Software > Advanced Settings >
VMFS3.HardwareAcceleratedLocking value to 0 and then 1.
If CAW is not supported or support is disabled, metro node returns CHECK CONDITION, ILLEGAL REQUEST, and INVALID
OP-CODE. The ESX server reverts to using SCSI RESERVE and the virtual machine operation continues.
Virtual machine operations may experience significant performance degradation if CAW is not enabled.
Metro node enables CAW to be enabled/disabled for all storage associated with metro node, using a single command. When
CAW is disabled on metro node, storage volumes, do not include CAW support information in their responses to inquiries from
hosts.
To mark storage CAW as disabled:
● VMFS3.HardwareAcceleratedLocking must be toggled, or
● Hosts may need to rescan their storage.
CAUTION:
Enabling/disabling CAW functionality supports exceptional situations such as assisting Dell EMC
Technical Support personnel to diagnose a problem. CAW is enabled by default and should be disabled only
by Dell EMC Technical Support.
Support for CAW can be enabled or disabled at two levels:
System Management
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