Chapter 10. Advanced features for storage efficiency 419
an increase in response times on volumes that use the same storage. Also, host formatting 
time is likely to increase, compared to a system that does not advertise support the SCSI 
Unmap command.
If you use Data Reduction Pools, thin-provisioned back-end that supports Unmap, or Flash 
Core Modules, it is recommended to turn on SCSI Unmap support. 
If only standard pools are configured and back-end is traditional (fully provisioned), you might 
consider keeping host Unmap support switched off because you do not see any 
improvement.
To check and modify current setting for host SCSI Unmap support, use the lssystem and 
chsystem CLI commands, as shown in Example 10-8.
Example 10-8   Turning host unmap support on
IBM_Storwize:ITSOV7K:superuser>lssystem | grep host_unmap
host_unmap off
IBM_Storwize:ITSOV7K:superuser>chsystem -hostunmap on
IBM_Storwize:ITSOV7K:superuser>
10.3.4  Offload IO throttle
Throttles are a mechanism to control the amount of resources that are used when the system 
is processing I/Os on supported objects. If a throttle limit is defined, the system processes the 
I/O for that object, or delays the processing of the I/O to free resources for more critical I/O 
operations. 
SCSI offload commands, such as Unmap and Xcopy, are used by hosts to format new file 
systems, or copy volumes without the host needing to read and then write data. 
Some host types might request large amounts of I/O on the storage system by issuing Write 
Same/Unmap commands. If the underlying storage cannot handle the amount of I/O that is 
generated, the performance of volumes can be affected. 
Spectrum Virtualize offload throttling limits the concurrent I/O that can be generated by such 
commands, which can prevent the MDisk overloading. This limits the rate at which host 
features, such as VMware VMotion, can copy data. 
Note: You can switch host Unmap support on and off nondisruptively on the system side. 
However, hosts might need to rediscover storage, or (in the worst case) be restarted. 
Note: For systems that fee managing any nearline storage, it might be recommended to 
set the offload throttle to 100 MBps.