Chapter 3. Planning 71
3.12.1  Queue depth
Typically, hosts issue subsequent I/O requests to storage systems without waiting for 
completion of previous ones. The number of outstanding requests is called queue depth. 
Sending multiple I/O requests in parallel (asynchronous I/O) provides significant performance 
benefits compared to sending them one-by-one (synchronous I/O). However, if the number of 
queued requests exceeds the maximum that is supported by the storage controller, you 
experience performance degradation.
Therefore, for large storage networks, plan for setting correct SCSI commands queue depth 
on your hosts. For this purpose, a large storage network is defined as one that contains at 
least 1000 volume mappings. For example, a deployment with 50 hosts with 20 volumes 
mapped to each of them is considered a large storage network. For more information about 
the queue depth calculations, search for “Queue depth in large SANs” at this IBM Knowledge 
Center web page.
3.12.2  Offloaded data transfer
If your Windows hosts are configured to use Microsoft Offloaded Data Transfer (ODX) to 
offload the copy workload to the storage controller, consider the benefits of this technology 
against extra load on storage controllers. The benefits and effects of enabling ODX are 
especially prominent in Microsoft Hyper-V environments with ODX enabled.
3.13  Host mapping and LUN masking
Host mapping is similar in concept to LUN mapping or masking. LUN mapping is the process 
of controlling which hosts have access to specific LUs within the disk controllers. LUN 
mapping is typically done at the storage system level. Host mapping is done at the software 
level.
LUN masking is usually implemented in the device driver software on each host. The host has 
visibility of more LUNs than it is intended to use. The device driver software masks the LUNs 
that are not to be used by this host. After the masking is complete, only some disks are visible 
to the operating system. The system can support this type of configuration by mapping all 
volumes to every host object and by using operating system-specific LUN masking 
technology. However, the default (and preferred) system behavior is to map only those 
volumes that the host is required to access.
The act of mapping a volume to a host makes the volume accessible to the WWPNs or iSCSI 
names such as iSCSI qualified names (IQNs) or extended-unique identifiers (EUIs) that are 
configured in the host object.
3.13.1  Planning for large deployments
Each I/O Group can have up to 512 host objects defined. This limit is the same whether hosts 
are attached by using FC, iSCSI, or a combination of both. To allow more than 512 hosts to 
access the storage, you must divide them into groups of 512 hosts or less, and map each 
group to single I/O Group only. This approach allows you to configure up to 2048 host objects 
on a system with four I/O Groups (eight nodes). 
For best performance, split each host group into two sets. For each set, configure the 
preferred access node for volumes presented to the host set to one of the I/O Group nodes. 
This approach helps to evenly distribute load between the I/O Group nodes.