Performance; Dual Hosting - IBM I VIRTUALIZATION - READ ME FIRST 7-9-2010 Manual

It is recommended to closely match the total size of the storage spaces for each client partition to
its initial disk requirements. As the storage needs of the client partition grow, additional storage
spaces can be dynamically created and linked to it on the host partition. On the client, the new
virtual disk will automatically be recognized as a non-configured drive and can be added to any
existing ASP. The only restriction to consider in this case is the maximum number of storage
spaces allowed per virtual SCSI connection for an IBM i client partition, which is 16. If more than
16 NWSSTGs are needed for a client LPAR, additional virtual SCSI connections can be created
dynamically in the HMC.

4.2. Performance

As described in section 2.2, disk I/O operations in an IBM i virtual client partition result in I/O
requests to the physical disk adapter(s) and drives assigned to the host partition. Therefore, the
best way to ensure good disk performance in the client LPAR is to create a well-performing disk
configuration in the host LPAR. Because the host partition is a standard IBM i partition, all the
recommendations in the Performance Capabilities Reference manual
(http://www.ibm.com/systems/i/solutions/perfmgmt/resource.html) will apply to it. Use the
manual's suggestions for maximizing IBM i disk performance for the type of physical storage used
in the host, whether it is integrated disk or SAN.
Note that if only the System ASP exists on the host partition, NWSSTG objects are created on the
same physical disk units as all other objects. If the host partition is running production
applications in addition to providing virtual storage to client partitions, there will be disk I/O
contention as both client partitions and IBM i workloads in the host send I/O requests to those
disk units. To minimize disk I/O contention, create storage space objects in a separate ASP on
the host (Independent ASPs are supported). Performance on the client(s) would then depend on
the disk adapter and disk configuration used for that ASP. If the host partition is providing virtual
storage to more than one client partition, consider using separate ASPs for the storage space
objects for each client. This recommendation should be weighed against the concern of ending
up with too few physical disk arms in each ASP to provide good performance.
Disk contention from IBM i workloads in the host LPAR and virtual client LPARs can be
eliminated if a separate IBM i LPAR is used just for hosting client LPARs. An additional benefit of
this configuration is the fact that an application or OS problem stemming from a different workload
on the host cannot negatively affect client partitions. These benefits should be weighed against:
•
The license cost associated with a separate IBM i partition
•
The maintenance time required for another partition, such as applying Program
Temporary Fixes (PTFs)
•
The ability to create well-performing physical disk configurations in both partitions that
meet the requirements of their workloads
If the host partition runs a heavy-I/O workload and the client partitions also have high disk
response requirements, it is strongly recommended to consider using a separate hosting partition,
unless separate ASPs on the host are used for storage space objects. If the host partition's
workload is light to moderate with respect to disk requirements and the client partitions are used
mostly for development, test or quality assurance (QA), it is acceptable to use one IBM i partition
for both tasks.

4.3. Dual hosting

An IBM i client partition has a dependency on its host: if the host partition fails, IBM i on the client
will lose contact with its disk units. The virtual disks would also become unavailable if the host
partition is brought down to restricted state or shut down for scheduled maintenance or to apply
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