IBM 170 Servers Manual page 232

In the test results that follow, we see the CPU required for IBM i operating system Virtual SCSI
server and the benefits of the IBM i operating system Virtual SCSI implementation should be
assessed for a given environment. Simultaneous multithreading should be enabled in a virtual
hosted disk environment. For most efficient virtual hosted disk implementation with larger IO
loads, it may be advantageous to keep the IBM i operating system Virtual SCSI Server partition
as a dedicated processor. Processor micro partitioning should be used with low IO loads or with
workloads which are not latency dependent.
Virtual storage can be created in an ASP using the CRTNWSSTG and linked using the
CRTNWSD commands. The disk can be manipulated in the client AIX or Linux partition the
same as an ordinary physical disk. Some performance considerations from dedicated storage are
still applicable when using virtual storage, such as spreading ASP's across multiple disks on
multiple RAID adapters so that parallel access is possible. From the server's point of view, a
virtual drive can be served using an entire ASP, or a portion of an ASP. If the server partition
provides the client with a partition of a drive, then the server decides the area of the drive to
serve to the client when the network storage space is created.
This allows reads and writes of an ASP to be shared among several virtual devices. If the entire
ASP is served to the client, then the rules and procedures apply on the client side as if the drive
were local
Consider the following general performance issues when using virtual SCSI:
Only use virtual hosted disk in low I/O loads
Virtual hosted disk is a client/server model, so the combined CPU cycles required on the I/O
client and the I/O server will always be higher than local I/O
If multiple partitions are competing for resources from a virtual hosted disk server, care must
be taken to ensure that enough server resources (processor, memory, and disk) are allocated
to do the job.
There is data read caching in memory on the Virtual hosted disk Server partition. Thus, all
I/Os that it services could benefit from effects of caching heavily used methods. Read
performance can be improved by increasing the memory in the virtual hosted disk server.
IBM i 6.1 Performance Capabilities Reference - January/April/October 2008
©
Copyright IBM Corp. 2008 Chapter 14 DASD Performance 232