High Availability Storage Options and Their Impact on Performance

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MPE Performance Environment
Lets spend a little time painting the picture of an environment where MPE was best designed to give
optimum performance. We will break this part up into a number of categories: memory, connection
technology and Ldev (spindle count/size). The order of the categories also indicates its impact on
performance, where the fastest access is memory and the slowest is disk. We will then go through the list
of available array products for the HP e3000.
Memory
The greater the amount of memory for the HP e3000 the more efficient it will run (to a point). MPE uses
available (server) memory as a cache to store user process information and also keeps a virtual copy of the
file. If there is enough memory to cache the user process information and data then MPE does not have to
wait for disk access when making changes to the file until either the user makes an explicit call to post-
and-wait or lack of memory causes MPE to push that data from memory to make room for other processes
that need to execute. The amount of memory needed depends on an understanding of the application’s
needs, the number of users and the amount of available disk space. This information can be supplied by
the application’s vender or with the help of an MPE performance specialist/consultant.
Connection Technology
The HP e3000 server comes in two flavors, an older NIO based machine, such as the 9x7, 9x8 and 9x9
and a newer PCI based machine such as the A-Class and N-Class machines. The NIO based machines are
limited to HVD-SCSI (High Voltage Differential) but can use the A5814 Router to connect to newer HP
fibre channel storage arrays.
Without playing too much with numbers, an NIO based machine can usually pump out about 12Mbytes
per second of data through the backplane and HVD-SCSI HBA (Host Based Adapter) card while the
theoretical limit of this SCSI specification is 20Mbytes. This sounds like a definite bottleneck compared
to fibre channel speeds but remember, performance is based on all the components (memory, connection
technology and disk) working at best case and disks are still a mechanical device limited to mechanical
speeds.
Another connection technology is the HVD-SCSI to Fibre Channel Fabric Router. This device connects to
an MPE (or HP-UX) HVD-SCSI HBA and converts SCSI to fibre channel (1Gbit) for connection to the
newer HP fibre channel portfolio of storage arrays. The router solution has been thoroughly tested and has
not been shown to increase any performance overhead by its presence as a connection solution.
The last connection technology for the HP e3000 is that of native fibre channel. This technology is only
available on PCI based servers, usually referred to as the A-Class and N-Class, running MPE/iX version
7.5.
This fibre channel technology is rated at 2Gbits per second (theoretical limit). This translates down to
about an effective rate of 200Mbytes per second but let me caution you right now. This speed limit or
bandwidth is no guarantee that the disks can keep up with this limit. Using a car story analogy (again),
while you can go 65mph on the freeway you still have to pull into your driveway and wait for the garage
door to open before you can enter.