Division Of Labor; The Compute Elements; The Ssdls; The I/O Processors - HP NetServer AA 4000 Reference Manual

Division of Labor

The Compute Elements

The SSDLs

The I/O Processors

Network Server Division
The compute elements and the I/O processors have very distinct
roles and therefore have different performance characteristics. If the
array was going to do nothing more than run Windows NT without
any applications, then the memory requirements are minimal. The
array can be functional with 64 MB of memory for each node. This
serves to prove that the AA 4000 software itself is not memory
intensive and does require a significant amount of server resources.
The compute element functions with just the CPU and the memory.
The server is sized for the maximum amounts of CPU and memory
required for the application that stays within the limits of the
memory support of the physical server and the fact that the HP AA
4000 software currently supports one CPU and a maximum of 2 GB
of system memory. The system will generate I/O requests, but they
are immediately intercepted by the HP AA 4000 software and
transferred to the IOP via the MICs, cables, and SSDLs. This creates
a very low overhead for I/O operations on the CE.
These two components are nothing more than I/O routers. By
requiring that the nodes be attached to specific ports, what little
software function there is in the SSDLs is easily maintained and does
not have to be concerned with any kind of routing scheme or mesh.
The SSDLs are for making sure the data is transferred from each CE
to both IOPs by simply providing a path. The HP AA 4000 software
is responsible for the integrity of the data between the nodes and
largely does this through checksums (EDCs) at the end of each
packet transfer.
Here a different kind of server activity takes place. The I/O
Processors do not run any applications other than simple
management software added by the customer. They are complex I/O
controllers that for lack of an easier solution happen to be running
Windows NT. They are not in lock-step with each other, but instead,
maintain disk synchronization through a HP AA 4000 software disk
partition on each logical disk the CE uses. These are not CPU or
memory intensive operations. As it turns out, the disk activity itself,
as in most optimized server environments, can be the bottleneck if
there is one.
CAUTION
It is possible to run applications on the IOPs. However, this
impacts the reliability of the overall system. HP strongly
recommends that other applications not be run on the IOPs
but will continue to support the array.
Ch 1: Architecture Overview and Terminology
1-9