1.6.7 Recommendations
•Use IDE on smaller systems
IDE actually outperforms SCSI on systems where only one or two devices are
attached. Several models of the IBM PC Server 300 and 320 lines implement
IDE as an integrated controller on the planar board. This is more than
adequate if no more than a couple of hard disks will be used.
•Distribute the workload on large systems
Research has shown that a single 66 MHz Pentium processor doing
database transactions needs as many as 6-10 drives to optimize system
performance. Therefore, do not determine the number of drives you need by
simply adding up your total storage requirements and dividing this by the
capacity of your drives. Instead, distribute the disk intensive workload from
a single physical disk drive to multiple disk drives and use the striping
features of RAID technology.
1.7 LAN Subsystem
The LAN adapter is another important component in the file server design.
While there are many different types of LAN adapters, for file servers they fall
into two main categories: bus master and shared RAM (non-bus master). The
following discussion centers on the benefits of using bus master LAN adapters,
although for small, lightly loaded LANs, non-bus master LAN adapters are quite
adequate.
1.7.1 Shared RAM Adapters
Shared RAM adapters derive their name from the fact that they carry on-board
RAM that is shared with the system processor. The memory on the adapter card
is mapped into a reserved block of system address space known as the upper
memory block (UMB) area. The UMB area is reserved for I/O adapters and is
addressed between the addresses of 640KB and 1MB. The server processor can
access this memory in the adapter in the same manner in which it accesses
system memory.
Shared RAM can be 8, 16, 32, or 64KB in size depending on which adapter is
used and how it is configured. Adapter cards with 64KB support RAM paging
which allows the system to view the 64KB of memory on the card in four 16KB
pages. This scenario only requires 16KB of contiguous system memory instead
of the 64KB required when not using RAM paging. All IBM NetBIOS products
support RAM paging.
The starting address of the shared RAM area is determined by the adapter
device driver, switch settings, or in the case of an EISA or MCA adapter, via the
setup utility or the reference diskette, respectively.
The main disadvantage of shared RAM architecture is that any data movement
between the shared RAM area and system memory must be done under direct
control of the system′s CPU. This movement of data to and from the shared
RAM must be done because applications cannot operate on data while it resides
in the shared RAM area. To compound matters, MOVE instructions from/to the
shared RAM are much slower than the same MOVE instruction from/to the
Chapter 1. IBM PC Server Technologies 31