IBM SG24-4576-00 manual Recommendations, LAN Subsystem, Shared RAM Adapters

1.6.7Recommendations

∙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

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