Specifications

62 Optimizing IBM Netfinity Servers for SAP R/3 and Windows NT
Figure 30. Set BIOS Compatibility Mapping
5. Press Esc twice to go in the configuration main menu.
Changing the RAID Parameters
There are two RAID parametersthat allow you to fine tune the RAID subsystem: s
the
Stripe Unit Size and the read ahead option.
Each of these options affects all arrays and all logical drives configured on the
adapter.
The Stripe Unit Size is the amount of data written to a given disk before writing to
the next disk in the array. The available choices are 8 KB, 16 KB, 32 KB and 64
KB. The default is 8 KB. If you set the stripe unit size to be 32 KB or 64 KB, you
can have at most eight disk drives per array. If you set the stripe unit size to 8 KB
or 16 KB, you can have at most 16 drives per array. The default is set to 8 KB.
This parameter should be chosen so that the stripe unit size is close to the size of
the system I/O requests or a multiple of it. This will depend on the type of
applications and modules you are running, cache strategies of file and database
systems, and the purpose of your server.
The read ahead option determines the way in which the ServeRAID adapter
processes I/O requests from the operating system. When read ahead is enabled,
the RAID adapter transfers data from a disk to its local cache in steps of stripe
unit size. This provides excellent overall performance when workloads tend to be
sequential. However, if the workload is random and system I/O requests are
smaller than stripe unit size, reading ahead to the end of the stripe unit will result
in a wasted SCSI bus bandwidth and wasted disk utilization. When read ahead is
disabled, the size of data transfer from the disk to the local cache is equal to the
Once data is stored in the logical drive, the stripe unit cannot be
changed without destroying data in the logical drives.
Data Will Be Lost