| |
Interface
Decisions & Implications
The
physical interface connecting the array members together, and the
connection between the array or RAID system and the host can have
a great impact on performance. The type of interface used is critical
to the performance.
For example, a RAID array constructed from hard disk drives ,ay
be attached to each other and the host systems by a number of different
interfaces: IDE, SCSI, Ultra-Wide SCSI,
Ultra2 SCSI, Fibre-Channel, SSA,
etc. each offers various benefits and expenses. To reach a proper
decision on the interface and type of device you will use, you need
to gather information on bus widths, interface choices, device types,
and draw up some data throughput comparisons.
- IDE
controllers offer little in the way of vast performance increases
due to the limitations in both the number of devices allowed per
bus, together with the strict limits on access.
- SCSI
controllers increase the number of devices to either 7 or
15 and allow multiple devices to communicate across the bus simultaneously.
When this multiple communication is used as part of a RAID system,
the combined data throughput of the drives is available to respond
to any single I/O request from the controller.
- Fibre
Channel can increase the number of permitted devices to 126
- all of which are allowed to communicate simultaneously although
in practical terms when using high data throughput devices this
would simply swamp the bus and introduce errors.
The ideal storage device and interface combination depends on the
environment. The number of drives on a bus should be in ratio to
the throughput of the interface. Simplistically for example, a SCSI
bus with a bandwidth of 80MB/sec can theoretically transfer
a maximum of 80MB/sec of data. This bus should therefore ideally
have the exact number of SCSI drives who's combined data transfer
rate equals 80MB/sec. If each drive is capable of transferring 10MB/sec
sustained, then eight of these drives would utilise the bus bandwidth
to the maximum. Nine drives would reduce performance, seven drives
would under use the available bandwidth. There are other overheads
such as interface control commands (see The
Pipeline Effect) to take into account but this simple rule of
thumb is quite effective in ensuring that the array is always near
to maximum performance.
Related
topics:
|
