Chapter 8. 11/23/09. IO System. Storage Must be dependable Networks Must tolerate faults in communications by including mechanisms to detect and recover form faults. Peripherals Extremely Diverse. I/O Systems. Emphasis is placed on dependability and cost.
The first two are easy to measure.
Average is open to wide interpretation because it depends on seek distance. The standard is
Sum of the time for all possible seeks divided by number of possible seeks.
Actual average may be considerably less.
Function of sector size, rotation speed and recording density.
30 – 80 MB/sec typical.
However, most disk controllers have a built-in cache that stores sectors as they are passed over. Resulting in higher transfer rates.
Today most disk transfers are multiple sectors lengths.
Controller time – Overhead imposed by controller in performing disk I/O.
Disk I/O time consists of the above times plus any wait time because other processes are using the disk.
Four stripes for sectors A, B, C, and D are shown.
Parity encoding is given by
Where Di represent a data byte in a sector on drive i.
If both sides of the above equation are exclusive ored with P, then
D5 for example can be recovered by
The Q parity is a Reed-Solomon code given by
Where is Galois Field (GF) multiplication and giis a constant. For i < 8 it turns out that gi = 2i. For larger i, it not as simple. For example g8 = 29.
But Q simplifies to
The problem is how to compute the GF multiplication.
See Xilinx application note XAPP731 “Hardware Accelerator for RADD 6 Parity Generation / Data Recovery Controller”.