Page 176 - DCAP403_Operating System

Unit 9: I/O & Secondary Storage Structure

9.5 Kernel I/O Sub-system Notes

I/O services provides by kernel are:

1. Scheduling
2. Buffering
3. Caching
4. Spooling

5. Device reservation
6. Error handling
7. I/O protection

I/O Scheduling

1. Some I/O request ordering via per-device queue
2. Some OSs try fairness – no one application receives poor service
3. To support asynchronous I/O, it must keep track of many I/O requests at the same time

(a) Use device-status table

Buffering
Store data in memory while transferring between devices

1. To cope with device speed mismatch

Example: File received via modem for storage on the hard disk via double
buffering (next slide shows the differences in device speeds)
2. To cope with device transfer size mismatch
(a) Common in computer networking
3. To maintain “copy semantics”

(a) The version of data written to disk is guaranteed to be the version on the time of the
application system call, independent of any subsequent changes in the application’s
buffer.
Caching

Fast memory holding copy of data
1. Always just a copy
2. Key to performance

Spooling

A buffer hat holds output for a device
1. If device can serve only one request at a time
2. Useful for printers and tape drives

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