Operating Systems Lecture Notes
22 February 2012 • Disk I-O
Outline
- Disks
- Disks controllers.
- Disk drivers.
- Disk scheduling.
Disk Structure
- Bulk secondary storage, up to terabytes per unit.
- A logical array of disk blocks.
- Disk structure.
- One or more platters coated with magnetic media on both sides.
- A rack of read-write heads on arms.
- Disk media partitioned into tracks, and tracks into sectors, and
tracks through disks into cylinders.
- Sectors are small, 256 to 1k bytes.
- Rotations from 4,000 to 15,000 rpm.
- Rotational latency, seek latency, transfer delay.
Disk Controllers
- The electronics between the disk hardware and the rest of the system.
- Gradual function shift operating systems to disk controllers (and
device controllers in general).
- Error correction, encryption, super blocking.
- Implements the bus protocol: scsi, sata (serial advanced technology attachment), usb (universal serial bus).
- And disk arm scheduling.
- But there still is a place for os control.
- Higher-level matters not identified with individual disk I-O
operations.
- Priority (os access vs user access).
- Specific use (paging or swapping I-O vs general disk I-O).
- Higher-level matters not identified with individual disk I-O
operations.
Disk Driver
- Translate from the application API to the disk controller API.
-
read()→ SCIS (for example) commands.
-
- Implement other, os-specific resource parts.
-
open()has no counterpart at the controller interface.
-
- Moderate other differences between the applicaion resource and the implementing device.
Disk Scheduling
- Disks are sharable above the block read-write level.
- Multiple independent io requests.
- How to send requests to the disk to maximize disk throughput: the
disk-schedulng problem.
- FCFS scheduling: send requests to the disk in the order received at
the driver.
- pluses: easy to implement; minuses: absuses the mechanicals to get terrible throughput.
- Shortest seek-time next scheduling: the next request is the one
closest to the disk heads.
- pluses: better throughput than fcfs; minuses: starvation.
- Scan (elevator) scheduling: the next request is the one closest to
the disk heads in the direction of travel, which changes only at the
cylinder extremes.
- plusses: no starvation; minuses: longer wait times.
- Look-Scan scheduling: the next request is the one closest to
the disk heads in the direction of travel, directions changes when there
are no more requests in the current direction.
- plusses: shorter (potentially) wait times; minuses: potential long wait times (round trip).
- Circular scan scheduling: pick a direction, scan in that direction,
then return without io to start again.
- shorter wait times (half-round trip)
- Look circular scan scheduling: pick a direction, scan in that
direction, then return without io to start again.
- shorter wait times (half-round trip)
- FCFS scheduling: send requests to the disk in the order received at
the driver.
- Picking a scheduler.
- The best scheduling policy depends on circumstances (surprise!).
- For lightly loaded disks, SSTN scheduling performs best.
- For heavily loaded disks, a look variant of either scan works best.
Buffering and Caching
- Buffering
- Smooth out differences between application model and device reality.
- Caching
- Buffers that exploit temporal or spatial locality.
- Temporal locality: if you touched it now, you’re probably
going to touch it in
. - Spatial locality: if you touched location i, you’re probably going to touch i ± 1 too.
- File-system metadata: information about the file system.
- Temporal locality: if you touched it now, you’re probably
going to touch it in
- Buffers that last longer than one application’s use.
- Buffers that exploit temporal or spatial locality.
Summary
| This page last modified on 2011 October 9. |
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