1. CPSC 457 Operating Systems
Lecture 12
Devices and File Systems

2. Last Time Deadlocks Resource Allocation Detecting Deadlocks
Resolving Deadlocks
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3. This Time Devices File Systems Hosts and Controllers I/O Drivers
File System Interface
Directory Structures
File System Implementation
File Meta Data
File Allocation
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4. Devices
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5. Devices and Hardware Host the CPU / Memory and OS, ”the computer”
Controller
the electronics and logic that operate the device, port or bus
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6. Communication with Devices
Registers Data – In / Out Control – Status (what is the device doing) - Control (what to do)
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7. Drivers Interface Define how a type of device should behave Driver
Write the software to conform the device to the interface the OS expects
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8. Device Characteristics
aspect
variation
example
Data-transfer mode
Character
block
Terminal
disk
Access method
Sequential
random
Modem
DVD
Transfer schedule
Synchronous
asynchronous
Tape
keyboard
sharing
Dedicated
shareable
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9. Device Characteristics (2)
Device speed
Latency
Seek time
Transfer rate
Delay between ops
Electronic? - memory
Physical? - disk
Mechanical? – tape-robot
I/O direction
Read-only
Write-only
read-write
DVD
Graphics controller
disk
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10. Working with Controllers
Registers
Keep registers available to pass data can control between host and controller
Memory Mapped I/O
Map information from/for the controller directly into virtual memory
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11. Controlling Controllers
Polling
OS Loops and waits for the device to work
Interrupts
Device does its work and interrupts the host
Direct Memory Access
Device moves a lot of memory then interrupts
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12. Device Interactions Block Character Socket Asynchronous
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13. Devices and the Kernel
Scheduling Buffering Caching Spooling Error Handling
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14. File System Interface
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15. File System Interface
How do we think about files? How do we organize files? How do we work with files? What to we track about files
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16. Directories
Logical Construct to organize information Link the logical to the physical Tree Structures – not graphs
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17. File System Implementation
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18. File System Structure
Layered – abstract to specific Applications Logical File System File Organization Module Basic File System Disk Drivers
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19. File System Data – on Disk
Boot Control Block (per volume) Volume Control Block (per volume) Directory Structure (per volume) File Control Block (per file)
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20. File System Data – in Memory
Mount Table Directory Cache System-Wide Open File Table Per-Process Open File Tables Read/Write Buffers
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21. Actions on Files
Creating a new file Opening a file Reading, writing, seeking in, a file Closing a file
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22. Virtual File System
Unify File System Types in an Abstract Whole Provide a clean interface to the OS Manage details at the low level
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23. File System Allocation
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24. Contiguous Allocation
Files are laid out in a sequence of neighbouring blocks
Fast
Limited
free space available
Need to know the size of the file
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25. Linked Allocation
Provide reference to the next block in each block of the file – put blocks wherever in memory
Don’t need to know the size
Waste space on pointers
Harder to seek to the middle of a file
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26. Indexed Allocation Keep a block of indexes to the blocks of the file
Easier to seek to a point in the file
Possibly wasteful of space
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27. File System Considerations
Space Time Failure How do we Recover Consistency Checking Transactional Systems
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28. File Systems
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29. Next Time
Disks
How do we organize data on secondary storage to make sense and access it quickly
Review
Multiprocessor Environments
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