1. The File Manager Implementation issues
This lecture:
From UFID to the physical storage
Data structures
File allocation
Reading: Bacon 7.5
13 August 2019
CS2051 Lecture 14

2. The File System is Layered
Application
fread(…), fopen(…), ...
Logical File System
Directory structures.
File-organisation
Links logical (user) view with physical view
Basic file system
Issues instructions to the right device driver
I/O control
Device drivers + interrupt handlers
Physical device
We consider hard disks only!
13 August 2019
CS2051 Lecture 14

3. Data Structures File metadata (Bacon) or File Control Block:
file size, permissions, owner, group, dates, ...
Information about where data on disk can be found.
Active file table in memory:
each entry contains
metadata for a file currently in use & number of readers/writers.
index to the table is the UFID
(Unix: file descriptor, Windows NT: file handle)
“open file” creates entry in the file table
“close file” writes metadata to disk
(assumes file is not used by anyone in the system)
Implementations vary: a per process table may exist too!
13 August 2019
CS2051 Lecture 14

4. Operating System Data Structures
Process A
Process B
O.S. 1 2 3 1 2 3
(file pointers)
System open file table
Active file information table
Buffer cache - disk blocks that have been recently read or written
Metadata
File Storage (files, directories, free blocks)
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CS2051 Lecture 14

5. File System Calls
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CS2051 Lecture 14

6. open(“/usr/ar/timetable”, O_RW) create(“/usr/ar/timetable.1”, 0755)
Find Metadata for root directory
read / directory to get SFID for /usr
find Metadata for /usr directory
read /usr directory to get SFID for ar
find Metadata for /usr/ar directory
read /usr/ar directory to get SFID for /usr/ar/timetable
read /usr/ar/timetable Metadata into Active File Information table
create System Open File table entry
find free entry in per-process table
return UFID
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CS2051 Lecture 14

7. read(4, &buf[0], 8192)
find System Open File table entry - get file offset
find Active File Information table entry
check permissions
calculate first file block number(s)
read file blocks into block cache
copy required data into user space
update position pointer in System Open File table entry
close(4)
release entry in per-process table
release entry in System Open File table
in Active File Information table:
if number of readers and writers is now zero,
write Metadata back to disk
release Active File Information table entry
13 August 2019
CS2051 Lecture 14

8. File Allocation Methods
Contiguous allocation
File occupies contiguous blocks on disk
Extent(s) allowed (Bacon, extent lists)
Linked Allocation
File is a linked list of data blocks (Bacon, chaining in the media)
Linked list is saved separately as a block
DOS, File Allocation Table (FAT) (Bacon, chaining in a map)
Indexed Allocation
Each file has a table of pointers to its blocks (Bacon, table...)
Unix, inodes (multilevel indexing)
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CS2051 Lecture 14

9. Contiguous Allocation
File …
start length
1 3
6 2
Pros: provides very good performance.
Cons:
hard to predict needs (cf. with memory allocation strategies); fragmentation; can’t eliminate all seeks if there are interleaved user requests.
Provide a way of splitting the file using extents.
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CS2051 Lecture 14

10. Linked Allocation FAT 1 1 2 FAT 2 3 3 4 File 1: …start at 4
FAT 1 1 2
FAT
1. eof 6. 3
3. eof 8. 6 2 3 3 4
File 1: …start at 4
File 2: …start at 5 4 5 5 6 6 7 7 8 8 9 9
1st approach: each block has a pointer to the next
Pros: no fragmentation, easy to extend a file
Cons: random access is difficult; if a block is corrupted the rest of the file is lost.
2nd approach: (DOS) File Allocation Table (FAT)
Provides a solution to the above cons (good to cache the FAT) 10 10
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CS2051 Lecture 14

11. Indexed Allocation 1 2 File1 Index Block: 9 3 4 5 6 7 8 9 101
Index block 9:
1. 4
2. 2
3. 8
4. 6
5. 3
6. NULL
7. NULL
8. NULL 2
File1
Index Block: 9 3 4 5 6 7 8 9 10
Pros: Facilitates random access to files
Cons: Too much space (for NULL pointers) may be wasted
The Unix approach: provide some indirect pointers too
13 August 2019
CS2051 Lecture 14

12. Conclusion
File System Interface provides a user-friendly view of the file system.
File System Implementation deals with file allocation and storage management.
Performance Issues
use disk cache for frequently accessed blocks
efficiency dependent on algorithms/types of data
Recovery
consistency checking (chkdsk in MS-DOS)
keep backups!
13 August 2019
CS2051 Lecture 14