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File Systems

Chapter 6

6.1 Files 6.2 Directories 6.3 File system implementation 6.4 Example file systems

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Long-term Information Storage

1. Must store large amounts of data

2. Information stored must survive the termination of the process using it

3. Multiple processes must be able to access the information concurrently. In short:

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Long-term Information Storage

Files: Good!

No Files: Bad!

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File Naming

Typical file extensions.

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File Structure

• Three kinds of files– byte sequence– record sequence– tree

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File Types: Text and Binary

(a) An executable file (b) An archive

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File Access• Sequential access

– read all bytes/records from the beginning– cannot jump around, could rewind or back up– convenient when medium was mag tape

• Random access– bytes/records read in any order– essential for data base systems– read can be …

• move file marker (seek), then read or …

• read and then move file marker

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File Attributes

Possible file attributes

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Example

Assume all counters are currently 0.

Consider the case when pages 0,2,4, and 5 are referenced between last interrupt.

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File Operations

1. Create

2. Delete

3. Open

4. Close

5. Read

6. Write

7. Append

8. Seek

9. Get attributes

10.Set Attributes

11.Rename

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• -rwxr-xr-x 1 dickens spcprj 20580000 Nov 16 2003 BM.Contention.Big• -rw-r--r-- 1 dickens spcprj 5832 Nov 14 2003 LR.Contention.Low• -rwxr-xr-x 1 dickens spcprj 5180000 Nov 14 2003 bitmap.contention.1• -rwxr-xr-x 1 dickens spcprj 12434 Nov 14 2003 Companion.4• -rw-r--r-- 1 dickens spcprj 2767 Nov 14 2003 temp.Swap• -rw-r--r-- 1 dickens spcp 2767 Nov 14 2003 temp• -rwxr-xr-x 1 dickens spc 16969 Nov 14 2003 ind_calc• -rw-r--r-- 1 dickens spcprj 1217 Nov 14 2003 Temp_File.contention

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An Example Program Using File System Calls (1/2)

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An Example Program Using File System Calls (2/2)

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Memory Mapped Files

• Use system calls like: – map (filename, starting address, size);

– unmap (filename, starting address, size) ;

• Implemented through paging mechanism.

• Advantages of this: ?

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DirectoriesSingle-Level Directory Systems

• A single level directory system– contains 4 files– owned by 3 different people, A, B, and C

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Two-level Directory Systems

Letters indicate owners of the directories and files

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Hierarchical Directory Systems

A hierarchical directory system

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A UNIX directory tree

Path Names

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To Open dict path is: /usr/jim/dict.

Path Names

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Directory Operations

1. Create

2. Delete

3. Opendir

4. Closedir

5. Readdir

6. Rename

7. Link

8. Unlink

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File System Implementation

A possible file system layout

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Implementing Files (1)

(a) Contiguous allocation of disk space for 7 files(b) State of the disk after files D and E have been removed

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Implementing Files (2)

Storing a file as a linked list of disk blocks

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Implementing Files (3)

Linked list allocation using a file allocation table in RAM

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Entry 4 bytes. Blocks 1K. 20 Million Entries == 80 MB for table.

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Implementing Files (4)

An example i-node

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An example i-node

Double Indirection

Triple Indirection

Disk block containing addresses of disk blocks containing addresses

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Implementing Directories (1)

(a) A simple directoryfixed size entriesdisk addresses and attributes in directory entry

(b) Directory in which each entry just refers to an i-node

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Implementing Directories (2)

• Two ways of handling long file names in directory– (a) In-line– (b) In a heap

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Shared Files (1)

File system containing a shared file

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Shared Files (1)

Cyclic Family Tree

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Links

(a) Situation prior to linking

(b) After the link is created

(c) After the original owner removes the file

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Symbolic Links

• Provide the path name of the target file in the linked file

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Disk Space Management (1)

• Dark line (left hand scale) gives data rate of a disk• Dotted line (right hand scale) gives disk space efficiency• All files 2KB

Block size

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Disk Space Management (2)

(a) Storing the free list on a linked list(b) A bit map

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Quotas for keeping track of each user’s disk use

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Consistency Issues

• File system can become inconsistent if there is a system crash and recent changes have not all been written to disk.

• Consider:– inode cached

– New block allocated to file

– Block filled, written to disk

– Free list updated and written to disk

– System crash

– Now block not on either free list or listed as part of a file

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• Consider:– inode cached

– Free list cached

– Block 10 deleted from file A. Cached inode updated.

– Block 10 now on free list and allocated to file B.

– File B closed, inode written to disk.

– System crash.

– Now block 10 listed as belonging to two files.

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File System Consistency

– Build two tables, each of which counts number of blocks.

– Table 1: Number of times block is in a file.

– Table 2: Number of times block is in the free list.

– Read all inodes.

– Increment tables.

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File System Reliability (3)

• File system states(a) consistent(b) missing block(c) duplicate block in free list(d) duplicate data block

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– Figure b- missing block.

• Adds it back to the free list.

– Figure c- block listed twice in free list

• Rebuilds the free list.

– Figure d- Same block belongs to two or more files.

• Make copy and insert in one of the files.

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File System Performance (1)

The block cache data structures

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Problem with LRU

• If frequently used block is inode or directory block, may want to write it out.

• Modify LRU to account for importance of block.

• Unix: synch()

• MS-DOS: Write-through cache

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• Most systems write critical blocks to disk immediately (but not necessarily non-critical blocks).

• If system crashes, file system likely intact (but users may become unglued).

• Unix: System call sync that causes all modified blocks to be written to disk.– An update daemon is running in the background and alternates between

sleeping and calling synch. Generally every 30 seconds.

• MS-DOS: Write-through cache. Every change is written through to disk.

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File System Performance (2)

• I-nodes placed at the start of the disk• Disk divided into cylinder groups

– each with its own blocks and i-nodes

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CP/M

• 8080 Chip

• Max 64K RAM

• 720K Floppy.

• Size of entire OS: 3584 bytes.

• Shell: 2K bytes.

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The CP/M File System (1)

Memory layout of CP/M

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Directory

• One directory

• Entries 32 bytes (fixed).

• After booting, reads in directory and computes free list.

• Does not save free list.

• Provides 38 systems calls, 17 File system related.

• File blocks are 1K.

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The CP/M File System (2)

The CP/M directory entry format

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MS-DOS

• Added hierarchical directory structure.

• Use fixed 32-byte directory entry

• Added attributes: read-only, archived, hidden, system.

• Time field: seconds (5 bits), minutes (6 bits), hours (5 bits).

• Date: day (5 bits), month (4 bits), year (7 bits– only good to 2107).

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The MS-DOS File System (1)

The MS-DOS directory entry

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File Allocation Table (FAT)

Entry 4 bytes. Blocks 1K. 20 Million Entries == 80 MB for table.

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The MS-DOS File System (2)

• Maximum partition for different block sizes• The empty boxes represent forbidden combinations

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Windows 98

• Used 32-bit FAT.

• Used the additional 10 bits from DOS entry.

• Allowed arbitrarily long file names

• Backward compatible to MS-DOS.

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The Windows 98 File System (1)

The extended MOS-DOS directory entry used in Windows 98

Bytes

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The Windows 98 File System (2)

An entry for (part of) a long file name in Windows 98

Bytes

Checksum

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The Windows 98 File System (3)

An example of how a long name is stored in Windows 98

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The UNIX V7 File System (1)

A UNIX V7 directory entry

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The UNIX V7 File System (1)

A UNIX V7 directory entry

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The UNIX V7 File System (2)

A UNIX i-node

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The UNIX V7 File System (3)

The steps in looking up /usr/ast/mbox