Chapter 6 File Systems

Essential requirements

1. Store very large amount of information2. Must survive the termination of processes

persistent

3. Concurrent access by multiple processes

File names

File name issues

Length Distinguish between upper and lower case Characters allowed

File structure types

(keyed)

File types

1. Regular files2. Directories (folders)3. Special

1. Character special (used for serial I/O: ports, printers, networks, etc.)

2. Block special (used for disks)

Regular files

ASCII or binary ASCII is easy to use

Record oriented (delimiter)? Binary is space efficient

Fixed or variable length

File access

1. Sequential2. Random

seek() and fseek() Binary

unbuffered: read(), write() Buffered: fread(), fwrite()

ASCII, buffered: fscanf(), fprintf()

File attributes

File operations

Create Delete Open Close Read Write

Append Seek Get attributes Set attributes Rename

Memory-mapped files

Mapping files into process virtual address space

Directories (folders)

1. Single level2. Two level3. Hierarchical

Two level directory systems

Hierarchical directory systems

Path names

We already described file names. How do we specify the “path” to a file i.e., how do

we navigate the directory structure? Path names:

1. Absolute c:\usr\ginger\mailbox\junk.cpp /usr/ginger/mailbox/junk.cpp

2. Relative (to the current working (default) directory)1. . = current directory: ./hw1/junk.cpp or hw1/junk.cpp2. .. = directory above current: ../music/mm.mp3

Directory operations

Create Delete Opendir Closedir

Readdir Rename Link Unlink

Same or similar so be careful!

File system implementation

Physical disks Divided into one or more “partitions” (logical, separate

disks). Each partition can have its own file system. Sector 0 = MBR (master boot record)

List of partitions (start and ends) Indicates boot partition Every partition has a boot block (although it may be empty) Boot steps:

1. boot code in MBR executes2. reads in boot block code of boot partition and executes it3. boot block code boots OS code in partition

File system layout

Implementing files

1. Contiguous allocation2. Linked list allocation3. Linked list allocation w/ table in memory4. I-nodes (index-nodes)

Implementing files: contiguous allocationGiven 1KB blocks, a 50KB file would be allocated 50

consecutive blocks+ simple: all we need to know if the disk address of the

first block and the number of blocks (or length of the file)

+ fast: only 1 seek + one read needed for the entire file+ sequential and random access are efficient- fragmentation (holes or compaction)- Must specify the size of the file ahead of time. Excellent for CDs and DVDs.

Implementing files: contiguous allocation

Implementing files: linked list allocation

Implementing files: linked list allocation+ no fragmentation- Sequential access is easy but requires

multiple seeks and reads.- Random access is slow (basically becomes

sequential access).

Implementing files: linked list w/ table in memory FAT = file allocation table+ random access requires only sequential

memory access (which is fast)- Need memory to store the table.

20GB and 1KB blocks requires a table with 20M entries. (20M x 4 bytes-per-entry = 80MB)

Size of table is proportional to disk size.

Implementing files: linked list w/ table in memory

Implementing files: i-nodes

i-node table for a file need only be in memory when the file is open.

Size of table is proportional to number of files we allow to be open at any time.

Implementing files: i-nodesfile

(disk)

Implementing directories (folders) We need to locate (the first block of) the file! We need to store file attributes (e.g., owner,

creation time, etc.).

i-nodes

Shared files (i.e., files in more than one directory)

Disk space management

Block size Page size? Sector, track, or cylinder size? What is the average size of a file? For Unix, 1KB is commonly used.

Keeping track of free blocks Linked list of free block numbers Bitmap

Disk quotas Limits on disk space usage by users.

File system reliability

Backups1. Full2. Incremental

Backups1. Physical dump2. Logical dump

Consistency When the system is not shut down properly.

File system performance

Caching (FIFO, second chance, LRU, etc.). Block read ahead. Reducing disk arm motion.

Example file systems

MS-DOS/Windows3.1/Windows95 Windows98 Unix V7

MS-DOS/Windows3.1/Windows95Attributes

1. Read-only2. Hidden3. System file4. Should be archived

Each entry is 32 bytes.

Windows98

Problem: We outgrew 8.3 file names (in 1988).

Windows98 & old MS-DOS file namesMS-DOS file name: “THEQUI~1.”

Long file name: “The quick brown fox jumps over the lazy dog.”

First byte is sequence number & Invalid attr’s 0x0f for all long file entries.

Unix V7 file system

simple

disk addr of file block 0disk addr of file block 9

disk addrs of file blocks 10-15

disk addrs of file blocks 16-51

disk addrs of file blocks 52…