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…