file systemscs-502 fall 20061 file systems cs-502 operating systems fall 2006 (slides include...

File SystemsCS-502 Fall 2006 1

File Systems

CS-502 Operating SystemsFall 2006

(Slides include materials from Operating System Concepts, 7th ed., by Silbershatz, Galvin, & Gagne and from Modern Operating Systems, 2nd ed., by Tanenbaum)


File (an abstraction)

• A (potentially) large amount of information or data that lives a (potentially) very long time

• Often much larger than the memory of the computer• Often much longer than any computation• Sometimes longer than life of machine itself

• (Usually) organized as a linear array of bytes or blocks

• Internal structure is imposed by application• (Occasionally) blocks may be variable length

• (Often) requiring concurrent access by multiple processes

• Even by processes on different machines!


File Systems and Disks

• User view– File is a named, persistent collection of data

• OS & file system view– File is collection of disk blocks– File System maps file names and offsets to disk blocks


Fundamental ambiguity

• Is the file the “container of the information” or the “information” itself?

• Almost all systems confuse the two.

• Almost all people confuse the two.


Example – Suppose that you e-mail me a document

• Later, how do either of us know that we are using the same version of the document?

• Windows/Outlook/Exchange/MacOS:• Time-stamp is a pretty good indication that they are

• Time-stamps preserved on copy, drag and drop, transmission via e-mail, etc.

• Unix/Linux• By default, time-stamps not preserved on copy, ftp, e-mail, etc.

• Time-stamp associated with container, not with information


Rule of Thumb

• Almost always, application thinks in terms of the information

• Most systems think in terms of containers

Professional Guidance: Be aware of the distinction, even when the system is not


Attributes of Files

• Name:– Although the name is not

always what you think it is!

• Type:– May be encoded in the

name (e.g., .cpp, .txt)

• Dates:– Creation, updated, last

accessed, etc.– (Usually) associated with

container– Better if associated with

content

• Size:– Length in number of bytes;

occasionally rounded up

• Protection:– Owner, group, etc.– Authority to read, update,

extend, etc.

• Locks:– For managing concurrent

access

• …


File Metadata

• Information about a file– Maintained by the file system– Separate from file itself– Possibly attached to the file

• E.g., in block # –1

– Some information visible to user/application• Dates, permissions, type, name, etc.

– Some information primarily for OS• Location on disk, locks, cached attributes


Example – Location

• Example 1:mv ~lauer/project1.doc ~cs502/public_html/S06

• Example 2:– System moves file from disk block 10,000 to disk block

20,000

– System restores a file from backup

• May or may not be reflected in metadata


Anomaly – Unix File Naming

ln ~lauer/project1.doc ~cs502/public_html/S06ln ~lauer/project1.doc NewProject1.doc

• Unix hard links allow one file to have more than one name and/or location– The real name of a Unix file is its i-node – an internal

name known only to the OS; points to metadata

• Hard links are not used very often in modern Unix practice– Exception: Copy-on-write of large directory trees!– Usually safe to regard last element of path as name


File Types


Operations on Files

• Open, Close• Gain or relinquish access to a file• OS returns a file handle – an internal data structure letting it

cache internal information needed for efficient file access

• Read, Write, Truncate• Read: return a sequence of n bytes from file• Write: replace n bytes in file, and/or append to end• Truncate: throw away all but the first n bytes of file

• Seek, Tell• Seek: reposition file pointer for subsequent reads and writes• Tell: get current file pointer

• Create, Delete:• Conjure up a new file; or blow away an existing one


File – a very powerful abstraction

• Documents, code• Databases

• Very large, possibly spanning multiple disks

• Streams• Input, output, keyboard, display• Pipes, network connections, …

• Virtual memory backing store• …

• Any time you need to remember something beyond the life of a particular process/computation


File Access Methods

• Sequential access– Read all bytes/records from the beginning– Cannot jump around, could possibly rewind or back up– Convenient when medium was magnetic tape or punched cards

• Random access– Bytes/records read in any order– Essential for data base systems– Read can be …

• move file pointer (seek), then read or …• read and then move file pointer

• Keyed or indexed access – access items in file based on the contents of an (part of an) item in the file– Provided in older commercial operating systems (IBM ISAM)– (Usually) handled separately by database applications in modern

systems


Questions?


Directory – A Special Kind of File

• A tool for users & applications to organize and find files

• User-friendly names

• Names that are meaningful over long periods of time

• The data structure for OS to locate files (i.e., containers) on disk


Directory structures

• Single level– One directory per system, one entry pointing to each file– Small, single-user or single-use systems

• PDA, cell phone, etc.

• Two-level– Single “master” directory per system– Each entry points to one single-level directory per user– Uncommon in modern operating systems

• Hierarchical– Any directory entry may point to

• Individual file• Another directory

– Common in most modern operating systems


Directory Considerations

• Efficiency – locating a file quickly.• Naming – convenient to users.

• Separate users can use same name for separate files.• The same file can have different names for different

users.• Names need only be unique within a directory

• Grouping – logical grouping of files by properties

• e.g., all Java programs, all games, …


Directory Organization – Hierarchical

• Most systems support idea of current (working) directory– Absolute names – fully qualified from root of file system

• /usr/group/foo.c

– Relative names – specified with respect to working directory• foo.c

– A special name – the working directory itself• “.”

• Modified Hierarchical – Acyclic Graph (no loops) and General Graph– Allow directories and files to have multiple names– Links are file names (directory entries) that point to existing

(source) files


Links

• Hard links: bi-directional relationship between file names and file – A hard link is directory entry that points to a source file’s metadata– Metadata maintains reference count of the number of hard links

pointing to it – link reference count – Link reference count is decremented when a hard link is deleted– File data is deleted and space freed when the link reference count

goes to zero

• Symbolic (soft) links: uni-directional relationship between a file name and the file– Directory entry contains text representing the

absolute or relative path name of source file– If the source file is deleted, the link pointer is invalid


Path Name Translation

• Assume that I want to open “/home/lauer/foo.c”fd = open(“/home/lauer/foo.c”, O_RDWR);

• File System does the following– Opens directory “/” – the root directory is in a known place on disk– Search root directory for the directory home and get its location– Open home and search for the directory lauer and get its location– Open lauer and search for the file foo.c and get its location– Open the file foo.c– Note that the process needs the appropriate permissions at every step

• File Systems spend a lot of time walking down directory paths– This is why open calls are separate from other file operations– File System attempts to cache prefix lookups to speed up common

searches– Once open, file system caches the metadata of the file


Directory Operations

• Create:• Make a new directory

• Add, Delete entry:• Invoked by file create & destroy, directory create & destroy

• Find, List:• Search or enumerate directory entries

• Rename:• Change name of an entry without changing anything else about it

• Link, Unlink:• Add or remove entry pointing to another entry elsewhere• Introduces possibility of loops in directory graph

• Destroy:• Removes directory; must be empty


More on Directories

• Fundamental mechanism for interpreting file names in an operating system

• Orphan: a file not named in any directory• Cannot be opened by any application (or even OS)• May not even have name!

• Tools• FSCK – check & repair file system, find orphans• Delete_on_close attribute (in metadata)

• Special directory entry: “..” parent in hierarchy• Essential for maintaining integrity of directory system• Useful for relative naming


Break


Implementation of Files

• Map file abstraction to physical disk blocks

• Some goals– Efficient in time, space, use of disk resources– Fast enough for application requirements– Scalable to a wide variety of file sizes

• Many small files (< 1 page)

• Huge files (100’s of gigabytes, terabytes, spanning disks)

• Everything in between


File Allocation Schemes

• Contiguous– Blocks of file stored in consecutive disk sectors– Directory points to first entry

• Linked– Blocks of file scattered across disk, as linked list– Directory points to first entry

• Indexed– Separate index block contains pointers to file blocks– Directory points to index block


Contiguous Allocation

• Ideal for large, static files– Databases, fixed system structures, OS code– Multi-media video and audio– CD-ROM, DVD

• Simple address calculation– Block address sector address

• Fast multi-block reads and writes– Minimize seeks between blocks

• Prone to fragmentation when …• Files come and go• Files change size

– Similar to unpaged virtual memory


Bad Block Management –Contiguous Allocation

• Bad blocks must be concealed• Foul up the block-to-sector calculation

• Methods• Spare sectors in each track, remapped by formatting

• Look-aside list of bad sectors– Check each sector request against hash table

– If present, substitute a replacement sector behind the scene

• Handling• Disk controller, invisible to OS

• Lower levels of OS file system; invisible to application


Contiguous Allocation – Extents

• Extent: a contiguously allocated subset of a file• Directory entry contains

– (For file with one extent) the extent itself– (For file with multiple extents) pointer to an extent block

describing multiple extents• Advantages

– Speed, ease of address calculation of contiguous file– Avoids (some of) the fragmentation issues– Can be extended to support files across multiple disks

• Disadvantages– Degenerates to indexed allocation in Unix-like systems

• Popular in 1960s & 70s, currently used for large files in NTFS


Linked Allocation

• Blocks scattered across disk– Each block contains

pointer to next block

– Directory points to first and last blocks

– Sector header:• Pointer to next block

• ID and block number of file

10

16

01

25


Linked Allocation

• Advantages– No space fragmentation!– Easy to create, extend files– Ideal for lots of small files

• Disadvantages– Lots of disk arm movement– Space taken up by links– Sequential access only!


Variation on Linked Allocation – File Allocation Table

• Instead of link on each block, put all links in one table– the FAT (File Allocation

Table)

• One entry per physical block in disk– Directory points to first &

last blocks of file– Each block points to next

block (or EOF)


FAT File Systems

• Advantages– Advantages of Linked File System– FAT can be cached in memory– Searchable at CPU speeds, pseudo-random access

• Disadvantages– Limited size, not suitable for very large disks– FAT cache describes entire disk, not just open files!– Not fast enough for large databases

• Used in MS-DOS, early Windows systems


Disk Defragmentation

• Re-organize blocks in disk so that file is (mostly) contiguous

• Link or FAT organization preserved

• Purpose:– To reduce disk arm movement during

sequential accesses


Bad Block Management –Linked and FAT Systems

• In OS:– format all sectors of disk• Don’t reserve any spare sectors

• Allocate bad blocks to a hidden file for the purpose

• If a block becomes bad, append to the hidden file

• Advantages• Very simple

• No look-aside or sector remapping needed

• Totally transparent without any hidden mechanism


Indexed Allocation

• i-node:– Part of file metadata– Data structure lists the

sector address of each block of file

• Advantages– True random access– Only i-nodes of open

files need to be cached– Supports small and

large files


Unix/Linux i-nodes

• Direct blocks:– Pointers to first n

sectors

• Single indirect table:– Extra block containing

pointers to blocks n+1 .. n+m

• Double indirect table:– Extra block containing

single indirect blocks

• …


Indexed Allocation

• Access to every block of file is via i-node

• Bad block management– Similar to Linked/FAT systems

• Disadvantage– Not as fast as contiguous allocation for large

databases• Requires reference to i-node for every access

vs.

• Simple calculation of block to sector address


Questions?


Scalability of File Systems

• Question: How large can a file be?• Answer: limited by

– Number of bits in length field in metadata

– Size & number of block entries in FAT or i-node

• Question: How large can file system be?• Answer: limited by

– Size & number of block entries in FAT or i-node


MS-DOS & Windows

• FAT-12 (primarily on floppy disks):

• 4096 512-byte blocks

• Only 4086 blocks usable!

• FAT-16 (early hard drives):

• 64 K blocks; block sizes up to 32 K bytes

• 2 GBytes max per partition, 4 partitions per disk

• FAT-32 (Windows 95)

• 228 blocks; up to 2 TBytes per disk

• Max size FAT requires 232 bytes in RAM!


MS-DOS File System (continued)

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


Classical Unix

• Maximum number of i-nodes = 64K!• How many files in a modern PC?

• I-node structure allows very large files, but …

• Limited by size of internal fields


Modern Operating Systems

• Need much larger, more flexible file systems

• Many terabytes per system

• Multi-terabyte files

• Suitable for both large and small

• Cache only open files in RAM


Examples of Modern File Systems

• Windows NTFS• Silbershatz §22.5

• Linux ext2fs• Silbershatz §21.7.2

• SUSE Administrator’s Manual, §20.2.2

• Linux “Reiser” file system• SUSE Administrator’s Manual, §20.2.1 & 20.2.5

• Max file = 246 bytes; max file system = 245 bytes


New Topic


Implementation of Directories

• A list of [name, information] pairs• Must be scalable from very few entries to very many

• Name:• User-friendly, variable length• Any language• Fast access by name

• Information:• File metadata (itself)• Pointer to file metadata block (or i-node) on disk• Pointer to first & last blocks of file• Pointer to extent block(s)• …


Very Simple Directory

• Short, fixed length names• Attribute & disk addresses contained in directory• MS-DOS, etc.

name1 attributes

name2 attributes

name3 attributes

name4 attributes

… …


Simple Directory

• Short, fixed length names• Attributes in separate blocks (e.g., i-nodes)

• Attribute pointers are disk addresses (or i-node numbers)

• Older Unix versions

name1

name2

name3

name4

…

i-node

i-node

i-node

i-node

Data structurescontaining attributes


More Interesting Directory

• Variable length file names– Stored in heap at end

• Modern Unix, Windows

• Linear or logarithmic search for name

• Compaction needed after– Deletion, Rename

attributes

attributes

attributes

attributes

… …

name1 longer_name3 very_long_name4 name2 …


Very Large Directories

• Hash-table implementation

• Each hash chain like a small directory with variable-length names

• Must be sorted for listing


Free Block Management

• Bitmap– Very compact on disk

– Expensive to search

– Supports contiguous allocation

• Free list– Linked list of free blocks

• Each block contains pointer to next free block

– Only head of list needs to be cached in memory

– Very fast to search and allocate

– Contiguous allocation vary difficult


Free Block ManagementBit Vector

…

0 1 2 n-1

bit[i] = 0 block[i] free

1 block[i] occupied

Free block number calculation

(number of bits per word) *(number of 0-value words) +offset of first 1 bit


Free Block ManagementBit Vector (continued)

• Bit map– Must be kept both in memory and on disk– Copy in memory and disk may differ– Cannot allow for block[i] to have a situation where

bit[i] = 1 in memory and bit[i] = 0 on disk• Solution:

– Set bit[i] = 1 in disk– Allocate block[i]– Set bit[i] = 1 in memory– Similarly for set of contiguous blocks

• Potential for lost blocks in event of crash!– Discussion:– How do we solve this problem?


Free Block ManagementLinked List

• Linked list of free blocks• Not in order!

• Cache first few free blocks in memory

• Head of list must be stored both

• On disk & in memory

• Each block written to disk when freed

• Potential for losing blocks?


Reading Assignment in Silbershatz

• File systems (general) – Chapter 11• Ignore §11.9, 11.10 for now!


Break

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file systemscs-502 fall 20061 file systems cs-502 operating systems fall 2006 (slides include...

Documents

file systemscs

file metadata information

file systems cs

information slide

disks user view file

anomaly unix file naming

blocks slide

metadata slide