Transcript File System Implementations

File System Implementations
CS-502, Operating Systems
Fall 2007
(Slides include materials from Operating System Concepts, 7th ed., by Silbershatz, Galvin,
& Gagne and from Modern Operating Systems, 2nd ed., by Tanenbaum)
CS-502 Fall 2007
File System
Implementations
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Implementation of Files
• Create file abstraction using physical disk devices
and disk blocks
– Efficient in time, space, use of disk resources
– Fast enough for application requirements
• Must be 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
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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
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File Allocation Schemes (continued)
• The allocation scheme is an attribute of a
file system, not of individual files within a
system.
• All files within a file system follow same
allocation model
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Volume
• The fundamental unit of a file system
• Physical volume may be a
• physical disk storage device
• physical partition of a single disk (aka minidisk)
• Logical Volume
• A physical volume
• A combination of other volumes
– Usually similar in size and characteristics
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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
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Contiguous Allocation
• Ideal for large, static files
– Databases, fixed system structures, OS code
– Multi-media video and audio
– CD-ROM, DVD
• Simple address calculation
– Directory entry points to first sector
– File block i  disk sector address
• Fast multi-block reads and writes
– Minimize seeks between blocks
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Contiguously Allocated Files
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Block-to-sector Calculation
• To find disk sector containing block i of file f
– Starting_block(f) + i
• Starting block of each file is named in
– Directory, or
– File metadata
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File Creation
(Contiguous File System)
• Search for an empty sequence of blocks
– First-fit
– Best-fit
• Prone to fragmentation when …
• Files come and go
• Files change size
• Similar to physical memory allocation in
base-limit type of virtual memory
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Digression: Bad Block Management
• Bad blocks on disks are inevitable
• Part of manufacturing process (less than 1%)
• Most are detected during formatting
• Occasionally, blocks become bad during operation
• Manufacturers typically add extra tracks to disks
• Physical capacity = (1 + x) * rated_capacity
• Who handles bad blocks?
• Disk controller: Bad block list maintained internally
– Automatically substitutes good blocks
• Formatter: Re-organize track to avoid bad blocks
• OS: Bad block list maintained by OS, bad blocks never used
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Bad Block Management in
Contiguous Allocation File Systems
• Bad blocks must be concealed
• Foul up the block-to-sector calculation
• Methods
• Look-aside list of bad sectors
– Check each sector request against hash table
– If present, substitute a replacement sector behind the scenes
• Spare sectors in each track, remapped by formatting
• Handling
• Disk controller, invisible to OS
• Lower levels of OS; concealed from higher layers of file
system and from application
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Contiguous Allocation – Extents
• Extent: a contiguously allocated subset of a file
• Directory entry points to
– (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 adapted to support files across multiple disks
• …
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Contiguous Allocation – Extents
• …
• Disadvantages
– Too many extents  degenerates to indexed allocation
• As in Unix-like systems, but not so well
• Popular in 1960s & 70s
– OS/360, other systems for commercial data processing
• Currently used for large files in NTFS
• Rarely mentioned in textbooks
• Silbershatz, §11.4.1 & 22.5.1
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Questions?
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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
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Linked Allocation
• Blocks scattered
across disk
• Each block contains
pointer to next block
• Directory points to
first and last blocks
• Sector header:
10
16
25
01
– Pointer to next block
– ID and block number
of file
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Linked Allocation (Note)
• This is Silbershatz
figure 11.5
• Links in the book are
incorrect
10
16
25
01
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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!
• Random access simulated by caching links
• Used in Xerox Alto file system
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Bad Block Management –
Linked File 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
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Variation on Linked Allocation –
File Allocation Table (FAT)
• Instead of link on each
block, put all links in
one table
– the File Allocation Table
— i.e., FAT
• Each entry corresponds
to physical block in disk
– Directory points to first &
last blocks of file
– Each block points to next
block (or EOF)
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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
– Also USB Flash drives, floppy disks, etc.
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Bad Block Management –
FAT File Systems
• Same as Linked File Systems
• I.e., 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
• Same advantages and disadvantages
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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
– Does not change the linked structure of the file
system!
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Exam Question (last spring)
• You have a humongous database stored in a
file on a 4 GB flash drive with a FAT file
system. What must the file system do to
locate block n of the database?
• Assume that database has not been defragmented, so
that its blocks are likely to be scattered randomly
across the flash drive.
• Given that the file system has found the
location of block n, what must it do to find
the location of block n+1? block n-1?
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Questions?
Linked and FAT File Systems
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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
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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
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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
• …
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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
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Indexed Allocation (continued)
• Widely used in Unix, Linux, Windows
NTFS
• Robust
• Has withstood the test of time
• Many variations
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Questions?
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Free Block Management in File Systems
• 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 very difficult
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Free Block Management
Bit 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
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Free Block Management
Bit 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
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Free Block Management
Bit Vector (continued)
• 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?
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Free Block Management
Linked List
• Linked list of free blocks
– Not necessarily in order!
• Cache first few free blocks in
memory
• Head of list must be stored
both
– On disk
– In memory
• Each block must be written to
disk when freed
• Potential for losing blocks?
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Reading Assignment
• Silbershatz, Chapter 11
• Ignore §11.8 – 11.10
• Tanenbaum (Modern Operating Systems),
Chapter 6
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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
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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!
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MS-DOS File System (continued)
• Maximum partition for different block sizes
• The empty boxes represent forbidden combinations
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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
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Modern Operating Systems
• Need much larger, more flexible file
systems
•
•
•
•
CS-502 Fall 2007
Many terabytes per system
Multi-terabyte files
Suitable for both large and small
Cache only open files in RAM
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Examples of Modern File Systems
• Windows NTFS
• Silbershatz §22.5
• Tanenbaum §11.7
• Linux ext2fs
• Silbershatz §21.7.2
• Other file systems …
• Consult your favorite Linux system documentation
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New Topic
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Mounting
mount –t type device pathname
• Attach device (which contains a file system
of type type) to the directory at pathname
• File system implementation for type gets loaded and
connected to the device
• Anything previously below pathname becomes
hidden until the device is un-mounted again
• The root of the file system on device is now
accessed as pathname
• E.g.,
mount –t iso9660 /dev/cdrom /myCD
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Mounting (continued)
• OS automatically mounts devices in mount table
at initialization time
• /etc/fstab in Linux
• Users or applications may mount devices at run
time, explicitly or implicitly — e.g.,
• Insert a floppy disk
• Plug in a USB flash drive
• Type may be implicit in device
• Windows equivalent
• Map drive
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Virtual File Systems
• Virtual File Systems (VFS) provide objectoriented way of implementing file systems.
• VFS allows same system call interface to be
used for different types of file systems.
• The API is to the VFS interface, rather than
any specific type of file system.
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Schematic View of Virtual File System
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Virtual File System (continued)
• Mounting: formal mechanism for attaching
a file system to the Virtual File interface
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Linux Virtual File System (VFS)
• A generic file system interface provided by
the kernel
• Common object framework
–
–
–
–
superblock: a specific, mounted file system
i-node object: a specific file in storage
d-entry object: a directory entry
file object: an open file associated with a
process
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Linux Virtual File System (continued)
• VFS operations
– super_operations:
• read_inode, sync_fs, etc.
– inode_operations:
• create, link, etc.
– d_entry_operations:
• d_compare, d_delete, etc.
– file_operations:
• read, write, seek, etc.
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Linux Virtual File System (continued)
• Individual file system implementations
conform to this architecture.
• May be linked to kernel or loaded as
modules
• Linux kernel 2.6 supports over 50 file
systems in official version
• E.g., minix, ext, ext2, ext3, iso9660, msdos, nfs,
smb, …
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Reading references
• Silbershatz, §11.2.3
• Robert Love, Chapter 12
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Questions?
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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:
•
•
•
•
•
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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)
…
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Very Simple Directory
name1
name2
attributes
attributes
name3
name4
…
attributes
attributes
…
• Short, fixed length names
• Attribute & disk addresses contained in directory
• MS-DOS, etc.
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Simple Directory
i-node
name1
name2
i-node
name3
name4
…
i-node
i-node
Data structures
containing attributes
• Short, fixed length names
• Attributes in separate blocks (e.g., i-nodes)
• Attribute pointers are disk addresses (or i-node numbers)
• Older Unix versions, MS-DOS, etc.
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More Interesting Directory
attributes
attributes
attributes
attributes
…
…
name1 longer_na
me3 very_long_n
ame4 name2 …
CS-502 Fall 2007
• Variable length file names
– Stored in heap at end
• Modern Unix, Windows
• Linear or logarithmic
search for name
• Compaction needed after
– Deletion, Rename
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Very Large Directories
• Hash-table implementation
• Each hash chain like a small directory with
variable-length names
• Must be sorted for listing
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Questions?
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