File System Implementation Sunu Wibirama Thursday, December 16, - - PowerPoint PPT Presentation
File System Implementation Sunu Wibirama Thursday, December 16, 2010 Outline File-System Structure File-System Implementation Directory Implementation Allocation Methods Free-Space Management Discussion 11. Thursday,
Sunu Wibirama
Thursday, December 16, 2010
11.
File-System Structure File-System Implementation Directory Implementation Allocation Methods Free-Space Management Discussion
Thursday, December 16, 2010
11.
File system is provided by OS to allow the data to be
stored, located, and retrieved easily
Two problems on file system design:
How the file system should look to the user Algorithms and data structures to map logical file
system onto the physical secondary-storage devices
File system organized into layers, uses features from
lower levels to create new features for use by higher levels.
I/O Control controls the physical device using
device driver
Basic file system needs only to issue generic
commands to the device driver to read and write physical block on the disk (ex. drive 1, cylinder 73, track 2, sector 11)
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File organization module knows physical
and logical blocks, translating logical block address to physical block address. It also manages free-space on the disk
Logical file system manages metadata
information (all file system structure except the actual data or contents of the file).
Logical file system maintains file structure
via file-control blocks (FCB)
File control block – storage structure
consisting of information about a file
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Boot control block contains info needed by system to boot
OS from that volume (UNIX: boot block, NTFS: partition boot sector)
Volume control block contains volume details (UNIX:
superblock, NTFS: master file table)
Directory structure organizes the files (UNIX: inode
numbers, NTFS: master file table)
Per-file File Control Block (FCB) contains many details
about the file
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It is used for file-system management and performance
improvement (via caching).
The data are loaded at mount time and discarded at
dismount.
The structures including:
In-memory mount table: information of each mounted
volume
In-memory directory structure System-wide open-file table: a copy of FCB of each open
file
Per-process open-file table: a pointer to the appropriate
entry in the system-wide open-file table, as well as other information based on process that uses the file.
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Application program calls the logical file
system
Logical file system knows the directory
The system then reads the appropriate
directory into memory, updates it with the new file name and FCB, and writes it back to the disk.
Now, the new created file can be used for
I/O operation, which will be explained in the next slide
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Directory-allocation and directory-management
algorithms significantly affects the efficiency, performance, and reliability of the file system.
Linear list of file names with pointer to the data
blocks.
simple to program time-consuming to execute, because it requires a
linear search to create or delete file.
Hash Table – linear list with hash data structure.
decreases directory search time problem: fixed size of hash table
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Many files are stored in the disk How to allocate space to these files so that disk space is utilized
effectively and files can be accessed quickly
An allocation method refers to how disk blocks are allocated for files:
Contiguous allocation Linked allocation Indexed allocation
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Each file occupies a set of contiguous blocks on the disk Simple – only starting location (block #) and length (number of blocks)
are required
Both sequential and direct access are supported Disadvantages:
Wasteful of space (dynamic storage-allocation problem) File cannot grow External fragmentation: free space is broken into chunks
One of several solutions: use a modified contiguous allocation scheme Extent-based file systems allocate disk blocks in extents An extent is a contiguous block of disks
Extents are allocated for file allocation A file consists of one or more extents
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Linked allocation solves all problems of
contiguous allocation
Each file is a linked list of disk blocks:
blocks may be scattered anywhere on the disk.
Ex: File “Jeep”
Start at block 9 Then: block 16, 1, 10 Finally end at block 25
pointer (4 bytes) 1 block (512 bytes)
508 bytes
visible part to user
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Advantages:
Free-space management system – no waste of space File can grow, depends on available free blocks
Disadvantages:
No random access (only sequential access) Space required for pointers (0.78 percent of the disk is being used for pointers, rather
than for information) ~> solution, uses clusters (unit of blocks)
Reliability, pointer damage will cause unlinked blocks in a file.
FAT (File Allocation Table): variation on linked allocation Located at the beginning of each volume
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To do random access:
to read the FAT
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Brings all pointers together into the
index block
Each file has its own index block, which
is an array of disk-block addresses
Directory contains the address of index
block
Support direct access without external
fragmentation
Each file has its allocation for all
pointers, so that it has wasted space greater than linked allocation (which contains one pointer per block).
We want the index block as small as
possible, then we have several mechanisms:
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Linked Scheme
An index block is normally one disk block Large files -> we can link together several index blocks Ex.: an index block contains:
Multilevel index
First-level index block points to second-level index blocks which in turn point to the
file blocks (see next slide)
Combined scheme
In unix, for example: 15 pointers in fileʼs inode 12 first pointer: direct blocks, for small file (no more than 12 blocks). If the block
size is 4KB, then up to 48KB (12 x 4KB) can be accessed directly
The next pointers point to indirect blocks, which implement multilevel index based
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1st-level index block
2nd-level index block
file
Back to Indexed Allocation
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Combined Scheme: UNIX UFS (4K bytes per block)
Back to Indexed Allocation
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Free-space list concept Bit vector (n blocks)
…
1 2 n-1 bit[i] =
0 ⇒ block[i] free 1 ⇒ block[i] occupied Block number calculation {(number of bits per word) *(number of 0-value words)} +offset of first 1 bit
Bit vector requires extra space. Example:
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Linked list
Link together all the free disk blocks Keeping a pointer to the first free block in a
special location on the disk and caching it in memory.
The first block contains a pointer to the next free
disk block...
Must read each block to traverse list, increase I/
O operation time.
Grouping
Storing the address of n blocks in the first free
block.
n-1 blocks are actually free blocks but the last
block contains the addresses of another n free blocks.
Counting
Keep the address of the first free block and n of
free contiguous blocks that follow the first block.
Each entry in free-space list consists of disk
address and a count
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Yes, but in very small quantity
‘clumps’ to manage small and large file (remember combined scheme in indexed allocation)
that are not predictably being fragmented in shorter time.
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disk using shake-fs
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http://geekblog.oneandoneis2.org/index.php/2006/08/17/ why_doesn_t_linux_need_defragmenting
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I have hello.txt
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I have hello.txt OK, now add bye.txt
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I have hello.txt OK, now add bye.txt I want to change hello.txt, dude...
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Just copy, delete the original content, and wrap it up in the larger space.....
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Just copy, delete the original content, and wrap it up in the larger space..... Or, Put your extended file content to the next space.....
If the first approach requires huge read and write operation, then the most possible approach is the second one. That’s why FAT suffers from large fragmentation
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