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FILE SYSTEM IMPLEMENTATIONSunu Wibirama
11.
Outline File-System Structure
File-System Implementation
Directory Implementation
Allocation Methods
Free-Space Management
Discussion
11.
Outline File-System Structure
File-System Implementation
Directory Implementation
Allocation Methods
Free-Space Management
Discussion
11.
File System Structure 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)
11.
Outline File-System Structure
File-System Implementation
Directory Implementation
Allocation Methods
Free-Space Management
Discussion
11.
File System Implementation 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
Basic file system and I/O control can be used by multiple file systems.
Recently, OS supports more than one FS
11.
A Typical File Control Block
11.
On-Disk File System Structures
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
11.
In-Memory File System Structures
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 structureSystem-wide open-file table: a copy of FCB of each open
filePer-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.
11.
New File Creation Process
Application program calls the logical file system
Logical file system knows the directory structures. It allocates a new FCB.
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
11.
In-Memory File System Structures
File open
File read
11.
Outline File-System Structure
File-System Implementation
Directory Implementation
Allocation Methods
Free-Space Management
Discussion
11.
Directory Implementation Directory-allocation and directory-management
algorithms significantly affects the efficiency, performance, and reliability of the file system. There are two impementations: linear list and hash table
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:
collisions (two file names hash to the same location)
fixed size of hash table (for 64 entries, the hash function converts filename into integersfrom 0 to 63)
11.
Outline File-System Structure
File-System Implementation
Directory Implementation
Allocation Methods
Free-Space Management
Discussion
Dynamic Storage Allocation and Fragmentation
Review:
11.
Dynamic Storage Allocation
At any given time, we have a set of free spaces (holes) of various sizes scattered through the disk.
Dynamic Storage Allocation problem: satisfying a request of size n from a list of free spaces
There are three solutions for dynamic storage allocation problem:First Fit: allocate the first hole that is big enough. Stop searching the
entire list as soon as we find a free hole that is large enoughBest Fit: Allocate the smallest hole that is big enough. We must
search the entire list in the storageWorst Fit: allocate the largest hole. Again, we must search the entire
list in the storage In term of speed and disk utilization : first fit and best fit are better then
worst fit
11.
External Fragmentation
First fit and best fit suffer from external fragmentation : there is enough total storage space to satisfy the request, but the available spaces are not contiguous
OS
File 2
File 3
File 8
50k
100k
File 9 125k ?
11.
Internal Fragmentation
Another approach : break the physical storage into fixed-sized blocks and allocate storage in units based on block size.
The storage space allocated to file may be slightly larger than the requested space, but we get internal fragmentation.
OS A (part 3)
A (part 2)
A (part 1)
Room for growth Allocated to A
11.
Fragmentation Solution?
Minimize external fragmentation? Large disk blocks (but internal fragmentation occurs) Choose the best allocation methods to allocate disk block Compaction:
Shuffle memory contents to place all free memory together in one large block (example: disk defragmenter)
Only for “execution time address binding” data (dynamic address relocation)
File 9 125k
OS
File 2
File 3
File 8
OS
File 2
File 3
File 8
50k
100k
OS
File 2
File 3
File 8
90k
60k
(a) (b)
Next, we want to allocate disk blocks effectively. But how?
11.
Allocation Methods
An allocation method refers to how disk blocks are allocated for files:
Contiguous allocation
Linked allocation
Indexed allocation
11.
Contiguous Allocation of Disk Space
11.
Contiguous Allocation OS : IBM VM/CMS 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) External fragmentation: free space is broken into chunksWe must determine the size of needed space before we allocated Preallocation may be inefficient if the file grow slowly over a long
period (months or years). The file therefore has a large amount of internal fragmentation
11.
Contiguous Allocation
(a) Contiguous allocation of disk space for 7 files. (b) The state of the disk after files D and F have been removed.
11.
Contiguous Allocation
One of several solutions: use a modified contiguous allocation scheme (ex. : Veritas file system, replacing standard UNIX UFS)
Initially, a contiguous block of space is allocated If it is not to be large enough, another block is added,
which is called extent. An extent is a contiguous block of disks A file consists of one or more extents
Another approach, we use Linked Allocation Method
11.
Linked Allocation
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
11.
Linked Allocation 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), so that pointers use much smaller percentage
Clusters operation increase internal fragmentationReliability, pointer damage will cause unlinked blocks in a file.
FAT (File Allocation Table): variation on linked allocation (ex.: MS-DOS and OS/2 operating systems)
Located at the beginning of each volume
11.
File-Allocation Table
To do random access: 1. The disk head move to the start of volumeto read the FAT2. Find the location of the desired block3. Move to the location of the block itself
11.
Indexed Allocation
Linked allocation: solves external fragmentation and size-declaration problem
FAT + Linked Allocation = efficient file retrieval using random access
Without FAT, linked allocation cannot support efficient direct access:
because the pointers to the blocks are scattered with the block themselves all over the disk
the pointers must be retrieved in order (sequential access).
How about collecting all pointers together into one location?
11.
Indexed Allocation 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: 1. Linked scheme2. Multilevel index3. Combined scheme
Index block
11.
Indexed Allocation Linked Scheme
An index block is normally one disk block Large files -> we can link together several index blocks Example: an index block contains:
- a small header of file name- a set of 100 disk-block addresses- nil (for small file) or a pointer to another index block (for a large file)
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 on their sequence (see next two slide)
11.
Multilevel Index
1st-level index block
2nd-level index block file
Back to Indexed Allocation
11.
Combined Scheme: UNIX UFS (4K bytes per block)
Back to Indexed Allocation
11.
Outline File-System Structure
File-System Implementation
Directory Implementation
Allocation Methods
Free-Space Management
Discussion
11.
Free-Space Management
Disk space is limited, we need to reuse the space from deleted files for new files, if possible
To keep track of free disk space, the system maintains a free-space list.
Free-space list records all free disk blocks: those not allocated to some file or directory
Creating file: search the free-space list for the required amount of space allocate that space to the new file remove that space from the free-space list
Deleting file: the disk space of the file is added to free-space list
11.
Free-Space Management Free-space list concept Bit vector (n blocks) Bit vector requires extra space.
Example:block size = 212 bytesdisk size = 230 bytes (1 gigabyte)n (amount of bits) = 230/212 = 218 bits
(or 32K bytes)
…
0 1 2 n-1
bit[i] =
0 ⇒ block[i] free
1 ⇒ block[i] occupied
Finding the free block number
{(number of bits per word) *(number of 0-value words)} +offset of first 1 bit
0 0 1 1 1 1 0 0 0 1 1 0
0 1 2 3 4 5 6 7 8 9 10 11
1st word 2nd Word 3rd word
Assume 1word = 3bits, then finding block 11: (3 X 3)+2
11.
Free-Space Management The other free-space management methods include:
(1) 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.
(2) Grouping Storing the address of n free 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.
(3) Counting (for contiguous free block) Several contiguous blocks may be freed simultaneously 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 (ex. address 4 + 15 --> we have 16 free blocks.1st block starts at address 4, followed by 15 free blocks)
::Discussion
Comparing File SystemFragmentation in Windows (FAT 32) and Linux (ext)
Defragmentation
http://en.wikipedia.org/wiki/Defragmentation
Why Linux rarely needs defragmentation tools?
• Does fragmentation occur in Linux? Yes, but in very small quantity
• Block Groups: group file-data together in ‘clumps’ to manage small and large file (remember combined scheme in indexed allocation)
• Only write files to unused portion of the disk that are not predictably being fragmented in shorter time.
Combined Scheme
Possible to allocate bigger blocks for a file
Unix System• Keep fragmentation level below 20%
• More than 20%? You certainly need to fix your hard disk using shake-fs
• Run : e2fsck -nv /dev/sda1 as root, resulting:
Fragmented Part
Implementation (*you should have known this before...)
http://geekblog.oneandoneis2.org/index.php/2006/08/17/why_doesn_t_linux_need_defragmenting
Empty hard disk (*simplified assumption)
Start with FAT file system....
I have hello.txt
OK, now add bye.txt
I want to change hello.txt, dude...?
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
1st approach
2nd approach
Initial condition
What About Linux?
What About Linux?
Add bye.txt
Change hello.txt
What About Linux?
Now you know... why people (again) prefer Unix based OS
for handling large dataDefragmentation = decreasing productivity
Thank You
