Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8 th Edition Threads.
CSCI-375 Operating Systems Lecture Note: Many slides and/or pictures in the following are adapted...
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Transcript of CSCI-375 Operating Systems Lecture Note: Many slides and/or pictures in the following are adapted...
CSCI-375Operating Systems
Lecture
Note: Many slides and/or pictures in the following are adapted from: slides ©2005 Silberschatz, Galvin, and Gagne Some slides and/or pictures in the following are adapted from: slides ©2007 UCB by Kubiatowicz
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Disk Formatting• A new magnetic disk is just a platter of a
magnetic recording material• Before disk can store data, must be divided into
sectors that the disk controller can read and write. This process is called low-level formatting or physical formatting. Fills disk with special data structure for each sector consisting of header, data area, and trailer– Header and trailer contain info used by disk controller
(e.g., sector number, error-correcting code (ECC))
Disk Formatting• To use a disk to hold files, OS still needs to
record its own data structures on the disk– First step is to partition the disk into one or more
groups of cylinders. The OS can treat each partition as though it were a separate disk
– Second step is logical formatting (or creation of a file system). The OS stores the initial file system data structures onto the disk
Building a File System• File System: layer of OS that transforms block
interface of disks (or other block devices) into files, directories, etc
• File System Components– Disk Management: collecting disk blocks into files– Naming: interface to find files by name, not by blocks– Protection: layers to keep data secure– Reliability/Durability: keeping of files durable despite
crashes, media failures, attacks, etc
Disk Management Policies• Basic entities on a disk
– File: user-visible group of blocks arranged sequentially in logical space
– Directory: user-visible index mapping names to files• Need way to structure files: File Header
– Track which blocks belong at which offsets within the logical file structure
– Optimize placement of files’ disk blocks to match access and usage patterns
Designing the File System: Access Patterns
• How do users access files?– Need to know type of access patterns user is likely to
throw at system• Sequential Access: bytes read in order (“give me
the next X bytes, then give me next, etc”)– Almost all file access are of this flavor
• Random Access: read/write element out of middle of array (“give me bytes i-j”)– Less frequent, but still important– Want this to be fast – don’t want to have to read all
bytes to get to the middle of the file
How to Organize Files on Disk• Goals:
– Maximize sequential performance– Easy random access to file– Easy management of file (growth, truncation, etc)
How to Organize Files on Disk• First Technique: Contiguous Allocation
– Use contiguous range of blocks– User says in advance how big file will be (disadvantage)
– Search for space using best fit/first fit• What if not enough contiguous space for new file?
– File header contains: First block in file, file size (# of blocks)– Pros?
• fast sequential access, easy random access
– Cons? • external fragmentation/hard to grow files
• Free holes get smaller and smaller
• Could compact space, but that would be really expensive
• Contiguous Allocation used by IBM 360– Result of allocation and management cost: people would create
a big file, put their file in the middle
Contiguous Allocation
How to Organize Files on Disk• Second Technique: Linked List Approach
– Each block, pointer to next on disk
– Pros: can grow files dynamically– Cons?
• Bad sequential access (seek between each block), unreliable (lose block, lose rest of file)
• Serious Con: bad random access!!!!
Null
File Header
Linked Allocation
Linked Allocation: File-Allocation Table (FAT)
• MSDOS links blocks together to create a file– Links not in blocks, but in the File Allocation Table
(FAT)• FAT contains an entry for each block on the disk
• FAT entries corresponding to blocks of file linked together– Access properties:
• Random access time is improved because the disk head can
find the location of any block by reading the FAT
Linked Allocation: File-Allocation Table (FAT)
Linked Allocation• Linked allocation solves the external
fragmentation and size declaration problems of contiguous allocation
• However, in the absence of a FAT, linked allocation cannot support efficient direct access
Indexed Allocation• Third technique: Indexed Files
– System allocates file header block to hold array of pointers big enough to point to all blocks
• User pre-declares max file size
Indexed Allocation• Direct access without external fragmentation, but
have overhead of index block– The pointer overhead of the index block is generally
greater than the pointer overhead of linked allocation
– Consider a common case of a file with only 1 or 2 blocks
• With linked allocation, lose space of only one pointer per
block
• With indexed allocation, an entire index block must be
allocated, even if only 1 or 2 pointers are non-nil
• How large should the index block be?
Multilevel Indexed Files (UNIX BSD 4.1)
• Multilevel Indexed Files: Like multilevel address translation (from UNIX 4.1 BSD)– Key idea: efficient for small files, but still allow big files– File header contains 13 pointers
• Fixed size table, pointers not all equivalent
• This header is called an inode in UNIX
– File Header format• First 10 pointers are to data blocks
• Block 11 points to indirect block containing 256 blocks
• Block 12 points to doubly indirect block containing 256
indirect blocks for total of 64K blocks
• Block 13 points to a triply indirect block (16M blocks)
Information in a UNIX Disk-Resident Inode
UNIX Inode• The first 10 addresses point to the first 10 data blocks of
the file• If the file is longer than 10 blocks long, then one or more
levels of indirection is used– The 11th address in the inode points to a block on disk that
contains the next portion of the index. This is the single indirect block, which contains the pointers to succeeding blocks in the file
– If the file contains more blocks, the 12th address in the inode points to a double indirect block, which contains a list of addresses of additional single indirect blocks, each contains pointers to file blocks
– If the file contains still more blocks, the 13th address in the inode points to a triple indirect block that is a third level of indexing, which points to additional double indirect blocks
UNIX Inode• Basic technique places an upper limit on file size
– Maximum size of a UNIX file? (DONE IN CLASS)– Designers thought this was bigger than anything
anyone would need. Much bigger than a disk at the time…
• Pointers get filled in dynamically: need to allocate indirect block only when file grows > 10 blocks – On small files, no indirection needed