Lecture 28
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Reminder: Homework 6 due today, case study
outlines with references due today.
Homework 7 is posted, due on Monday at the
Exam 2 review.
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Exam 2 is next Wednesday.
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Questions?
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Outline
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File system implementation
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Allocation methods
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Free space management
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Other issues
Recovery
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Allocation Methods
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As with any resource, there are various ways to
allocate disk space into files. Disk logically is a
linear block array. Want a method that has
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effective disk utilization
efficient access for a variety of operations; random
vs. sequential; read vs. write vs. append
Common methods are contiguous, linked, and
indexed
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Contiguous Allocation
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Each file occupies contiguous blocks on the
disk. The directory entry is <fileID,
startBlockAddr, length>
What are the advantages of this organization?
What are the disadvantages of this
organization?
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Linked Allocation
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File is a linked list of disk blocks. Directory
entry is <fileID, startBlockAddr, endBlockAddr>.
Must read the start block to find the next block
number.
What are the advantages of this organization?
What are the disadvantages of this
organization?
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Linked Allocation
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To mitigate some of the disadvantages, can
use a file allocation table (FAT - used in
DOS/Win). Directory entry is <fileID,
startBlock#>
FAT is indexed by block number and links the
table entries rather than the blocks themselves.
Often the FAT is small enough to cache, so can
find out what block a random access it in
without reading disk.
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Indexed Allocation
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Each file has an index block that is an array of
disk addresses to the blocks that make up the
file.
What are the advantages of this organization?
What are the disadvantages of this
organization?
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Indexed Allocation
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Several ways to handle large files
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Link index blocks - lose a lot of advantages
Multiple levels of indexing - most files are small, can
always have a very, very large file.
UNIX UFS - most of the index block is direct
address, but third to last entry is two-level (i.e. it
points to another index block), the second to last
block is three-level, and the last entry is four-level.
Good compromise: small files do not pay for multilevel, but very, very large files are supported.
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Indexed Allocation - UNIX UFS
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Free Space Management
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Allocating space starting with an empty disk is
straightforward. Keeping track of disk space as
files are deleted is more problematic.
Disk data structure is the free-space list that
initially consists of most of the disk. When files
are created, blocks are allocated off this list.
When a file is deleted, its disk blocks are put
back on the free-space list. The implementation of the free-space list must support
whatever allocation method is being used.
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Bit Map
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When disks are small, the free-space list often
is a bit map (or bit vector). Each block is
represented by a bit in the map. Bit is set to 1 if
block is free and set to 0 if block is allocated.
E.g., if blocks 2, 3, 4, 5, 8, 9, 10, 11, 12, 13, 17,
18, 25, 26, 27 are free and the rest allocated
will have as a bit map:
001111001111110001100000011100000...
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Bit Map
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Advantages:
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Relatively simple to implement. Most architectures
have bit manipulation instructions.
Efficient to find first free block or n consecutive free
blocks (for contiguous allocation). Look for the first
word of the bit map that is not 0, find the first set bit
in the word.
block # = bits/word * (# 0-value words) + offset of first set bit
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Bit Map
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Disadvantages:
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To be efficient, need bit map in memory.
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1.3GB disk with 512B blocks => >332KB bit map
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Can cluster blocks into groups, e.g. 4 blocks/bit =>
~83KB bit map
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40GB disk with 1K blocks => >5MB bit map
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Not feasible for large disks
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Linked List
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Can use a regular linked-list approach. Head
pointer is put in a special place on the disk and
cached in memory. Each free block contains
pointer to the next free block.
Disadvantages: traversing list is not efficient,
but not done often; allocating a large file is
cumbersome
Advantages: can be incorporated into FAT data
structure for no extra overhead.
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Other Approaches
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Grouping: can store addresses of n (not
necessarily contiguous) free blocks in the first
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free block. The n entry points to the next
block of addresses. Allows system to find large
numbers of free blocks much faster than linked
approach
Counting: for contiguous or clustered
allocation, freeing happens simultaneously to a
contiguous set of blocks. Keep track of this
with free space entry <diskaddr, count>.
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Performance Enhancements
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Page caching - files are cached as pages in
the VM to streamline access
Asynchronous writes - most writes are
buffered and control returned to the caller. The
buffer is written out to disk at a later time.
However, certain metadata must be written
synchronously to maintain system integrity or to
support atomic transactions.
Free-behind or read-ahead during sequential
access.
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Other Issues
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Where are FCBs and index blocks allocated?
Unix inodes are preallocated across a partition.
Allocator tries to keep data blocks near their
inode block.
What is the size of a file pointer (disk address)
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16-bits => 64KB maximum
32-bits => 4GB maximum
64-bits => very, very large...
But as file pointer gets larger so do the data
structures that hold them. FAT16->FAT32...
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Recovery
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Sometimes computers crash and we would like
to make sure file system is intact. Two issues.
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Consistency checking: in-memory cache
generally is more up-to-date than on-disk data,
since updates are not necessarily written to disk
immediately. What happens when computer
crashes?
Most systems have a consistency checker
program (fsck on Unix, chkdisk on Win) that is/can
be run after a crash. These programs compare the
directory structure with data blocks and free space
information.
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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Recovery
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Backup and restore: keep backup "copies" of data
so that in the event of a failure can restore the data.
Typical schedule, n is 7 to 30:
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Day 0: full backup, copy all files
Day 1: incremental backup, copy only those files that have
changed since Day 0
Day 2: incremental backup, copy only those files that have
changed since Day 1
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Day n-1: incremental backup...
Repeat cycle
Sometimes weekly incrementals or "mirror"
backups that can be "hot-swapped". Main issue is
storage requirements.
Wednesday, March 21
CS 470 Operating Systems - Lecture 28
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