Lecture 27
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Any issues with finding case study references?
Reminder: Homework 6, case study outline with
references due on Wednesday
Questions?
Monday, March 19
CS 470 Operating Systems - Lecture 27
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Outline
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Directory ADT
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Finish up directory structures
File system implementation
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Disk and memory structures
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Operation implementations
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Allocation methods
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CS 470 Operating Systems - Lecture 27
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Directory Structures
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Recall as discussed last class, first issue
regarding files is name clashes. Do not want to
require every file in system to have a unique
name.
The generalized directory structure is the
familiar tree structure. Users can create
subdirectories in their UFD. Names become
path names that can be absolute or relative.
Introduce new directory operations: current
directory, change directory. New issue: what
happens when try to delete a directory?
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Directory Structures
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Sometimes users may want to share
subdirectories (i.e., have them mapped into
their UFDs). Acyclic-graph structure provides
this functionality. UNIX allows this through the
use of links. Links may be hard, creating a full
directory entry to the shared data, or soft,
creating a directory entry that is simply a path
name to the shared data.
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Directory Structures
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Main issues are in directory traversal - an entry
may appear more than once, and in deletion who has the right to delete the shared data.
In UNIX, hard links are counted and a file is
removed from the physical disk only when there
are no hard links to it. Deleting a file simply
removes its directory entry and decrements the
link count. Soft links can become "dangling
pointers", if the named directory entry is
removed.
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CS 470 Operating Systems - Lecture 27
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File System Implementation
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How is the physical storage of files organized?
Most file systems (FS) are on disks. Disks
have a natural efficient transfer size called a
block. Block sizes range from 32 bytes to 4KB.
A common size is 512 bytes.
Disks are good for file storage
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Can rewrite data in place
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Can randomly access blocks
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CS 470 Operating Systems - Lecture 27
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File System Implementation
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Under the OS API, there are several layers to
get to the actual disk:
Monday, March 19
User access
Symbolic name
Logical filesystem
Symbolic name -> File control block
(FCB): logical address, directory, other
metadata, protection (Chapter 14)
File organization
module
Logical blocks -> Physical blocks, free
space manager (Chapter 11)
Basic FS
Calls to drivers, physical address may be
a VFS or <drive,cylinder,track,sector>
I/O Driver
Driver hides differences in actual
hardware (Chapter 13)
Device (e.g. disk)
Actually the controller to the device
(Chapter 12)
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File System Implementation
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There are many file system types
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ISO 9660 for CD-ROMs
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U(nix)FS
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FAT - FAT8, FAT12, FAT16, FAT32...
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NTFS
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ext(ended FS) 2/3/4
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...
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CS 470 Operating Systems - Lecture 27
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File System Disk Structures
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File systems have both on-disk and in-memory
data structures. Depend on particular OS and
FS.
Most commonly on disk:
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Boot information, total number of blocks, number
and location of free blocks, directory structure, and
of course, the actual files.
Boot control block per volume - information to
boot OS from this volume. Typically the first block
of the volume. Also called boot block (UFS) and
partition boot sector (NTFS)
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File System Disk Structures
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On-disk structures, cont'd
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Volume control block - contains partition/volume
details including number of blocks in partition, size
of blocks, free-block count and pointers, free FCB
count, etc. Also called superblock (UFS), master
file table (NTFS).
Directory structure per FS. UFS include names and
associated inode numbers. This is stored inside
the master file table in NTFS.
Per file FCB with file attributes. Called an inode in
UFS. NTFS stores in the master file table.
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File System Memory Structures
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In-memory data structures are often used for
caching to improve efficiency. Data is loaded at
mount time and discarded at unmount.
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Mount table - information about each mounted
volume
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Directory-structure cache
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System OFT - copy of FCB of each open file
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Process local OFT
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CS 470 Operating Systems - Lecture 27
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Creating a File
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Applications call the logical FS interface. The
LFS knows the format of the directory
structures. To create a file it does the following:
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Allocates/obtains a new FCB
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Reads the appropriate directory into memory
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Updates directory with new file name and FCB
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Writes directory back to disk
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CS 470 Operating Systems - Lecture 27
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Opening a File
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If a file has been opened already, it has an
entry in the system OFT. Otherwise, the LFS
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Reads in the appropriate directory into memory
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Reads in the FCB and stores it in the system OFT
The rest of this operation does the following:
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Sets entry in the process local OFT to point to the
system OFT entry and increment the open count
Return index of the entry in the process local OFT.
Called a file descriptor (UFS), file handle (Win)
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Reading from a File
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To read from a file, the LFS
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Follows the file entry in the process local OFT
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Follows the entry in the system OFT
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Reads the data blocks
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Closing a File
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The close operations does the following
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Removes the file entry in the process local OFT
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Decrements the open count in the system OFT
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If the open count becomes 0, writes the metadata
back to disk and removes the file entry from the
system OFT.
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CS 470 Operating Systems - Lecture 27
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Virtual File System
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The VFS is an interface layer between the LFS
and an actual concrete FS. It is used to allow
applications to operate on any type of file
without knowing exactly what kind of file it is.
Typically, it is implemented as a table of
function pointers. For each FS type there is a
table indexed by LFS operation that points to
the function that does the actual FS operation.
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Virtual File System
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Allocation Methods
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As with any resource, there are various ways to
allocate disk space into files. 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
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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?
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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?
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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). 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.
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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?
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CS 470 Operating Systems - Lecture 27
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Indexed Allocation
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Several ways to handle large files
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Link index blocks
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Multiple levels of indexing
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UNIX UFS - most of the index block is direct
address, but second to last entry is two-level (i.e. it
points to another index block) and the last entry is
three-level. Good compromise: small files do not
pay for multi-level, but large files are supported.
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Indexed Allocation - UNIX UFS
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