The document discusses file system implementation and mass storage structures. It describes the on-disk and in-memory structures used to manage files and free space on disks. These include the boot block, volume control block, file control blocks, directory structures, allocation methods like contiguous, linked and indexed allocation, and free space management using bitmaps, linked lists and counting. It also covers disk organization, scheduling algorithms like FCFS, SSTF, SCAN and CSCAN, and failure modes and consistency in networked file systems.

1. UNIT – 5
FILE SYSTEM IMPLEMENTATION
AND MASS STORAGE STRUCTURE

2. FILE SYSTEM STRUCTURE
• Disks provide the bulk of secondary storage on
which a file system is maintained.
Characteristics
• A disk can be rewritten in place; it is possible to
read a block from the disk, modify the block, and
write it back into the same place.
• A disk can access directly any given block of
information it contains.

3. Layered File System

4. • Application program layer
• Logical file system layer – Meta data(File
structure)
• File organization modules layer – Information
about files and logical blocks(Mapping)
• Basic file system layer – generates commands
for device driver(To read and write data)
• I/O control interface – consist of device driver
and interrupts.
• Physical h/w devices layer

5. FILE SYSTEM IMPLEMENTATION
• File-System Implementation are done by the
following
• On-disk structure
• In-memory structure
On-disk
• On-disk is the file system may contain
information about
» How to boot an operating system stored there
» The total number of blocks
» The number and location of free blocks
» The directory structure
» Individual files

6. On-disk structure:
• The On-disk structure include
» Boot control block
• Contain information needed by the system to boot an
operating system from the volume
• First block of a volume.
» Volume control block
• Contains volume details, such as the number of blocks in the
partition, size of the blocks, free block count and free-block
pointers, and free FCB count and FCB pointers.
» Directory structure
• Used to organize the files.
» FCB
• Contains the details about the file, including file permissions,
ownership, size, and location of the data blocks.
File data blocks
File permissions
File dates (create, access,
write)
File owner, group, ACL
File Size

7. • In-memory Structure
• The structures may include the ones described
below
» In-memory mount table
• Contains information about each mounted volume
» In-memory directory-structure
• Holds the directory information of recently accessed
directories
» System-wide open-file table
• Contains a copy of the FCB of each open file as well as other
information
» Per-process open file table
• Contains a pointer to the appropriate entry in the system-
wide open file table as well as other information

8. • In-memory file-system structures for File open.

9. • In-memory file-system structures for File read

10. Directory Implementation
• Linear list of file names with pointer to the data blocks
– Simple to program
– Time-consuming to execute
• Linear search time
• Could keep ordered alphabetically via linked list or
use B+ tree
• Hash Table – linear list with hash data structure
– Decreases directory search time
• Collisions – A collision is a situation where two file names
hash to the same location. Alternatively, a chained-overflow
hash table can be used to avoid collisions

11. Allocation methods
• Allocation methods is a methods used to
allocate space to the files for
-Utilizing the disk space effectively
-Accessing the files quickly
Methods
• Contiguous allocation
• Linked allocation
• Indexed allocation

12. Allocation Methods - Contiguous
• An allocation method refers to how disk blocks are
allocated for files:
• Contiguous allocation – each file occupies set of
contiguous blocks
– Best performance in most cases
– Simple – only starting location (block #) and
length (number of blocks) are required
– Problems include finding space for file, knowing
file size, external fragmentation, need for
compaction off-line (downtime) or on-line

13. Contiguous Allocation
• Mapping from logical to physical

14. Allocation Methods - Linked
• Linked allocation – each file a linked list
of blocks
– File ends at nil pointer
– No external fragmentation
– Each block contains pointer to next block
– No compaction, external fragmentation

15. Linked Allocation

16. File-Allocation Table

17. Allocation Methods - Indexed
• Indexed allocation
– Each file has its own index block(s) of pointers to
its data blocks
• Logical view
index table

18. Example of Indexed Allocation

19. Indexed Allocation – Mapping
(Cont.)

20. Free-Space Management
• Maintaining the free-space list to keep track of free disk space.
• File system maintains free-space list to track available blocks/clusters
– (Using term “block” for simplicity)
• Bit vector or bit map (n blocks)
…
0 1 2 n-1
bit[i] =

1  block[i] free
0  block[i] occupied
3,4,6 free block

21. • File Creation
– Search the free space list for the required amount of space
– Space is removed from the free space list and allocate it to
the new file
• File Deletion
– Disk space is added to the free space list
• Types of mechanism
• The Free space list can be maintained in the following
ways
– Bit Vector
– Linked List
– Grouping
– Counting

22. Free-Space Management (Cont.)
• Bit map requires extra space
• Each block is represented by 1 bit. If the block is free the bit is 1 if the
block is allocated the bit is 0.

23. Linked Free Space List on Disk
 Linked list (free list)
 Cannot get contiguous
space easily
 No waste of space
 No need to traverse the
entire list (if # free blocks
recorded)

24. Free-Space Management (Cont.)
• Grouping
• A modification of the free list approach is to store the
addresses of n free blocks in the first free block.
• plus a pointer to next block that contains free-block-pointers
(like this one)
• Counting
• Keep address of first free block and count of following
free blocks
• Free space list then has entries containing addresses and
counts

25. DISK STRUCTURE
• The traditional head-sector-cylinder, HSC
numbers are mapped to linear block
addresses by numbering the first sector on the
first head on the outermost track as sector 0.
• Numbering proceeds with the rest of the
sectors on that same track, and then the rest
of the tracks on the same cylinder before
proceeding through the rest of the cylinders to
the centre of the disk

26. Disk Scheduling
• Minimize seek time
• Seek time  seek distance
• Disk bandwidth is the total number of
bytes transferred, divided by the total
time between the first request for
service and the completion of the last
transfer

27. Disk Scheduling (Cont.)
• To service a requset,a disk system requires that
the head to be moved the desired track,then a
wait for latency and finally transfer it
• Several algorithms exist to schedule the
servicing of disk I/O requests
1. FIFO
2. Shortest seek time first
3. Scan
4. Circular scan
5. LOOK
6. Circular LOOK

28. • The analysis is true for one or many platters
• We illustrate scheduling algorithms with a
request queue (0-199)
98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53

29. FCFS
Illustration shows total head movement of 640 cylinders

31. SSTF
• Shortest Seek Time First selects the request
with the minimum seek time from the
current head position
• SSTF scheduling is a form of SJF scheduling
• Illustration shows total head movement of
236 cylinders

34. SCAN
• The disk arm starts at one end of the
disk, and moves toward the other end,
servicing requests until it gets to the
other end of the disk, where the head
movement is reversed and servicing
continues.
• SCAN algorithm Sometimes called the
elevator algorithm

35. SCAN (Cont.)

37. C-SCAN
• The head moves from one end of the disk to
the other, servicing requests as it goes
– When it reaches the other end, however, it
immediately returns to the beginning of the
disk, without servicing any requests on the
return trip
• Treats the cylinders as a circular list that
wraps around from the last cylinder to the
first one
• Total number of cylinders?

38. C-SCAN (Cont.)

39. LOOK
• Similar to SCAN beginning direction
• it reverses the direction immediately,
without going all the way to the end of
the disk

40. C-LOOK
• C-LOOK a version of C-SCAN
• Arm only goes as far as the last
request in each direction, then
reverses direction immediately,
without first going all the way to the
end of the disk

41. C-LOOK (Cont.)

42. Linux
File system
• Computer network is used to support RFS
• File transfer protocol(FTP) and Secure shell
protocol(SSP)
Client-server model:
1.Client
2.Server
3.Service

43. • Clients -mount remote file system from server
• Client open communication channel using IP
address of the remote host and port address
• Server opens to start service
• Network File system(NFS) – standard for UNIX
client server file sharing protocol
• Common Internet File system – WINDOWS

44. Failure modes
Reason for local file system failure
• Media fails where the file system is stored
• Corruption of directory structure
• Power cable and hard disk cable failure
• Disk controller failure
• Host adapter failure

45. Consistency semantics
• Related with file sharing
• When does the changes of original file reflect to
other users
1.Unix – Writes to open file immediately visible
2.Session - Writes to open file not visible. Once the
file is closed, the changes are visible only to new
sessions
3.Immutable – Becomes immutable and read only
4.Transaction like semantics

46. Record blocking
• File is a collection of data records
1.Fixed blocking
2.Variable length spanned blocking – records are
packed into blocks without wastage of space
3.Variable length unspanned – Large wastage