The document discusses a new file system developed for Unix, which addresses inefficiencies in traditional file systems by optimizing block sizes and utilizing cylinder groups for better data allocation and management. It highlights key improvements in throughput, storage utilization, and functional enhancements such as long file names and quotas. Additionally, it presents layout policies that enhance data locality and performance while reducing fragmentation.

1. A Fast File System for Unix
Marshall K. Mckusick, William N. Joy,
Samual J. Leffler and Robert S. Fabry
Computer Systems Research Group, UCB
Presented By:
CS 5204: Operating Systems, Virginia Tech Parang Saraf

2. About the Paper
• Considered as one of the most fundamental papers in
operating systems
• Have been cited around 930 times
• Describes a new file system
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3. Traditional File System
• File System developed at Bell Laboratories
• A file system is described by its Super-Block
o Number of Data Blocks
o Count of maximum number of files
o Pointer to free list (linked list to all free blocks)
• Disk drive is divided into partitions
o Each disk partition may contain one file system
o A file system never spans multiple partitions
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4. Traditional File System
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5. Traditional File System – Inode
• Each file has a descriptor associated with it – Inode.
• Information includes:
o Ownership of the file
o Time stamps marking last modification and access time
o Array of indices pointing to the data blocks
 Direct Blocks – 8
 Indirect Blocks – Singly, Doubly and Triply
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6. Traditional File System – Inode
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7. Traditional File System – Inode
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8. Traditional File System – Problem
• Inode information segregated from Data
o Long seek time from inode to its data
• Files in single directory are not typically allocated consecutive
slots for inode information
o Many non-consecutive blocks of inodes are accessed when executing
operations on inodes of several files in a directory
• Sub-optimum allocation of data blocks
o Small Block size – 512 bytes
o Many Seeks – Next sequential block is not on the same cylinder
o Limited read-ahead
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9. Old File System
• Developed at Berkeley
• Increased Throughput
• Changing the basic block size from 512 bytes to 1024 bytes
• Each disk transfer accessed twice as much data
• Less number to indirect blocks used
• Increased Reliability
• Staging modifications to critical file system information so that they could
either be completed or repaired cleanly after a crash
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10. Old File System – Problem
• Old file system was still using just 4% of disk bandwidth
• Main problem – Scrambled Free List
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11. Old File System – Problem
• Old file system was still using just 4% of disk bandwidth
• Main problem – Scrambled Free List
o Initially ordered for optimal access
o Scrambled because files were created and removed
o Eventually becomes entirely random – blocks allocated randomly
o On creation provides transfer rates up to 175 kbps
o Rate deteriorates to 30 kbps after a few weeks of moderate use
• Possible Solution – Dump, rebuild and restore / Fragmentation
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12. New File System
• Each disk drive contains one or more file systems
• A File System is described by its super-block, located at
the beginning of the disk partition
• Super-block is replicated to protect against catastrophic
loss
• Block size is any power of two >= 4096 bytes
o Decided at the time of file system creation and can’t be changed
o File Systems can have different block sizes
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13. New File System – Cylinder Groups
• Comprises of one or more consecutive cylinders
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14. New File System – Cylinder Groups
• Comprises of one or more consecutive cylinders
• Disk partition is divided into one or more cylinder groups
• Has associated book-keeping information:
o A redundant copy of super-block
o Space for inodes
o A bit map describing available blocks – replaces free list
o Summary information describing usage of data blocks
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15. New File System – Cylinder Groups
• Contains static number of inodes:
o Allocated at file system creation time
o Default policy – one inode for each 2048 bytes
• Book-keeping information begins at varying offset from the
beginning of the cylinder group
o Redundant information spirals down into the cylinder
o Any single track, cylinder or platter can be lost without losing copies of the
super-block
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16. New File System – Structure
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17. New File System – Key Contributions
• Optimizing storage utilization
• File System Parameterization
• Layout Policies
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18. Optimizing Storage Utilization
• New 4096 size blocks – transfers 4 times more
• Problem with large blocks:
o Wasted space due to small files
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19. Optimizing Storage Utilization
• Solution:
o Divide the 4096 block into 2, 4 or 8 fragments to accommodate small files
o Fragment size is specified at the time file system is created
o Block map records the space available at fragment level
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20. Optimizing Storage Utilization
• Free List vs Bitmap
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21. Optimizing Storage Utilization
• Space allocation:
o Space is allocated when a program does a write system call
o Three possible conditions:
 Enough space left in an already allocated block or fragment
 File contains no fragmented blocks – allocate new blocks and fragments
 File contains one or more fragmented blocks but has insufficient space
to hold new data – new block is allocated, old fragments are copied and
new fragments are appended
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22. Optimizing Storage Utilization
• Free space reserve
o Minimum acceptable percentage of file system blocks that should
be free – 90%
o Only system administrator can allocate blocks after that
o Important for the layout policies to be effective
o After this the file system throughput is cut in half because of the
inability to localize blocks in a file
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23. Optimizing Storage Utilization
• Wasted space comparison
o Space wasted by 4096/1024 byte new file system is same as 1024
byte Old File System
o New file system uses less space for indexing large files
o Uses same amount of space for small files
o Free space reserve should also be counted as wasted space
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24. File System Parameterization
• Optimum block allocation based on hardware parameters
o Speed of Processor
o Hardware support for mass storage transfers
o Characteristics of the mass storage devices
• Blocks are allocated on the same cylinder
• Block allocation depends on whether the processor has
an input/output channel or not
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25. File System Parameterization
Accessing which data is faster?
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26. File System Parameterization
Accessing which data is faster?
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Depends whether processor has I/O channel or not

27. File System Parameterization
• Rotationally Optimal Blocks
o Processors without I/O channels must field an interrupt and then prepare for a
new disk transfer
o Disk rotates during this time
o Place blocks such that disk rotation is taken into account before the start of a
new disk transfer operation
• Cylinder group summary information includes count of
blocks based on different rotational positions – 8 positions
• Super-block contains a vector of lists called as
Rotational Layout Tables – Used by system when
allocating new blocks
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28. File System Parameterization
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29. Layout Policies
• Layout policies divided into two distinct parts:
o Global Policies
o Local Allocation Routines
• Two allocable resources:
o Inodes
o Data Blocks
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30. Layout Policies
• Global Policies
o Uses file system wide summary information to make decisions
regarding the placement of new inodes and data blocks
o Tries to localize data that is concurrently accessed while spreads
out unrelated data
o Inodes:
 Places all inodes of files in a directory in the same cylinder group
 A new directory is placed in a cylinder group that has a greater than
average number of free inodes and the smallest number of directories
already in it – ensures that files are distributed throughout the disk
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31. Layout Policies
• Global Policies
o Data Blocks:
 Tries to place all data blocks for a file in the same cylinder group
 None of the cylinder groups should ever become completely full
 Heuristic Solution – redirect block allocation to a different cylinder group
when a file exceeds 48 kb and at every MB thereafter
 Ensures that cost of one long seek per MB is small
 New cylinder groups are chosen from those cylinder groups that have a
greater than average number of free blocks left
 Finally it calls Local Allocation Routines for block allocation
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32. Layout Policies
• Local Allocation Routines
o Allocates a free block as requested by the Global layout policies
o Uses a four level allocation
o First Level – use the next free block that is rotationally closest to the requested
block on the same cylinder
Cylinder 0
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33. Layout Policies
• Local Allocation Routines
o Second Level – if there are no free blocks on the same cylinder, a free block
in the same cylinder group is selected
Cylinder 0
Cylinder Group
Cylinder 1
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34. Layout Policies
• Local Allocation Routines
o Third Level – if the cylinder group is full, use the quadratic hash function to
hash the cylinder group number to find another cylinder group to look for a
free block
o Fourth Level – if the hash fails, use an exhaustive search on all cylinder
groups
o Quadratic Hash
o is used because of its speed in finding unused slots in nearly full hash tables
o File systems parameterized to maintain 10% free space rarely use this
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35. Performance
• Measured Throughput
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36. Performance
• List Directory command performance
o For large directories containing many directories, disk access for inodes is cut
by a factor of two
o For large directories containing only files, disk access for inodes is cut by a
factor of eight
• Both reads and writes are faster in new file system
o Because larger block sizes are used
o The overhead of allocating is more but cost per byte allocation is same
o Reading rate is always at least as fast as writing rate
 Writes are slower for 4096 byte block as compared to 8096 byte block
 In old file system writing was 50% faster than reading
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37. New File System - Limitations
• Limited by memory to memory copy operations required
to move data from disk buffers in the system’s address
space to data buffers in the user’s address space
o Buffer alignment of both address space
• One block is allocated to a file at a time
o Pre-allocate several blocks at once and releasing unused ones on file closing
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38. Functional Enhancements
• Long File Name
• File Locking
• Symbolic Links
• Rename
• Quotas
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39. Long File Name
• Maximum length of file name is 255 characters
• Directories are allocated 512 byte units called chunks
• Chunks are broken into Directory Entries:
o Contains information necessary to map the name of file with inode
o First three fields are fixed length – inode number, size of entry and length of
file name
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40. File Locking
• Hard Lock – always enforced when a program tries to
access a file
• Advisory shared or exclusive locks – requested by the
programs
• System administrator privilege can override locks
• No deadlock detection is attempted
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41. Symbolic Links
• A symbolic link is implemented as a file that contains a
pathname
• Pathname can be relative or absolute
• On encountering a symbolic link while interpreting a
component of a pathname, the contents of the symbolic
link is prepended to the rest of the pathname
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42. Rename
• Old file system required three system calls for renaming
• Target file could be left with temporary name due to crash
• New rename system call added that guarantees the
existence of the target name
• Renaming works both on directory and files
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43. Quotas
• Old file system – any single user can allocate all the
available space in the file system
• Quota restricts the amount of file system resources that a
user can obtain
• Sets limits to both inodes and number of disk blocks
• Hard and soft limits
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44. Key Take-Away points
• Substantially higher throughput rates – large block size
• Flexible allocation policies
o Better locality of reference
o Less wastage
• Adapted to wide range of peripheral and processor
characteristics
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45. References
• Presentation on “A Fast File System” by:
o Zhifei Wang : www.cs.pdx.edu/~walpole/class/cs533/spring2006/slides/191.ppt
o pdc-amd01.poly.edu/~wein/cs6243/ppts/fastfile.ppt
o Sean Mondesire and Subramanian Kasi :
www.cs.ucf.edu/courses/cop5611/spring05/item/FFS.ppt
o www.scs.ryerson.ca/~aabhari/File_System.ppt
• http://flylib.com/books/en/3.224.1.79/1/
• http://osr507doc.sco.com/en/HANDBOOK/graphics/harddisk.gif
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