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: Parang Saraf CS 5204: Operating Systems, Virginia Tech

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 2

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 3

Traditional File System 4

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 5

Traditional File System – Inode 6

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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 8

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 9

Old File System – Problem Old file system was still using just 4% of disk bandwidth Main problem – Scrambled Free List 10

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 11

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 12

New File System – Cylinder Groups Comprises of one or more consecutive cylinders 13

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 14

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 15

New File System – Structure 16

New File System – Key Contributions Optimizing storage utilization File System Parameterization Layout Policies 17

Optimizing Storage Utilization New 4096 size blocks – transfers 4 times more Problem with large blocks: o Wasted space due to small files 18

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 19

Optimizing Storage Utilization Free List vs Bitmap 20

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 21

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 22

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 23

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 24

File System Parameterization 25 Accessing which data is faster?

File System Parameterization 26 Accessing which data is faster? Depends whether processor has I/O channel or not

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 27

File System Parameterization 28

Layout Policies Layout policies divided into two distinct parts: o Global Policies o Local Allocation Routines Two allocable resources: o Inodes o Data Blocks 29

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 30

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 31

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 32 Cylinder 0

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 33 Cylinder Group Cylinder 0 Cylinder 1

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 34

Performance Measured Throughput 35

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 36

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 37

Functional Enhancements Long File Name File Locking Symbolic Links Rename Quotas 38

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 39

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 40

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 41

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 42

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 43

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 44

References Presentation on “A Fast File System” by: o Zhifei Wang : o pdc-amd01.poly.edu/~wein/cs6243/ppts/fastfile.ppt o Sean Mondesire and Subramanian Kasi : o