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Enhancing File System Efficiency with Fast File System Policies

Learn how Fast File System (FFS) addresses Unix file system shortcomings, improves data allocation, enhances file locality, and minimizes fragmentation. Explore the benefits of cylinder groups, allocation policies, and disk-aware design.

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Enhancing File System Efficiency with Fast File System Policies

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  1. Chapter 41.Locality and The Fast File System Sangchul Kim(stdio@snu.ac.kr) School of Computer Science and Engineering Seoul National University

  2. Contents • Background • Problem • Fast File System • The Cylinder Group • Policies • A Few Other Things • Summary

  3. Background S i-nodes Data • Unix file system • Super block (S) • Information about the entire file system • How big the volume is • How many i-nodes there are • A pointer to the head of a free list of blocks • The Advantage of this file system • Simple • Support the basic abstractions • Files and the directory hierarchy

  4. Problem A A E B E B C C D E D E • The performance is terrible • Data blocks are allocated randomly • File system ends up getting fragmented • Transferring data from the disk is inefficient • Block size is too small (512 bytes) • i-nodes are allocated far from data blocks • All i-nodes at beginning of disk is far from the data • Going back and forth from i-nodes to data blocks

  5. Fast File System (FFS) url: https://www.bitrix24.com/about/blogs/tips-n-trick/how-to-set-up-an-inspiration-system-and-never-run-out-of-ideas.php (retrieved on 2015/06/02) • The ideas • Design the file system structures • The cylinder group • Design allocation policies to be “disk aware” • File locality • Change the internal implementation • Keep the same interface to the file system

  6. Fast File System (FFS) url: http://osr507doc.sco.com/en/HANDBOOK/hdi_dkinit.html (Retrived on 2015/06/02) • The cylinder group (block group) • Comprise of one or more consecutive cylinders • Disk partition is divided into one or more cylinder groups

  7. Fast File System (FFS) Disk drive File system • The cylinder group (block group) • Super block copy(S) • For reliability reasons (e.g., corruption or scratch) • Inode bitmap (ib) and data bitmap (db) • Manage free space • i-node and data block • Unix file system

  8. Fast File System (FFS) • Policies • The core idea of policies • “related” data are placed in the same cylinder group • The files that are in same directory • Place in the same cylinder group • How to allocate files and directories • Placement of files • Allocatethe data blocks of a file in the same group as its i-node • Prevent long seeks • Placement of directories • A low number of allocated directories for balance • A high number of free i-nodes to be able to allocate bunch of files

  9. Fast File System (FFS) G0 G1 G2 G3 G4 G5 G6 G7 G8 G9 6 7 8 9 4 5 2 3 0 1 0 1 2 3 4 5 6 7 8 9 • Policies • The large-file exception • Without a different rule, a large file would entirely fill the block group • It prevents subsequent “related” files from being placed within this block group • Choose the chunk size carefully • Affect the performance

  10. A Few Other Things 10 9 5 4 10 8 11 9 7 11 0 0 3 6 1 8 6 5 2 2 1 4 7 3 • Internal fragmentation • The block size is 4KB, but many files are under that size • Divide a single file system block into fragments (sub-blocks) • Continue allocating 512 byte blocks • After acquiring a 4KB of data, the sub-blocks copy in to 4KB block • Using buffers, the redundant I/O is avoided. • Parameterization

  11. A Few Other Things • Long file names • File names can be of arbitrary length. • Symbolic link • Implemented as a file that contains a pathname. • Atomic rename() operation • Previous file system required three calls to the system • Target file could be left with only its temporary name.

  12. Summary • The ideas to solve the Unix file system problem • The cylinder group and “disk aware” policy • To solve the allocation problem of data blocks • To solve the allocation of i-nodes far from data blocks • The expansion of the block size • To solve the bad performance of transferring data from the disk • The introduction of a few other things • Internal fragmentation • Parameterization • Long file names • Symbolic link • Atomic rename() operation

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