File system internals

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File system internals

• Traditional Unix (Bach)
• Linux block cache
• Linux memory-mapping (mmap)

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Traditional Unix (Bach)

• Lower Level File System Algorithms

Name-to-inode
Allocating inodes
Allocating buffers
Handling inodes
Buffer allocation algorithms
Buffer manipulation algorithms getblk brelse
bread breada bwrite
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Interpreting theprevious picture

• This shows the low level algorithms used to
  implement the system calls

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Linux specialties

• Virtual file system
• Allows a common interface to a large variety of
  file systems
• Implemented as an abstraction layer
• File system implementations are in
  /usr/src/linux/fs
• Uses the fops structure of pointers to methods

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Linux specialties

• Ext2 file system
• Clustered implementation for efficiency
• Has hooks, but not implementation for undelete
• mmap idea
• A file being accessed randomly can be treated as
  virtual memory
• Accessing a block is handled through the virtual
  memory subsystem, not the disk block cache
• Dentry (directory entry) objects
• Cached
• Accessed via hashing

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The file-operations structure (half)
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The file-operations structure (half)
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Internal linux structures

• Directory objects
• These are artificially constructed for file
  systems without directories (FAT)
• Dentry objects
• Multipurpose
• Acts as a controller for the inode cache
• In Linux it is effectively an entry in the system
  open file table
• In-core inodes
• These are artificially constructed for file
  systems without inodes (FAT)
• These are always cached hence the term in-core

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The ext2 file system

• Efficiency features
• Block size 1024, 2048, 4096
• Partitioned into groups to shorten seek time
• Choice of blocks/inode
• Preallocated contiguous spaces for regular files
• Fast symbolic links (link info stored in inode)
• Safety features
• Doing operations in repairable order
• Making new hard link done by
• Increment refcount in inode
• Then add new name to directory
• If crash happens between these two operations,
  fsck finds and fixes
• Support for (even root cant override these)
• Immutable files
• Append-only files

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Planned new features for ext2

• Block fragmentation
• Same block can contain fragments of several file
  ends
• Access control lists
• Fine-grained and temporary control of access
• Handling compressed and encrypted files
• Note compress first, then encrypt -
• Logical deletion (supports undelete)
• Reason is obvious

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Structure of an ext2 cluster

• Features and size
• Bitmaps are all block size so number of items
  ltbits per block
• Looking at bitmaps word-by-word allows fast
  searching for holes
• Superblock and group descriptors are copies of
  those in other groups
• Number of groups/file system gt
  partition_size/group_size

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Contents of superblock and group
descriptors(partial list)

• Superblock
• Must fit in 1024 bytes (why?)
• Flexible version number, etc.
• Size and current number of everything global
• Group descriptor (24 bytes)
• Location and current number of
• Bitmaps, inode tables, directories, and data
  blocks
• Information on each group is found in each
  groups descriptor area each group knows about
  all the others

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Inodes and the inode table

• Table size is implied in group descriptor
• Inode size is a fixed 128
• Structure is almost like all other unix
  filesystems also
• Fragment address
• ACL for descriptor

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Usage of blocks regular files and directories

• Directory
• Names can be up to 255 bytes must contain
  length
• End of entry is padded to multiple of 4
• Symbolic link
• Destination (if less than 60 char) is in inode,
  else in a data block
• Device file, pipe, and socket
• Everything fits into inode
• Bitmap caches are used
• One each for inode and data blocks

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Disk space management

• Goals
• Avoid fragmentation
• Time efficiency
• Methods for achieving these goals
• Allocation
• Directories should be evenly distributed among
  groups
• Files should be near their directories
• Preallocation allocates adjacent blocks near any
  new one