OS Notes

This article was originally published on bmf-tech.com.

Overview

Notes on what I researched while studying about operating systems.

Swap

A feature that moves the contents of memory to the hard disk when memory is insufficient.

What is Page Cache

• The CPU cannot read data directly from storage.
• Data needs to be loaded into memory first.
• Data loaded into memory can be reused as page cache.
• Cache used when accessing files.

Buffer Cache

• Cache used when directly accessing block devices.

Slab Cache

• A memory area within the kernel that caches structures like dentry, which stores directory metadata, and inode, which stores file metadata.
• Memory used by the kernel such as inode and dentry.
• The kernel has a mechanism to cache various memory resources within the kernel space by resource to improve memory utilization efficiency = slab cache.
• Dentry Cache
  • Directory entry cache
  • A structure that associates hard links, parent-child relationships of directories and file names, and inode with directory names and file names.
  • What is a Hard Link
  • The association of resources like files and directories on a computer's file system with the names given to those resources, or the association itself.
  • Allows an existing file to be referenced by a different file name (path name).
  • Referenced by inode.
  • Limited to the same file system.
  • File (or path)
  • Not affected by moving the original file.
  • Does not disappear even if the original file is deleted.
  • Symbolic Link
  • Specifies an existing file by path instead of inode, allowing files from different file systems to be referenced.
  • Referenced by path.
  • Can be referenced from different file systems.
  • Can be specified for directories as well.
  • Cannot be referenced if the original file is moved.
  • Disappears if the original file is deleted.
  • Directory Entry
  • A special file within each directory that lists the names of files, their actual storage locations, creation dates, update dates, file attributes, sizes, etc.
• Slab Allocator
  • An algorithm discovered by Jeff Bonwick.
• Inode
  • A storage area for file information created for each file.
  • Records the name, size, permissions, owner, update information, etc., of files stored on the disk.