File Systems Basics, Page Cache, & I/O Buffering
Understand how operating systems store and cache data: Inodes, OS Page Cache, write-back flushing (fsync), direct I/O, and Write-Ahead Logging (WAL).
01.1. Inodes and the UNIX File System Layout
In UNIX-like operating systems (ext4, XFS, APFS, ZFS), a file is represented on disk by an Inode (Index Node).
An Inode contains metadata:
- File size, permissions (rwxrwxrwx), owner UID/GID.
- Timestamps:
atime(access),mtime(modify),ctime(inode change). - Pointers to physical storage blocks (direct, indirect, double-indirect block extents).
The Critical Insight: The filename itself is NOT stored inside the inode. A directory is simply a special file containing a table that maps human-readable strings (filenames) to Inode numbers. This explains why Hard Links can point multiple filenames to the exact same underlying inode without duplicating data.
Linux Page Cache Write Pipeline & fsync Durability Guarantee 💾
Linux Page Cache Write Pipeline & fsync Durability Guarantee 💾
How writes travel through user space, kernel page cache, and drive hardware buffers before reaching persistent non-volatile media.
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