Writing Filesystems

From Genunix

How to write a Solaris filesystem

Finally fulfilling a promise I gave a while ago, I'm going to braindump a little mini-series here on what I know about the sourcecode structure of Solaris filesystems, the undocumented interfaces that filesystem code in Solaris must use to actually work, how these interfaces work and how locking issues / typical design faults that Solaris filesystems have had in the past can be avoided in new implementations. As this unspools, a reimplementation of the FAT filesystem driver will be written, explaining step-by-step how the skeleton code grows into a functional implementation.

As an editorial note, I had a title "Writing a Solaris filesystem in 21 days" for this series originally. I must admit I failed at doing it in 21 days - so beware, it might take longer ...

Table of contents

Introduction 

What does it involve writing a (disk-based) filesystem for Solaris ? Why this article ?

Sourcecode Structure 

The sourcefile tree for a Solaris filesystem driver and associated utilities

Build Environment 

How to set up an OpenSolaris workspace for developing your filesystem

Module glue 

Filesystem drivers are kernel modules - but not 'ordinary' kernel modules ...

Mount option handling 

See how mount option parsing can be handed off to the framework

VFS and Vnode interfaces 

Provides an overview of per-mountpoint (VFS) and per-file (Vnode) operations that a filesystem driver must/may implement

Specifics about VFS Operations

VFS_MOUNT(), VFS_UNMOUNT(), and VFS_FREEVFS()

VFS_STATVFS()

VFS_ROOT()

VFS_SYNC()

VFS_VGET() - Requesting a file node from a filesystem instance

Userdata I/O 

Excursion on how read/write and mmap-based I/O

A simple locking protocol for a read/write filesystem 

Reentrancy in filesystems and the need for shared/exclusive access for directory and file updates

Specifics about Vnode interfaces for file I/O

Opening and closing files - VOP_OPEN(), VOP_CLOSE()

Reading and writing files via system calls - VOP_READ(), VOP_WRITE()

Support for mmap'ed IO - VOP_MAP(), VOP_ADDMAP() and VOP_DELMAP()

Backend for mapped IO - VOP_GETPAGE(), VOP_PUTPAGE()

File and directory attributes - VOP_GETATTR(), VOP_SETATTR(), VOP_ACCESS()

ACLs and security attributes - VOP_GETSECATTR(), VOP_SETSECATTR()

ioctl on files - VOP_IOCTL()

Specifics about directory-related Vnode operations

Reading directory contents - VOP_READDIR() and VOP_LOOKUP()

File and directory creation/removal/rename - VOP_CREAT(), VOP_REMOVE(), VOP_RENAME(), VOP_MKDIR(), VOP_RMDIR()

VOP_INACTIVE() and a vnode's lifecycle

Shows how VFS_VGET() and VOP_INACTIVE() complement each other, and describes support for forced umount

Generic directory walking support code 

demonstrates how readdir/lookup codepaths can be unified using a generic directory walker mechanism

Mapping file/directory offsets to disk blocks 

one of the most important tasks of a filesystem - where's my data, dude ?

Timestamps 

POSIX atime, mtime, ctime, and their (non)-equivalents in a given filesystem

Filesystem Utilities

mount / unmount

fsck

mkfs