Simplefs

A simple filesystem to understand things.

This is a Work In Progress. Do not use this yet.

If you are planning to learn filesystems, start from the scratch. You can look from the first commit in this repository and move the way up.

The source files are licensed under Creative Commons Zero License. More information at: http://creativecommons.org/publicdomain/zero/1.0/ Full license text at: http://creativecommons.org/publicdomain/zero/1.0/legalcode

simplefs 1.0 Architecture + Notes

Block Zero = Super block Block One = Inode store Block Two = Occupied by the initial file that is created as part of the mkfs.

Only a limited number of filesystem objects are supported. Files and Directories can be created. Support for .create and .mkdir is implemented. Nested directories can be created. A file cannot grow beyond one block. ENOSPC will be returned as an error on attempting to do. Directories store the children inode number and name in their data blocks. Read support is implemented. Basic write support is implemented. Writes may not succeed if done in an offset. Works when you overwrite the entire block. Locks are not well thought-out. The current locking scheme works but needs more analysis + code reviews. Memory leaks may (will ?) exist.

Credits

The initial source code is written by psankar and patches are contributed by other github members. azat being the first contributor.

An O_LARGETHANKS to the guidance of VijaiBabu M and Santosh Venugopal. Their excellent talks on filesystems motivated me to implement a file system from the scratch. Without their inspirational speeches, I would not have focussed on filesystems.

A big thanks should go to the kernelnewbies mailing list for helping me with clarifying the nitty-gritties of the linux kernel, especially people like Mulyadi Santosa, Valdis Kletnieks, Manish Katiyar, Rajat Sharma etc.

Special thanks to Ankit Jain who provides interesting conversations to help my intellectual curiosities.

Patch Submission

Please send a merge request only if you are ready to publish your changes in the same creative commons zero license.

We would like to keep the filesystem simple and minimal so that it can be used as a good teaching material.

We are not planning to use this filesystem in a production machine. So some design choices are driven by learning/teaching simplicity over performance/scalability. This is intentional. So do not try to fix those things :)

TODO

• After the 1.0 release, start with support for extents, which on completion will be 2.0
• After the 2.0 release, start with journalling support, which on completion will be 3.0

To compile:

install linux kernel sources and run make from the checkedout directory.

To test:

And here is the build environment for simplefs: azat/simplefs-bld

psankar@linux-9dni:~/src/simplefs> make

psankar@linux-9dni:/src/simplefs> dd bs=4096 count=100 if=/dev/zero of=image 100+0 records in 100+0 records out 409600 bytes (410 kB) copied, 0.00106827 s, 383 MB/s psankar@linux-9dni:/src/simplefs> ./mkfs-simplefs image Super block written succesfully root directory inode written succesfully welcomefile inode written succesfully inode store padding bytes (after the two inodes) written sucessfully root directory datablocks (name+inode_no pair for welcomefile) written succesfully padding after the rootdirectory children written succesfully welcomefilebody has been written succesfully

psankar@linux-9dni:~/src/simplefs>

Now as root: linux-9dni:/home/psankar/src/simplefs # insmod simplefs.ko ; mount -o loop -t simplefs image /home/psankar/src/simplefs/mount/ ; dmesg [41932.281196] Sucessfully registered simplefs [41932.284811] The magic number obtained in disk is: [268640275] [41932.284813] simplefs filesystem of version [1] formatted with a block size of [4096] detected in the device. [41932.284823] simplefs is succesfully mounted on [/dev/loop16] linux-9dni:/home/psankar/src/simplefs # cd mount/ linux-9dni:/home/psankar/src/simplefs/mount # ls vanakkam linux-9dni:/home/psankar/src/simplefs/mount # cat vanakkam Love is God. God is Love. Anbe Murugan. linux-9dni:/home/psankar/src/simplefs/mount # cp vanakkam hello linux-9dni:/home/psankar/src/simplefs/mount # cat hello Love is God. God is Love. Anbe Murugan. linux-9dni:/home/psankar/src/simplefs/mount # echo "Hello World" > hello linux-9dni:/home/psankar/src/simplefs/mount # cat hello Hello World linux-9dni:/home/psankar/src/simplefs/mount # mkdir dir1 linux-9dni:/home/psankar/src/simplefs/mount # cd dir1 linux-9dni:/home/psankar/src/simplefs/mount/dir1 # cp ../hello . linux-9dni:/home/psankar/src/simplefs/mount/dir1 # cat hello Hello World linux-9dni:/home/psankar/src/simplefs/mount/dir1 # echo "First level directory" > hello linux-9dni:/home/psankar/src/simplefs/mount/dir1 # cat hello First level directory linux-9dni:/home/psankar/src/simplefs/mount/dir1 # mkdir dir2 linux-9dni:/home/psankar/src/simplefs/mount/dir1 # cd dir2 linux-9dni:/home/psankar/src/simplefs/mount/dir1/dir2 # touch hello linux-9dni:/home/psankar/src/simplefs/mount/dir1/dir2 # cat hello linux-9dni:/home/psankar/src/simplefs/mount/dir1/dir2 # echo "Second level directory" > hello linux-9dni:/home/psankar/src/simplefs/mount/dir1/dir2 # cat hello Second level directory linux-9dni:/home/psankar/src/simplefs/mount/dir1/dir2 # ... linux-9dni:/home/psankar/src/simplefs/mount # .. linux-9dni:/home/psankar/src/simplefs # umount mount ; rmmod simplefs.ko ; linux-9dni:/home/psankar/src/simplefs # insmod simplefs.ko ; mount -o loop -t simplefs image /home/psankar/src/simplefs/mount/ linux-9dni:/home/psankar/src/simplefs # cd mount linux-9dni:/home/psankar/src/simplefs/mount # ls -lR .: total 0 drwxr-xr-x 1 root root 0 Aug 5 22:43 dir1 ---------- 1 root root 0 Aug 5 22:43 hello ---------- 1 root root 0 Aug 5 22:43 vanakkam

./dir1: total 0 drwxr-xr-x 1 root root 0 Aug 5 22:43 dir2 ---------- 1 root root 0 Aug 5 22:43 hello

./dir1/dir2: total 0 -rw-r--r-- 1 root root 0 Aug 5 22:43 hello linux-9dni:/home/psankar/src/simplefs/mount # cat hello Hello World linux-9dni:/home/psankar/src/simplefs/mount # cd dir1 linux-9dni:/home/psankar/src/simplefs/mount/dir1 # cat hello First level directory linux-9dni:/home/psankar/src/simplefs/mount/dir1 # cd dir2/ linux-9dni:/home/psankar/src/simplefs/mount/dir1/dir2 # cat hello Second level directory linux-9dni:/home/psankar/src/simplefs/mount/dir1/dir2 # ... linux-9dni:/home/psankar/src/simplefs/mount # .. linux-9dni:/home/psankar/src/simplefs # umount mount ; rmmod simplefs.ko ; linux-9dni:/home/psankar/src/simplefs # dmesg [41932.281196] Sucessfully registered simplefs [41932.284811] The magic number obtained in disk is: [268640275] [41932.284813] simplefs filesystem of version [1] formatted with a block size of [4096] detected in the device. [41932.284823] simplefs is succesfully mounted on [/dev/loop16] [41951.102689] No inode found for the filename [hello] [41951.102706] No inode found for the filename [hello] [41951.102724] No inode found for the filename [hello] [41951.102731] New file creation request [41951.102986] The new filesize that is written is: [41] and len was: [41] [41961.195423] The new filesize that is written is: [12] and len was: [12] [41968.055946] No inode found for the filename [dir1] [41968.055959] New directory creation request [41980.265026] No inode found for the filename [hello] [41980.265073] No inode found for the filename [hello] [41980.265082] New file creation request [41980.265168] The new filesize that is written is: [12] and len was: [12] [41991.857978] The new filesize that is written is: [22] and len was: [22] [41999.993579] No inode found for the filename [dir2] [41999.993592] New directory creation request [42005.461745] No inode found for the filename [hello] [42005.461756] New file creation request [42016.694882] The new filesize that is written is: [23] and len was: [23] [42024.480843] simplefs superblock is destroyed. Unmount succesful. [42024.485339] Sucessfully unregistered simplefs [42031.157983] Sucessfully registered simplefs [42031.162331] The magic number obtained in disk is: [268640275] [42031.162334] simplefs filesystem of version [1] formatted with a block size of [4096] detected in the device. [42031.162360] simplefs is succesfully mounted on [/dev/loop17] [42104.389807] simplefs superblock is destroyed. Unmount succesful. [42104.395575] Sucessfully unregistered simplefs linux-9dni:/home/psankar/src/simplefs #