File System FAQ

1
File System FAQ
2
Fs_init() vs. fs_mkfs()

• What need to be done where
• FS_init() takes care of system wise FS related
  initialization (Related to the running system).
• FS_mkfs() is doing the initialization of the FS
  itself. (Related to the specifics of FS on disk)
• How to tell
• Think what will happen if you have two disks in
  your system (How you separate work)

3
Fs_init() vs. fs_mkfs() (cont.)

• When to init file descriptor table.
• When to set bitmaps and inodes to be free.
• When to initialize currentInode (correspond to
  /)

4
Directory structure

• Disk layout vs memory layout common.h

typedef struct char nameMAX_NAME int
inodeNo char dummyDIRECTORY_ENTRY_SIZE -
MAX_NAME - 4 dirEntry
define MAX_NAME 32 define MAX_INODE_DIGITS
31 define DIRECTORY_ENTRY_SIZE
(MAX_NAMEMAX_INODE_DIGITS1) define INVALID -1
5
Inode contents

• Opens To keep track of concurrent opens for this
  file. (No need to put them on disk)
• Links To keep track of number of hard links.

6
Fs_lseek() semantic.

• Our fs_lseek() only takes two arguments
• fd, offset
• Unix lseek() takes three
• fd, offset, whence
• Whence SEEK_SET, SEEK_CUR, SEEK_END
• So our fs_lseek() will assume one of them
• SEEK_SET, if you prefer other, put that in your
  README file.
• What if lseek() want to seek after end of file?

7
Pathname (Absolute pathname?)

• Do we need to support absolute pathname
• Eg When I am in /foo, do I need to support
  chdir /bar/tmp/
• You do not need to do that for this project.
  (Otherwise, you would have to support absolute
  pathname everywhere open, link, mkdir etc)

8
Remove directory

• Do I need to support removing directory that are
  not empty?
• Ie Remove all the subdirectories and files
  contained in the parent directory when removing
  parent directory?
• No. Just return error when the directory to be
  removed is not empty.

9
Treat directory as file?

• What does it mean why?
• If you treat directory as file, then you can use
  your existing function to handle file I/O to do
  directory manipulation. (Avoid duplicate efforts
  internally.)
• Say if you want to implement indirect blocks
  (both for file and directory)
• If you treat directory same as file, then you
  would not need to worry about indirect blocks
  when you are working on the directory.

10
Can I modify fs_fsck and others

• Yes. If you have different disk layout or
  assumptions than ours. (But make sure you still
  have a correct fs_fsck! Also document them in
  README.)
• Also if you implement extra credits, you would
  have to make fs_fsck correct as well.

11
Synchronous operation

• Can I cache directory, inode, bitmap etc to
  speed up FS?
• Yes. But for this project, your first priority is
  to make it correct. (ie, if power failure occurs,
  your FS should still be in consistent state.) So
  better make them synchronous.
• If you make them work, you can get extra credit!!

12
Want some more for Christmas?!

• If you think you have fancy ideas to improve our
  FS, you can get extra credits. Please email me on
  what you want to do and we can discuss them.
• Potential ideas (Other than on web)
• Implement buffer cache (1 credit?)
• Implement write ahead log (2 credits?)
• Implement a good automated test suite using
  expect? (1 credit?)