Operating Systems 2017F Lecture 7

Video

Video from the lecture given on September 28, 2017 is now available.

Code

Code and files from the lecture (captured as they were at the end) are available here.

Notes

3000 Sep28 Next week: Anil is away from Sunday to Thursday morning. Review section.

Topic today: File systems

What is a file?

It is a key - value pair.

• Key: hierarchical filename (pathname)
• Value: arbitrary number of bytes

In principle, you can use files to store very small values

• most filesystems have a minimum file size of at least 1k, often 4K or more
• a file containing one byte of data wastes a lot of real disk space

A set of files 'stored' together, sharing a common hierarchical root, is a filesystem. Why quote 'stored'?

• Because there is no storage for virtual filesystems (such as /proc and /sys)
• when you first start a UNIX system, you start with the ‘root’ filesystem. (have nothing to do with root user.)
• other filesystems can be put on top by "mount"-ing them.

Demonstration

Run mount, it shows

 /dev/vda1 on / type ext4 (rw, relatetime,data=ordered)

• /dev is root filesystem
• / is the top of hierarchy
• ‘rw’ means read write
• /def/vda1 is a Special file, files in /dev are special files.

(Special files are actual files, historically they might have been stored on disk, nowadays they are not.)

Run df . (df is for displaying the amount of available disk space for file systems on which the invoking user has appropriate read access.)

• There is a filesystem called ‘udev’, which is a virtual file system with zero space used, since it is created automatically on run time

Run who in another terminal

• it displays soma tty7 2017-09-28 12:59 (:0)

Run ls /dev

• Instead of vda, it shows sda. The ‘v’ in vda stands for ‘virtual’. The ‘s’ in sda stands for SCSI (The Small Computer System Interface).
• Any device you want to access on unix system has /dev

Run ls -a in /dev

• Looking at the permissions (e.g ‘crw--rw----’), first letter for normal file is blank , directory is ‘d’, ‘c’ is for Character Devices, ‘b’ is for Block Devices

• Character devices example: keyboard, mouse, /dev/random

-read/write a single byte (character) at a time

• What makes b-disk different? Caches.

-Hard drives are optical drives are examples of block devices

• Character and block devices are accessed via special files, denoted by 'c' and 'b' in the file metadata
• Block devices are for block storage, to cache results

-It is faster to read or write large chunks of data (e.g. 4KiB) than it would be one byte at a time

-Traditional filesystems are stored in block devices

• However, a filesystem can be anything that provides a filesystem interface.

file interface basics:

• open, read, write, seek, close
• open directory, read/write directory, close directory

Block size issues:

• larger blocks, less overhead

-there is a fixed cost for each block access

-large files span many blocks

• smaller blocks, more efficient space usage

-you end up with fewer files that are smaller than one block

filesystem interface basics:

-mount and unmount

filesystem block size is a multiple of disk block (sometimes called sector or cluster) size

 e.g. an SSD has a 512 byte sectors with an EXT3 filesystem with 8,192 byte (8K) blocks

Inodes

How do you structure a filesystem? First, you have to understand inodes.

Demonstration

1. Run cp lec07.txt test.txt

- There is no difference between those files

2. Run ln test.txt duplicate.txt

- There is difference between duplicate.txt and test.txt: A number for test.txt changed from ‘1’ to ‘2’. That number is inode count

- ln stands for link

3. Run ln duplicate.txt alsodup.txt

- Inode count number changed ‘2’ from ‘3’

4. Run ln -s duplicate.txt another.txt

- There is no different between another.txt lec07.txt

5. Run ls -lahas diffrence’

- It shows another.txt -> duplicate.txt (symbolic link pointing one to the other

6.Run rm duplicate.txt

- The arrow is still pointing, but inode count number goes down. (From ‘3’ to ‘2’)

7. Run cat another.txt

- It shows there is such file, because the file that pointing to is not exist

• ‘ln’ creates hard links to an item by default.
• File name is a pointer to an inodes.
• ‘rm’ is not remove, just unlink, which reduces the link count for inode
• When you clear storage, when inode inbound number go to 0, is not link to other file,system will remove it.
• When the user open a file, reference count is incremented.
• A file sys doesn’t have to be a block device? It can store anywhere as long as you provide the api.
• A file system can be made in a file (the basic of virtual machine)

One other file api: mmap

Inode store permission

mmap, munmap: map or unmap files or devices into memory, this is the piece that unify the file sys with process memory.

What ‘read’ does: read directly into RAM

Run top:

-There are ‘VIRT’, ‘RES’ and ‘SHR’:

VIRT stands for the virtual size of a process
RES stands for the resident size, which is an accurate representation of how much actual physical memory a process is consuming.
SHR indicates how much of the VIRT size is actually sharable (memory or libraries).