Title: Class 16: Storage
Date: 2014-03-27
Category: Classes
Tags: operating systems, kernel, kernel programming, storage, delay lines
Author: David Evans
[Exam 2](http://goo.gl/41PmXp) is due at **11:59pm tonight**.
[PS4](|filename|../../pages/ps/ps4/ps4.md) is due **Sunday, 6 April**.
# Storage Systems
## Delay Lines
> _I do not imagine that many of the Turing lecturers who will follow me
will be people who were acquainted with Alan Turing... Although a
mathematician, Turing took quite an interest in the engineering side of
computer design… Turing’s contribution to this discussion was to
advocate the use of gin, which he said contained alcohol and water in
just the right proportions..._
> Sir Maurice Wilkes (1913-2010), [_Computers Then and Now_](|filename|./a1967-wilkes.pdf) (1967 Turing
Award Lecture)
Why does it matter what you put in the delay line?
Kingston KVR16N11/4 4GB 2Rx8 512M x 64-Bit PC3-12800 [newegg](http://www.newegg.com/Product/Product.aspx?Item=N82E16820239263)
[Datasheet](http://www.kingston.com/datasheets/KVR16N11_4.pdf)
J. Alex Halderman, Seth D. Schoen, Nadia Heninger, William Clarkson, William Paul, Joseph A. Calandrino (UVa BSCS 2004), Ariel J. Feldman, Jacob Appelbaum, and Edward W. Felten. [_Lest We Remember: Cold Boot Attacks on Encryption Keys_](https://citp.princeton.edu/research/memory/). USENIX Security 2008.
# Storage Abstractions
# Unix File System
An **inode** represents a file. It includes:
- size (in bytes)
- user ID, group ID, permissions
- link count
- diskmap
and many other properties (see [fs.h](https://github.com/torvalds/linux/blob/master/include/linux/fs.h)).
`stat` (Unix command) provides information about an inode.
```bash
> stat -x class16.pptx
```
Why doesn't the inode include the filename?
What commands would make the value of Links become 2? Can it ever be 0?
### Unix System 5 File System
Diskmap is 13 entries: 0-9 are direct pointers to disk blocks (each disk
block is 1K bytes); 10 is a pointer to an indirect block (each entry
points to a block); 11 is a pointer to a double indirect block (each
entry points to an indirect block); 12 is a pointer to a triple indirect
block (each entry points to a double indirect block).
What is the largest file that can be store using this system?
How would you test if your file system uses a similar structure?
`df -i` gives information about the device inodes:
```bash
> df -i
Filesystem 512-blocks Used Available Capacity iused ifree %iused Mounted on
/dev/disk0s2 975425848 503107888 471805960 52% 62952484 58975745 52% /
devfs 369 369 0 100% 641 0 100% /dev
map -hosts 0 0 0 100% 0 0 100% /net
map auto_home 0 0 0 100% 0 0 100% /home
```
Could I run out of inodes before I run out of storage space?