Week 11

Question 1

What is persistent storage?

Answer 1

Holds data regardless of whether the system is powered on/off.

Persistent storage can take several forms, commonly is based on disks.

All components of the OS and applications are kept there and loaded in main memory (RAM) when executed.

Persistent storage tends to be order of magnitude cheaper and slower than RAM so it is not used directly for program execution.

Question 2

What are the requirements for OS storage management?

Answer 2

Performance

Support for multi-processing

Security/access control

Question 3

How does the OS keep track of how many inodes and datablocks are in the file system.

Answer 3

We can use superblocks.

Note that they depend on the disk size.

I need to use a block (typically the first so it is easy to find) to store them!

Question 4

How does persistent storage look like from the point of view of the OS?

Answer 4

It typically looks like an array of blocks where data can be written or read.

Question 5

How is the file system organized?

Answer 5

It looks like a tree which organizes units of storage (files) into a hierarchial set of location (directories).

Question 6

How can we keep track of free blocks?

Answer 6

Possible solutions:
Free block list
Bitmap: array of bits, one per block (if bit = 0 then the block is free. If bit = 1 it is occupied.)

Question 7

How does the OS keep track of how many inodes and datablocks are in the filesystem?

Answer 7

We will use superblocks.

The size will depend on the disk size.

Typically we put the block first so its easy to find.

Question 8

What is an inode?

Answer 8

In UNIX systems, inode tends to be a generic name for structures that hold file information.

It stands for index mode.

Inodes are typically arranged as part of a data structure on disk

Question 9

What is the inode number?

Answer 9

Specifies a specific node within this structure.

This is called the i-number.

Question 10

What is a sector?

Answer 10

A disk is internally partitioned into sectors, and a whole sector must be retrieved at a time.

Note sector size is determined by the disk hardware.

Question 11

What is a block?

Answer 11

A block is a unit of allocation, the minimum of space the OS can allocate to a file.

Typically, block size = sector size * 2^n with n determined by the OS.

Question 12

How does the OS retrieve content of the disk?

Answer 12

Compute which sector within the block the content is.

Retrieve the sector.

Retrieve the content from within the sector.

Question 13

What is an indirect pointer.

Answer 13

An indirect pointer points to a disk block storing direct pointer.

A double indirect pointer points to a block containing indirect pointers.

Question 14

In UNIX/LINUX there are two main interfaces to access files from programs

Answer 14

Libc I/O functions: fopen, fclose, fread, fwrite…

POSIX system calls: open, close, read, write…

Question 15

What do the following do?

O_CREAT

O_WRONLY

O_TRUNC

Answer 15

O_CREAT: create the file if it does not exist

O_WRONLY: onlu open the file for writing

O_TRUNC: truncate the file to 0 bytes if it exists

Returns a file descriptor representing the file.

Question 16

What is strace?

Answer 16

run a program while tracing all the system calls it issues.

Question 17

What does the following do?

ssize_t read(int fd, void buf, sizet count);

Answer 17

ssize_t read(int fd, void buf, sizet count);
* The first parameter is an open file descriptor
* The second parameter is a pointer to a buffer to store data
* The third parameter describes how many bytes should read at most
* Returns the number of bytes read

Question 18

What does the following do?
~~~
ssizet write(int fd, const void *buf, sizet count);
~~~

Answer 18

The first parameter is an open file descriptor

The second parameter is a pointer to a buffer containing data to be written

The third parameter describes how many bytes should written

Returns the number of bytes written