Lecture 12 - Filesystems

Question 1

What is a filesystem?

Answer 1

Method of long term storage

Stores large amounts of data

Organised as data and metadata

Usually on block devices

Question 2

What is metadata?

Answer 2

Data about data

Question 3

What is structural metadata?

Answer 3

About the design and specification of data structures

“Data about the containers of data”

i.e. where are blocks stored on disk

how long are blocks

is it fast or slow

Question 4

What is descriptive metadata?

Answer 4

Data about the actual content

e.g. when was this file created, by whom, whats the encoding, can it be trusted

Question 5

What are some design choices about filesystems?

Answer 5

Namespace - flat, hierarchical

Types of files - structured/unstructured

what metadata to use

how to store on the device

Question 6

What are some important file attributes?

Answer 6

Name - human readable

Identifier - for machine use

Type

Location - both within the filesystem and on the device

Size

Protection

Time, date, user info

Question 7

What are the two ways of accessing a file?

Answer 7

sequential access - read from beginning - cannot randomly pick and choose a bit to read

Random access - read in any order, pick and choose as you like

Question 8

What are some typical file operations?

Answer 8

Create

Delete

Open

Close

Read

Write

Append

Seek

Get/set attributes

Rename

Question 9

What are file descriptors?

Answer 9

An integer the kernel uses to name a file when it is open

In unix also applies to devices since they are represented as files

Question 10

What does the POSIX file api do?

Answer 10

Forms an interface between OS kernel and software in user mode

Question 11

What is the difference between mandatory and advisory file locking?

Answer 11

Mandatory - access is denied depending on locks held and requested

Advisory - processes can find status of locks and decide what to do

Question 12

What is a hard link?

Answer 12

Link to low-level reference to the file, i.e. points to the actual file itself on disk

Question 13

What is a soft link?

Answer 13

Symbolic link

High level reference to file, i.e. where it is in the hierarchy

Question 14

How are files accessed in filesystem?

Answer 14

By name: name identifies unique file (including path)

By content or attribute: could index by the actual content of a file - would need to search to find it

Question 15

What are the 5 layers in file systems?

Answer 15

Application programs

Logical file system (hierarchy - files, diretcories etc.)

Basic file system (block read/write)

I/O control (drivers, interrupts)

Hardware devices

Question 16

Why is a layer of abstraction necessary when dealing with disks?

Answer 16

Otherwise would need to know about many different types of disk - so many details to know and things to handle it would be a nightmare

Question 17

What is device abstraction?

Answer 17

Making all devices of the same type look the same to the programmer

e.g. make all hdds have the same interface, and ssds, etc even though they work in very different ways