chapter 15: file system internals

What is a volume?

A partition or set of partitions typically formatted with a file system.

Why can one computer have multiple file systems?

Because it can have multiple storage devices and partitions.

What is a partition?

A slice of a storage device.

What usually happens to a volume before use?

It is formatted into a file system.

Examples of common UNIX/Linux file systems

ext2, ext3, ext4, zfs, ufs.

Examples of common Windows file systems

NTFS, FAT, FAT32.

What is tmpfs?

A special-purpose file system for temporary I/O.

What is procfs?

A file system in UNIX/Linux that presents process/system information as files.

What must happen before a file system can be used?

It must be mounted.

What is mounting?

Attaching a file system to a mount point so it becomes accessible.

What is a mount point?

A location in the directory tree, usually an empty directory, where a file system is attached.

What two things is the OS given when mounting?

The device name and the mount point.

What happens if the file system is already mounted?

The OS must decide whether multiple mounts are allowed.

What issue happens if the mount point is not empty?

Existing files may be hidden/obscured until the file system is unmounted.

What is a raw partition?

A partition with no file system, just blocks.

What is a raw partition good for?

Swap space or custom workloads like databases and RAID.

What is a cooked partition?

A partition containing a structured file system.

What is a bootable partition?

A partition with enough code to find and mount other file systems.

What can a bootable partition enable if it understands multiple OSs?

Dual booting.

What is a root partition?

The partition containing the operating system and the ability to mount other partitions.

Why is file sharing important?

User-oriented systems need users to share files.

Where is file-permission metadata kept?

In the FCB.

Why can sharing across systems be tricky?

User IDs may not match between systems.

What is a virtual file system (VFS)?

A layer that separates generic file-system operations from implementation details.

What does the VFS do?

Dispatches the correct operation to the correct file-system type.

Why is VFS useful?

It makes multiple file systems appear seamless to users/programs.

What is a remote file system?

A file system accessed over a network from another machine.

What early protocol was used to transfer files between machines?

FTP.

What does a distributed file system (DFS) do?

Makes remote file systems visible on local machines.

What Internet protocol is commonly used for file transfer today?

HTTP.

In the DFS client-server model, what is the client?

The machine trying to mount/use the remote file system.

In the DFS client-server model, what is the server?

The machine sharing its local file system.

What must the server do in a remote file system?

Authenticate and authorize clients.

What information does the client typically send?

User ID information along with operations.

Why are remote file systems harder than local ones?

They must handle more failure conditions.

What is one benefit of keeping state on both client and server?

Better error detection and more security.

What is one downside of keeping state on both client and server?

More overhead.

What are consistency semantics?

Rules for what happens when multiple users share a file at the same time.

Why not use normal process synchronization over the network?

It would generate too much network traffic.

What is a file session?

All actions between open() and close().

What can the server do during a file session?

Block other client requests until the session completes.

What is another simple consistency approach?

Share the file read-only.

What does NFS stand for?

Network File System.

What is NFS?

A popular remote file system using a client-server model.

What does Windows more commonly use instead of NFS?

CIFS.

How does an NFS share appear to the client?

As though it is part of the local file system.

Why is NFS good in heterogeneous environments?

It works across different OSs, machines, and network protocols.

What happens first in the NFS mount protocol?

The client requests a remote directory.

What does the server check during NFS mount?

Whether the client has permission to mount it, including read-only or read-write access.

What happens if the client is allowed to mount?

The mount is recorded in an export table.

What is the export table used for?

Tracking mounts, notifying clients of downtime, and revoking access.

What does the NFS remote operations protocol use?

Remote procedure calls (RPCs).

What can NFS RPCs do?

Search directories, read directory entries, manipulate links/directories, access attributes, and read/write files.

What is special about NFS version 3?

It is stateless.

What does stateless mean in NFSv3?

Each operation must provide all needed arguments because no session state is kept.

Main advantage of NFSv3 statelessness

Faster and less overhead.

Main disadvantage of NFSv3 statelessness

Less secure.

What is special about NFS version 4?

It is stateful.

Main advantage of NFSv4

More secure.

Main disadvantage of NFSv4

More overhead.

Why can NFS writes be delayed?

Because data must be safely written to disk and other clients/sessions may interfere.