CS 346 – Chapter 11

• File system– Files– Access– Directories– Mounting– Sharing– Protection

Files

• What is a file?• Attributes

– Name, internal ID, type, location on device, size, permissions, modification/creation time

• Operations– Create, read, write, reposition file pointer (seek), delete, truncate (i.e.

to zero)– Less essential: append, rename, copy– The first time we refer to a file, need to search for it: “open”

• Active file tables. What is stored in each?– Table per process– System-wide table

• The “open count” for a file

Type and structure

• Policy question – should OS be aware of file types?• How file type determined

– filename extension– Keep track of which application created file– Magic number

• File type determines its structure– At a minimum: bits and bytes– e.g. OS expects executable file to have certain format– Text file: recognize meaning of certain ASCII codes

• Files stored in “blocks” on a device– Each I/O operation can grab one block (~ 1KB <= page size)– Can start a new file on a new block, or do some “packing”

Accessing data

• Sequential access– Read, write, rewind operations– We almost always utilize files this way

• Direct access– More complex system calls: Allow arbitrary access to any byte

in file on demand– What kind of application needs this functionality?– Read/write operations may specify a relative or absolute block

number

• Indexed access– Another file stores pointers to appropriate blocks in some large

file

Directories

• File system resides on some “volume”– A volume may be a device, part of a device, multiple devices:– So, can have multiple file systems on the same device (partition)– A file system can use multiple devices, but this adds complexity

• Can have specialized “file systems” to allow certain devices to be treated as files, with file I/O commands

• Volume must keep around info about all files– Confusingly called a directory

• Directory operations on files:– Search, create, delete, list, rename, traverse

File organization

• How are files logically organized in the directory?

• Single-level directory: one flat list– File names must be unique

– Excellent if everyone is sharing files

• Two-level directory– Each user has a separate directory: Figure 11.9

– System maintains a master file directory: pointers to each user’s file directory

– Allows user’s work to be isolated

– Can specify file by absolute or relative path name

– Special “system user” for system files. Why necessary?

– Search path: sequence of directories to use when searching for a file. Look here, look in system folder, etc.

File org (2)

• Tree-based directory: Files can be arbitrarily deep• Allows user to impose local structure on files• Each process has a current working directory

– To access file, need to specify path name or change the current directory

• Policy on deleting an entire directory• Acyclic directory: support links to existing files

– In effect, the same file has multiple path names– Same file exists in multiple directories– But there is just 1 file, not a copy– When traversing, need to ignore the links– What happens when we delete file? Links now point to …– Can count the # of references to file (like garbage collection)

Mounting

• Mount = make volume/device available to file system.• Assign a name to its root so that all files will have a specific

path name.

• Mount point = position in existing file system in which we insert the new volume.– Think of inserting a subtree at a new child of an existing node.– E.g. You plug in a USB drive, and immediately it acquires the name E:

so you can access its files– In UNIX, a new “volume” may appear under /

• Unused volumes may be temporarily unmounted if file system desires

File sharing

• In multi-user system, desirable to have some files accessible by multiple users!

• File system must have more info– Owner of each file– Assign unique ID numbers for users and groups of users– When you access file, we check your IDs first

• Remote file system access– Manually transfer files via FTP– Distributed file system: see a file system on another computer

on the network– Anonymous browsing on the Web

Remote file system

• We’d like to mount a remote file system on our machine.– In other words, be able to give (path) names to remote files to

manipulate them.

• Client-server relationship: a file server accepts requests for remote machines to mount – E.g. You are logged into ultrax2, but ultrax1 is the file server.– NFS is a standard UNIX file sharing protocol– OS file system calls are translated into remote calls

• One challenge – to authenticate the client.– Typically the client & server share same set of user IDs. When you

get a computer account, your user ID is good everywhere.– Or, provide your password the first time you access server.

• What is role of distributed naming service, e.g. DNS ?

Consistency

• Policy decisions concerning how we handle multiple users accessing the same file– Reminiscent of synchronization

• When do changes made by one user become observable to others?– Immediately, or not until you reopen the file?

• Should we allow 2 users to read/write concurrently?– As in a database access

• System may define immutable shared file– Like a CD-R– Cannot be modified, name cannot be resused.– No constraints on reading

Protection

• Owner/creator of file should set capabilities for– What can by done– By whom

• Types of access– Read– Write– Execute

Could also distinguish other access capabilities: – Delete– List

Specifying permissions

• Establish classes of users, each with a possibly distinct set of permissions– Classes can be: owner, group, rest of world

• For each level of users:– ‘r’ = Can I read the file?– ‘w’ = Can I write to (or delete) the file?– ‘x’ = Can I execute the file?

• Examples– rw-rw-r-- (664)– rwxr-xr-- (754)– rw-r----- (640)

• If no groups, can set group permission = rest of world.• Use chmod command