File Systems: Organization, Access, and Protection in Operating Systems, Study notes of Operating Systems

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Lecture 13:Lecture 13:

File Systems File Systems

CSE 120: Principles of Operating Systems

Alex C. Snoeren

HW 3 Due 11/9, Lab 2 Due 11/

File Systems File Systems

 First we’ll discuss properties of physical disks

◆ Structure ◆ Performance ◆ Scheduling

 Then we’ll discuss how we build file systems on them

◆ Files ◆ Directories ◆ Sharing ◆ Protection ◆ File System Layouts ◆ File Buffer Cache ◆ Read Ahead

Physical Disk Structure Physical Disk Structure

 Disk components ◆ Platters ◆ Surfaces ◆ Tracks ◆ Sectors ◆ Cylinders ◆ Arm ◆ Heads  Logically, disk broken down into sectors ◆ Addressed by cylinder, head, sector (CHS) Arm Heads Track Platter Surface Cylinder Sector

Disk Interaction Disk Interaction

 Specifying disk requests requires a lot of info:

◆ Cylinder #, surface #, track #, sector #, transfer size…

 Older disks required the OS to specify all of this

◆ The OS needed to know all disk parameters

 Modern disks are more complicated

◆ Not all sectors are the same size, sectors are remapped, etc.

 Current disks provide a higher-level interface (SCSI)

◆ The disk exports its data as a logical array of blocks [0…N] » Disk maps logical blocks to cylinder/surface/track/sector ◆ Only need to specify the logical block # to read/write ◆ But now the disk parameters are hidden from the OS

Disk Performance Disk Performance

 Disk request performance depends upon a number of

steps

◆ Seek – moving the disk arm to the correct cylinder » Depends on how fast disk arm can move (increasing very slowly) ◆ Rotation – waiting for the sector to rotate under the head » Depends on rotation rate of disk (increasing, but slowly) ◆ Transfer – transferring data from surface into disk controller electronics, sending it back to the host » Depends on density (increasing quickly)

 When the OS uses the disk, it tries to minimize the cost

of all of these steps

◆ Particularly seeks and rotation

Disk Scheduling Disk Scheduling

 Because seeks are so expensive (milliseconds!), the

OS tries to schedule disk requests that are queued

waiting for the disk

◆ FCFS (do nothing) » Reasonable when load is low » Long waiting times for long request queues ◆ SSTF (shortest seek time first) » Minimize arm movement (seek time), maximize request rate » Favors middle blocks ◆ SCAN (elevator) » Service requests in one direction until done, then reverse ◆ C-SCAN » Like SCAN, but only go in one direction (typewriter)

File Systems File Systems

 File systems

◆ Implement an abstraction (files) for secondary storage ◆ Organize files logically (directories) ◆ Permit sharing of data between processes, people, and machines ◆ Protect data from unwanted access (security)

Files Files

 A file is data with some properties

◆ Contents, size, owner, last read/write time, protection, etc.

 A file can also have a type

◆ Understood by the file system » Block, character, device, portal, link, etc. ◆ Understood by other parts of the OS or runtime libraries » Executable, dll, souce, object, text, etc.

 A file’s type can be encoded in its name or contents

◆ Windows encodes type in name » .com, .exe, .bat, .dll, .jpg, etc. ◆ Unix encodes type in contents » Magic numbers, initial characters (e.g., #! for shell scripts)

File Access Methods File Access Methods

 Some file systems provide different access methods

that specify different ways for accessing data in a file

◆ Sequential access – read bytes one at a time, in order ◆ Direct access – random access given block/byte number ◆ Record access – file is array of fixed- or variable-length records, read/written sequentially or randomly by record # ◆ Indexed access – file system contains an index to a particular field of each record in a file, reads specify a value for that field and the system finds the record via the index (DBs)

 What file access method does Unix, NT provide?

 Older systems provide more complicated methods

Directories Directories

 Directories serve two purposes

◆ For users, they provide a structured way to organize files ◆ For the file system, they provide a convenient naming interface that allows the implementation to separate logical file organization from physical file placement on the disk

 Most file systems support multi-level directories

◆ Naming hierarchies (/, /usr, /usr/local/, …)

 Most file systems support the notion of a current

directory

◆ Relative names specified with respect to current directory ◆ Absolute names start from the root of directory tree

Basic Directory Operations Basic Directory Operations

Unix  Directories implemented in files ◆ Use file ops to create dirs  C runtime library provides a higher-level abstraction for reading directories ◆ opendir(name) ◆ readdir(DIR) ◆ seekdir(DIR) ◆ closedir(DIR)

NT

 Explicit dir operations ◆ CreateDirectory(name) ◆ RemoveDirectory(name)  Very different method for reading directory entries ◆ FindFirstFile(pattern) ◆ FindNextFile()

Path Name Translation Path Name Translation

 Let’s say you want to open “/one/two/three”

 What does the file system do?

◆ Open directory “/” (well known, can always find) ◆ Search for the entry “one”, get location of “one” (in dir entry) ◆ Open directory “one”, search for “two”, get location of “two” ◆ Open directory “two”, search for “three”, get location of “three” ◆ Open file “three”

 Systems spend a lot of time walking directory paths

◆ This is why open is separate from read/write ◆ OS will cache prefix lookups for performance » /a/b, /a/bb, /a/bbb, etc., all share “/a” prefix

Protection Protection

 File systems implement some kind of protection

system

◆ Who can access a file ◆ How they can access it

 More generally…

◆ Objects are “what”, subjects are “who”, actions are “how”

 A protection system dictates whether a given action

performed by a given subject on a given object should

be allowed

◆ You can read and/or write your files, but others cannot ◆ You can read “/etc/motd”, but you cannot write it

Representing Protection Representing Protection

Access Control Lists (ACL)  For each object, maintain a list of subjects and their permitted actions Capabilities  For each subject, maintain a list of objects and their permitted actions Charlie w r rw Bob w - r Alice rw - rw /one /two /three Subjects Objects ACL Capability