File Systems: Organizing Data on Secondary Storage - Prof. Geoffrey M. Voelker, Study notes of Computer Science

Download File Systems: Organizing Data on Secondary Storage - Prof. Geoffrey M. Voelker and more Study notes Computer Science in PDF only on Docsity! 1
        Lecture 12: File Systems Geoffrey M. Voelker November 12, 2001 CSE 120 – Lecture 12 – File Systems 2  
Last time we talked about the physical characteristics of disks and their performance
Today we’ll talk about file systems  Files  Directories  Sharing  Protection  File System Layouts  File Buffer Cache  Read Ahead 2 November 12, 2001 CSE 120 – Lecture 12 – File Systems 3  
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) November 12, 2001 CSE 120 – Lecture 12 – File Systems 4  
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) 5 November 12, 2001 CSE 120 – Lecture 12 – File Systems 9          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() November 12, 2001 CSE 120 – Lecture 12 – File Systems 10 !"   
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 6 November 12, 2001 CSE 120 – Lecture 12 – File Systems 11   
File sharing has been around since timesharing  Easy to do on a single machine  PCs, workstations, and networks get us there (mostly)
File sharing is incredibly important for getting work done  Basis for communication and synchronization
Two key issues when sharing files  Semantics of concurrent access » What happens when one process reads while another writes? » What happens when two processes open a file for writing?  Protection November 12, 2001 CSE 120 – Lecture 12 – File Systems 12    
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 7 November 12, 2001 CSE 120 – Lecture 12 – File Systems 13 #       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 rwrwCharlie r-wBob rw-rwAlice /three/two/one Subjects Objects ACL Capability November 12, 2001 CSE 120 – Lecture 12 – File Systems 14 
$ 
 %   
The approaches differ only in how the table is represented  What approach does Unix use?
Capabilities are easier to transfer  They are like keys, can handoff, does not depend on subject
In practice, ACLs are easier to manage  Object-centric, easy to grant, revoke  To revoke capabilities, have to keep track of all subjects that have the capability – a challenging problem
ACLs have a problem when objects are heavily shared  The ACLs become very large  Use groups (e.g., Unix) 10 November 12, 2001 CSE 120 – Lecture 12 – File Systems 19  & 
 
Applications exhibit significant locality for reading and writing files
Idea: Cache file blocks in memory to capture locality  This is called the file buffer cache  Cache is system wide, used and shared by all processes  Reading from the cache makes a disk perform like memory  Even a 4 MB cache can be very effective
Issues  The file buffer cache competes with VM (tradeoff here)  Like VM, it has limited size  Need replacement algorithms again (LRU usually used) November 12, 2001 CSE 120 – Lecture 12 – File Systems 20
  * 
On a write, some applications assume that data makes it through the buffer cache and onto the disk  As a result, writes are often slow even with caching
Several ways to compensate for this  “write-behind” » Maintain a queue of uncommitted blocks » Periodically flush the queue to disk » Unreliable  Battery backed-up RAM (NVRAM) » As with write-behind, but maintain queue in NVRAM » Expensive  Log-structured file system » Always write next block after last block written » Complicated 11 November 12, 2001 CSE 120 – Lecture 12 – File Systems 21 #
Many file systems implement “read ahead”  FS predicts that the process will request next block  FS goes ahead and requests it from the disk  This can happen while the process is computing on previous block » Overlap I/O with execution  When the process requests block, it will be in cache  Compliments the disk cache, which also is doing read ahead
For sequentially accessed files, can be a big win  Unless blocks for the file are scattered across the disk  File systems try to prevent that, though (during allocation) November 12, 2001 CSE 120 – Lecture 12 – File Systems 22 & 
Files  Operations, access methods
Directories  Operations, using directories to do path searches
Sharing
Protection  ACLs vs. capabilities
File System Layouts  Unix inodes
File Buffer Cache  Strategies for handling writes
Read Ahead 12 November 12, 2001 CSE 120 – Lecture 12 – File Systems 23 !) +
Read Sections 6.3.8 and 5.4.1