Class 18
CS 372H
30 March 2010

On the board
------------

1. file systems
2. crash recovery [next time]

---------------------------------------------------------------------------

recall from last time:

    file systems
    A. Intro
    B. Files
    C. Implementing files
	1. contiguous
	2. linked files
	3. FAT 
   -->	4. indexed files
    D. Directories
    E. FS performance (case study: FFS)

    remember, we're assuming meta-data provided, and the question is
    what does it look like? we'll discuss in "directories" below where
    the metadata came from.

	4. indexed files

	    [DRAW PICTURE]

	    --Each file has an array holding all of its block pointers
		--like a page table, so similar issues crop up
	    --Allocate this array on file creation
	    --Allocate blocks on demand (using free list)
	   
	    +: sequential and random access are both easy
	    -: need to somehow store the array

	    --large possible file size --> lots of unused entries in the
	    block array

	    --large actual file size --> huge contiguous disk chunk
	    needed

	    --solve the problem the same way we did for page tables:

			[............]

		[..........]	    [.........]

	    [ block     block                         block]

	    --okay, so now we're not wasting disk blocks, but what's the
	    problem? (answer: equivalent issues as for page tables:
	    here, it's extra disk accesses to look up the blocks)

	5. indexed files, take two

	    --classic Unix file system 

	    --inode contains:

		permisssions
		times for file access, file modification, and inode-change
		link count (# directories containing file)
		ptr 1  --> data block
		ptr 2  --> data block
		ptr 3  --> data block
		.....
		ptr 11  --> indirect block 
				      ptr  --> 
				      ptr --> 
				      ptr --> 
				      ptr -->
				      ptr -->
		ptr 12 --> indirect block
		ptr 13 --> double indirect block
		ptr 14 --> triple indirect block

	    +: Simple, easy to build, fast access to small files
	    +: Maximum file length can be enormous, with
	       multiple levels of indirection 
	    -: worst case # of accesses pretty bad
	    -: worst case overhead (such as 13 block file) pretty bad
	    -: Because you allocate blocks by taking them off unordered
	       freelist, meta data and data get strewn across disk
 

	    Notes about inodes:

	    --stored in a fixed-size array

	    --Size of array fixed when disk is initialized; can't be changed

	    --Lives in known location, originally at one side of disk,
	    now lives in pieces across disk (helps keep metadata close
	    to data)

	    --The index of an inode in the inode array is called an
	    ***i-number***

	    --Internally, the OS refers to files by i-number

	    --When a file is opened, the inode brought in memory
   
	    --Written back when modified and file closed or time elapses

    D. Directories

    --Problem: "Spend all day generating data, come back the next
    morning, want to use it."  F. Corbato, on why files/dirs
    invented.

    --Approach 0: Have users remember where on disk their files are

	--like remembering your social security or bank account #)
  
	--Disgusting. People want human digestible names

    --So use directories to map names to file blocks, somehow
	
	--But what is in directory?

    --A short history of directories

	--Approach 1: Single directory for entire system

	    --Put directory at known location on disk

	    --Directory contains <name,inumber> pairs

	    --If one user uses a name, no one else can

	    --Many ancient personal computers work this way

	--Approach 2: Single directory for each user

	    --Still clumsy, and "ls" on 10,000 files is a real pain
	    --(But some oldtimers still work this way)

	--Approach 3: Hierarchical name spaces. 

	    --Allow directory to map names to files ***or other dirs***

	    --File system forms a tree (or graph, if links allowed)

	    --Large name spaces tend to be hierarchical

		--examples: ip addresses, domain names, scoping in
		programming languages, etc.)

		--more generally, the concept of hierarchy is everywhere
		in computer systems

    --Hierarchial Unix

	--used since CTSS (1960s), and Unix picked it up and used it
	nicely
	--structure like:
		            "/"
	     bin  cdrom    dev       sbin           tmp
			        awk chmod ....

	--directories stored on disk just like regular files

	    --here's the data in a directory file:

	      [<name, inode#>]
	       <bin, 1021>
	       <dev, 1001>
	       <sbin, 2011>
	       ....

	    --i-node for directory contains a special flag bit

		--only special users can write directory files

	--key point: i-number might reference another directory

	    --this neatly turns the FS into a hierarchical tree, with almost
	    no work

	--another nice thing about this: if you speed up file
	operations, you also speed up directory operations, because
	directories are just like files

	--bootstrapping: where do you start looking?

	    --root dir always inode #2 (0 and 1 reserved)

	    --and, voila, we have a namespace!


	--special names: "/", ".", ".."

	--given those names, we need only two operations to navigate the
	entire name space:

	    --"cd name": (change context to directory "name")
	    --"ls": (list all names in current directory)


	--example:

	    [DRAW PICTURE]


	--links:

	    --hard link: multiple dir entries point to same inode; inode
	    contains refcount

		"ln a b": creates a synonym ("b") for file ("a")

		--how do we avoid cycles in the graph? (answer: can't
		hard link to directories)

	    --soft link: synonym for a *name*

		"ln -s /d/a b": 

		--creates a new inode, not just a new directory entry

		--new inode has "sym link" bit set

		--contents of that new file:

		    "/d/a"

    E. FS Performance

	[began talking about FFS; notes will be in notes for next class]