Class 18
CS 202
7 April 2020

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

1. Last time
2. File systems: Directories
3. File systems: Performance

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1. Last time
    - mmap() continued
    - file systems: intro and implementation of file structures
    - today: directories, performance
    - next time: crash recovery

    See notes from last time for more about inodes: 
        --stored in a fixed-size array
        --Size of array fixed when disk is initialized; can't be changed
        --Multiple inodes in a disk block
        --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


2.  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 #
  
	--yuck. (people want human-friendly 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     dvdrom  dev       sbin           tmp    usr
          ls, grep	             tcpdump 

	--directories stored on disk just like regular files

	    --here's the data in a directory file; this data can be in the
	    *data blocks* of the directory or else in the inode of the
	    directory.

	      [<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

	--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:

                /a/foo.c
                /b/c/essay.txt
           
            what does the file system look like?

            [ i0 ... i7 || [block ] [ block ] [block ] .....]

	    [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"


3. File systems: performance

    Case study: FFS

	--Unix FS was simple, elegant and ... slow

	    --blocks too small

	    --inode array all at the beginning of
	    the disk

	    --inode had:
		--too many layers of mapping indirection
		--transfer rate low (they were getting one block at a time)

            --free blocks were stored in a linked list on the disk

	    --poor clustering of related objects

		--consecutive file blocks not close together

		--Inodes far from data blocks

		--Inodes for a given directory not close together

		--result: poor enumeration performance, meaning things like:
			"ls" and "grep foo *.c" were slowwwww

	    --other problems:
		--14 character names were the limit
		--can't atomically update file in crash-proof way
 
 
       --FFS (fast file system) fixes these problems to a degree.
	
	    [Reference: "M. K. McKusik, W. N. Joy, S. J. Leffler, and R.
	    S.  Fabry. A Fast File System for UNIX. ACM Trans. on
	    Computer Systems, Vol. 2, No. 3, Aug. 1984, pp. 181-197.]

      what can we do to above?

      [ask for suggestions]

      * make block size bigger (4 KB, 8KB, or 16 KB)

      * cluster related objects

	  "cylinder groups" (one or more consecutive cylinders)

	[superblock | bookkeeping info | inodes | bitmap | data blocks (512 bytes each) ]

	        [note: it's 512 above, not 4KB, because the file system
	        doesn't exactly _insist_ that data blocks are larger.
	        there can be _fragments_. this is a way to group
	        larger writes to together but to not waste space if 
	        file size isn't a multiple of 4KB.]
	
	    --try to put inodes and data blocks in the same cylinder group

	    --try to put all inodes of files in the same directory in
	    the same cylinder group
	    
	    --new directories placed in cylinder group with greater than
	    average number of free inodes

	    --as files are allocated, use a heuristic: spill to next
	    cylinder group after 48 KB of file (which would be the point
	    at which an indirect block would be required, assuming
	    4096-byte blocks) and at every megabyte thereafter.
	    
      * bitmaps (to track free blocks)

	    --Easier to find contiguous blocks

	    --Can keep the entire thing in memory 

	    --500 GB disk / 4KB disk blocks = 125,000,000 entries = 15MB.
	    not outrageous these days.

      * reserve space
	   --but don't tell users. (df makes full disk look 110% full)

      * atomic "rename"

      * symbolic links

      * total performance

	--20-40% of disk bandwidth for large files

	--10-20x of original Unix file system!

	--still not the best we can do
	    (meta-data writes happen synchronously, which really hurts
	    performance. but making asynchronous requires story for
	    crash recovery.)

      Others:

	--Most obvious: big file cache

	    --kernel maintains a *buffer cache* in memory

	    --internally, all uses of ReadDisk(blockNum, readbuf)
	    replaced with:

		ReadDiskCache(blockNum, readbuf) {
		    ptr = buffercache.get(blockNum); 
		    if (ptr) {
			copy BLKSIZE bytes from ptr to readbuf
		    } else {
			newBuf = malloc(BLKSIZE);
			ReadDisk(blockNum, newBuf);
			buffercache.insert(blockNum, newBuf);
			copy BLKSIZE bytes from newBuf to readbuf
		    }

	--no rotation delay if you're reading the whole track.
	    --so try to read the whole track

	--more generally, try to work with big chunks (lots of disk
	blocks) 
	    --write in big chunks
	    --read ahead in big chunks (64 KB)

	--why not just read/write 1 MB at a time?
	    --(for writes: may not get data to disk often enough)
	    --(for reads: may waste read bandwidth)