File System Internals

1
File System Internals

• Sunny Gleason
• COM S 414
• November 29, 2001

2
In this Lecture

• The Hard Disk
• Architecture
• Performance
• File System Structures
• Local File systems
• Like, FAT, UFS, Ext2, Ext3

3
Where to Find More Info

• Hard Drive Manufacturers
• http//www.storage.ibm.com/hdd/index.htm
• http//www.seagate.com/newsinfo/technology/
• http//www.westerndigital.com/library/
• Windows File Systems
• http//www.microsoft.com/hwdev/download/hardware/f
  atgen103.pdf
• http//msdn.microsoft.com/library/default.asp?url
  /library/en-us/fileio/fsys_10ku.asp

4
Where to Find More Info

• Unix File Systems
• http//www-106.ibm.com/developerworks/library/l-fs
  .html
• NFS Version 4
• http//nfsv4.org/
• The Actual Code -
• Linux Kernel Source
• http//www.kernel.org/
• (look in the fs directory of any 2.4 kernel)
• BSD Kernel Source
• http//www.openbsd.org/
• (Look in the sys/ufs directory)

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Where to Find More Info

• The Book
• Chapters 11, 12,
• CS414 Spring 2001 Web Site
• http//www.cs.cornell.edu/Courses/cs414/2001sp/
• (from which these slides are mostly stolen)
• CS414 Fall 2000 Web Site
• http//www.cs.cornell.edu/Courses/cs414/2000fa/
• (other useful slide sets available)

6
The Memory Hierarchy

• Memory is arranged as a hierarchy
• Close to CPU
• Registers, L1 cache
• L2 Cache
• RAM (primary memory)
• Disk Storage (secondary memory)
• Tape or Optical Storage (tertiary mem.)
• Higher higher speed, higher cost

7
Hard Disk Architecture

• A disk drive has several physical components
• spindle
• surface (one side in the pack)
• read/write arm and head
• track (cylinder is vertical set of tracks)
• Sector

8
Physical Disk Access

• Delays associated with accessing a sector on the
  disk
• Seek delay (biggest)
• Moving the read/write head
• Rotational delay
• Waiting for the sector to spin under the head
• Transfer delay (smallest)
• Transferring the bits from the disk

9
Physical Disks

• O/S goal provide file system API
• Problems with disks
• Read errors
• Bad blocks
• Missed seeks
• O/S Disk API may have many levels
• Physical disk block ltsurf, cyl, secgt
• Disk (volume) logical block ltblockgt
• File logical ltfile block, record, or bytegt

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Logical Disks

• A single hard disk may contain multiple file
  systems

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Making the HD Usable

• The hard disk must be partitioned
• Partitions are formatted with specific
  filesystems
• In some cases, can quick format instead of full
  reformat
• Multiple partitions are useful
• (Limited) protection against crashes
• If one partition fills up, the rest are still
  usable
• Dual-booting - in general, ability to load
  multiple operating systems

12
Some Typical Numbers

• Sector Size 512 bytes
• Cylinders per disk 6962
• Platters 3 - 12
• Rotational Speed 10,000 rpm
• Storage size 12 - 120 GB
• Seek time 5 - 12ms
• Latency 3ms
• Transfer Rate 14-20 MB/sec

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Disk Structure

• Bare disk interface cylinders, sectors
• O/S imposes structure on disks
• Disk contents
• Data user files
• Metadata structural / administrative info
• Any ideas?
• Free list structure indicating which blocks are
  unused
• Typically maintained as a bitmap an array of
  bits, representing blocks

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Dealing with Mechanical Latencies

• Caches
• Locality in file access
• RAM disk
• Reserve RAM as a fast! filesystem
• RAID
• Exploiting parallelism
• Clever layouts and scheduling algorithms
• Head scheduling
• Meta-information layout

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Bad Blocks

• All disks have some bad blocks
• Blocks go bad as time goes on
• O/S removes these blocks from the allocation map
• On some disks, some cylinders have reserve blocks
  that can be remapped to replace bad blocks

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The File System

• File system supports the abstraction of file
  objects
• Create, delete, read, write, rename
• File a named collection of data
• Typical abstraction a vector of bytes
• O/S knows about special file types
• Directories, symlinks, executable files
• For data files, applications decide internal file
  structure (data file format)

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Accessing Files

• Files can be accessed in different ways
• Sequential Access
• Read bytes one at a time, in order
• Direct access
• Random access, given block/byte number
• Record access
• Some higher-level structure, instead of byte
• Indexed access
• Uses map from index field to corresponding file
  record

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Storing Files

• Files can be allocated in different ways
• Contiguous allocation
• All bytes together, in order
• Linked Structure
• Each block points to the next block
• Indexed Structure
• An index block contains pointer to many other
  blocks
• Rhetorical Questions -- which is best?
• For sequential access? Random access?
• Large files? Small files? Mixed?

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Linked-list allocation

• Each data block contains pointer to the next data
  block
• Advantages?
• Disadvantages?

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Linked-List Allocation

• A single pointer is sufficient to locate all the
  blocks of the file
• Seeking takes O(n) time, where n is the size of
  the file
• A single corrupt pointer can cause the entire
  file to be lost

21
MS-DOS Filesystem

• MS-DOS uses a File Allocation Table (FAT)
• Like a linked structure, except pointers are kept
  in a separate table
• For every block, the FAT keeps track of whether
  or not it is allocated, and if so, which block it
  points to
• Two copies of the FAT on disk

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Indexed Allocation

• Index block contains pointers to each data block
• Pros?
• Cons?

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Combined Scheme UFS

• Unix File System
• An inode contains the metadata for UNIX files
• Contains control and allocation information
• Each inode contains 15 block pointers
• 12 direct
• 1 single, 1 double, 1 triple indirect
• Kind of tricky -- see the diagram!

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UNIX Inode
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UNIX Inode

• If data blocks are 4K
• First 48K reachable from the inode
• Next 4MB available from single-indirect
• Next 4GB available from double-indirect
• Next 4TB (!) available through the
  triple-indirect block
• Any block can be found with at most 3 disk
  accesses

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UNIX Directories

• Directories are just like regular files
• They contain ltfilename, inodegt tuples
• Filename is usually filename filename_length

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UNIX Disk Layout

• Boot block provides information on how to boot
  the computer (tiny bootstrap program)
• Superblock contains the file system layout of
  inodes, block size, location of the free list

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File System Problems

• Fragmentation
• When the blocks of a file are located all over
  the physical disk
• Causes undesirable seeking
• Use defragmentation utility to compact the
  filesystem, consolidate free space
• See the pictures!

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Fragmentation
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Defragmentation
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File System Problems

• Unreliability
• Historically, disks have been among the most
  unreliable components
• Develop bad blocks
• Modern disks detect such faults, and have
  replacement blocks that can be remapped to
  replace bad blocks
• Filesystems still need to track bad blocks and
  avoid using them
• Inode 1 is a special inode that keeps track of
  where all the bad blocks are

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File System Problems

• System crashes or power failures can occur at any
  time
• Any disk operation can be interrupted at any time
• Need to ensure that the filesystem is consistent
  throughout updates
• Data that is being modified may be lost, but that
  should not compromise entire file system

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File System Problems

• Crashes can occur at any time
• A write in UNIX involves
• Writing the new data
• Updating the inode
• Updating the free list
• Is there a correct order? What can go wrong if
  the FS does not respect the order?

34
Disk Scheduling

• To minimize mechanical delays, the O/S looks at
  multiple pending disk requests
• FCFS (first come, first serve)
• Ok when load is low
• Long waiting times for long request queues
• SSTF (shortest seek time first)
• Always minimize arm movement, maximizes
  throughput
• Favors middle blocks
• SCAN (elevator)
• Continue in same direction until done, then
  reverse direction and service in that order
• C-SCAN like scan, but return to 0 at end

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Disk Scheduling

• In general, unless there are request queues, it
  doesnt matter
• The O/S may locate files strategically for
  performance reasons
• The Organ Pipe distribution locates heavily-used
  files towards the center of the disk
• The Ext2 Filesystem places groups of inodes
  around the disk, closer to the data blocks that
  they reference

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Conclusion

• Hard disks provide vast amounts of slow, cheap
  storage
• Operating Systems layer file system services on
  top of the raw disk API
• The O/S must find ways to work around the slow
  performance and unreliability of disk storage

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Thanks!

• Any questions?
• Review session - Tuesday, 12/04530 - 730pm