FILE

1
FILE SYSTEM STRUCTURE (CHAPTER 11)
2
TERMINOLOGY

• Computers represent data as a sequence of zero
  and ones, termed bits
• A byte is eight contiguous bits

0101111110011010101010110000000000..00000
0101111110011010101010110000000000..00000
3
TERMINOLOGY

• Computers represent data as a sequence of zero
  and ones, termed bits
• A byte is eight contiguous bits

0101111110011010101010110000000000..00000
0101111110011010101010110000000000..00000
4
TERMINOLOGY

• Computers represent data as a sequence of zero
  and ones, termed bits
• A byte is eight contiguous bits

0101111110011010101010110000000000..00000
0101111110011010101010110000000000..00000
5
TERMINOLOGY

• Computers represent data as a sequence of zero
  and ones, termed bits
• A byte is eight contiguous bits
• 1024 bytes is one Kilobyte (KB), e.g., your
  textbook.

0101111110011010101010110000000000..00000
0101111110011010101010110000000000..00000
6
TERMINOLOGY

• Computers represent data as a sequence of zero
  and ones, termed bits
• A byte is eight contiguous bits
• 1024 bytes is one Kilobyte (KB).
• 1024 KB is one Megabyte (MB), a high resolution
  photograph.

0101111110011010101010110000000000..00000
0101111110011010101010110000000000..00000
7
TERMINOLOGY

• Computers represent data as a sequence of zero
  and ones, termed bits
• A byte is eight contiguous bits
• 1024 bytes is one Kilobyte.
• 1024 KB is one Megabyte (MB).
• 1024 MB is one Gigabyte (GB), e.g., a DVD quality
  movie.

0101111110011010101010110000000000..00000
0101111110011010101010110000000000..00000
8
TERMINOLOGY

• Computers represent data as a sequence of zero
  and ones, termed bits
• A byte is eight contiguous bits
• 1024 bytes is one Kilobyte.
• 1024 KB is one Megabyte.
• 1024 MB is one Gigabyte.
• 1024 GB is one Terabyte (TB), all text in the
  library of congress.

0101111110011010101010110000000000..00000
0101111110011010101010110000000000..00000
9
TERMINOLOGY

• Computers represent data as a sequence of zero
  and ones, termed bits
• A byte is eight contiguous bits
• 1024 bytes is one Kilobyte.
• 1024 KB is one Megabyte.
• 1024 MB is one Gigabyte.
• 1024 GB is one Terabyte (TB).
• 1024 TB is one Petabyte (PB), entire multimedia
  collection at LoC.

0101111110011010101010110000000000..00000
0101111110011010101010110000000000..00000
10
TERMINOLOGY

• Computers represent data as a sequence of zero
  and ones, termed bits
• A byte is eight contiguous bits
• 1024 bytes is one Kilobyte.
• 1024 KB is one Megabyte.
• 1024 MB is one Gigabyte.
• 1024 GB is one Terabyte.
• 1024 TB is one Petabyte (PB).
• 1024 PB is one Exabyte (XB), record all phone
  conversations in a year.

0101111110011010101010110000000000..00000
0101111110011010101010110000000000..00000
11
TERMINOLOGY

• Computers represent data as a sequence of zero
  and ones, termed bits
• A byte is eight contiguous bits
• 1024 bytes is one Kilobyte.
• 1024 KB is one Megabyte.
• 1024 MB is one Gigabyte.
• 1024 GB is one Terabyte.
• 1024 TB is one Petabyte.
• 1024 PB is one Exabyte.
• 1024 XB is one Zetabyte (ZB), all uncompressed
  medical data.

0101111110011010101010110000000000..00000
0101111110011010101010110000000000..00000
12
HOW MUCH DATA IS THERE?

• Approximately 5000 films are made each year
  (worldwide)
• Two hour display time at 240 mbps 900 TB
• Approximately 52 billion photographs are taken
  each year
• _at_ 10 KB per photograph, 520 PB
• Library of congress
• 20 million books _at_ 1MB 20 TB
• 15 million photographs _at_ 1 MB 13 TB
• 4 million maps _at_ 100 MB 400 TB
• 500,000 movies _at_ 10 GB 5 PB
• 3.5 million sound recordings at library of
  congress _at_ 1 audio per CD 2 PB

13
Physical Storage Media

• A system consists of several forms of storage
• Cache fastest and most costly form of storage
  volatile managed by the computer system
  hardware.
• Main memory
• fast access (10ns to 100ns 1 nanosecond 109
  seconds)
• generally too small (or too expensive) to store
  the entire database
• capacities of up to a few Gigabytes widely used
  currently
• Capacities have gone up and per-byte costs have
  decreased steadily and rapidly (roughly factor
  of 2 every 2 to 3 years)
• Volatile contents of main memory are usually
  lost if a power failure or system crash occurs.

14
Physical Storage Media (Cont.)

• Magnetic-disk
• Data is stored on spinning disk, and read/written
  magnetically
• Primary medium for the long-term storage of data
  typically stores entire database.
• Data must be moved from disk to main memory for
  access, and written back for storage
• Much slower access than main memory (more on this
  later)
• direct-access possible to read data on disk in
  any order, unlike magnetic tape
• Capacities range up to roughly ? GB currently
• Much larger capacity and cost/byte than main
  memory
• Growing constantly and rapidly with technology
  improvements (factor of 2 to 3 every 2 years)
• Survives power failures and system crashes
• disk failure can destroy data, but is very rare

15
Magnetic Hard Disk Mechanism
NOTE Diagram is schematic, and simplifies the
structure of actual disk drives
16
Magnetic Disks

• Read-write head
• Positioned very close to the platter surface
  (almost touching it)
• Reads or writes magnetically encoded information.
• Surface of platter divided into circular tracks
• Over 16,000 tracks per platter on typical hard
  disks
• Each track is divided into sectors.
• A sector is the smallest unit of data that can be
  read or written.
• Sector size typically 512 bytes
• Typical sectors per track 200 (on inner tracks)
  to 400 (on outer tracks)
• To read/write a sector
• disk arm swings to position head on right track
• platter spins continually data is read/written
  as sector passes under head
• Head-disk assemblies
• multiple disk platters on a single spindle
  (typically 2 to 4)
• one head per platter, mounted on a common arm.
• Cylinder i consists of ith track of all the
  platters

17
Magnetic Disks (Cont.)

• Earlier generation disks were susceptible to
  head-crashes
• Surface of earlier generation disks had
  metal-oxide coatings which would disintegrate on
  head crash and damage all data on disk
• Current generation disks are less susceptible to
  such disastrous failures, although individual
  sectors may get corrupted
• Disk controller interfaces between the computer
  system and the disk drive hardware.
• accepts high-level commands to read or write a
  sector
• initiates actions such as moving the disk arm to
  the right track and actually reading or writing
  the data
• Computes and attaches checksums to each sector to
  verify that data is read back correctly
• If data is corrupted, with very high probability
  stored checksum wont match recomputed checksum
• Ensures successful writing by reading back sector
  after writing it
• Performs remapping of bad sectors

18
Disk Subsystem

• Multiple disks connected to a computer system
  through a controller
• Controllers functionality (checksum, bad sector
  remapping) often carried out by individual disks
  reduces load on controller
• Disk interface standards families
• ATA (AT adaptor) range of standards
• SCSI (Small Computer System Interconnect) range
  of standards
• Several variants of each standard (different
  speeds and capabilities)

19
Performance Measures of Disks

• Access time the time it takes from when a read
  or write request is issued to when data transfer
  begins. Consists of
• Seek time time it takes to reposition the arm
  over the correct track.
• Average seek time is 1/2 the worst case seek
  time.
• Would be 1/3 if all tracks had the same number of
  sectors, and we ignore the time to start and stop
  arm movement
• 4 to 10 milliseconds on typical disks
• Rotational latency time it takes for the sector
  to be accessed to appear under the head.
• Average latency is 1/2 of the worst case
  latency.
• 4 to 11 milliseconds on typical disks (5400 to
  15000 r.p.m.)
• Data-transfer rate the rate at which data can
  be retrieved from or stored to the disk.
• 4 to 8 MB per second is typical
• Multiple disks may share a controller, so rate
  that controller can handle is also important
• E.g. ATA-5 66 MB/second, SCSI-3 40 MB/s
• Fiber Channel 256 MB/s

20
Optimization of Disk-Block Access

• Block a contiguous sequence of sectors from a
  single track
• data is transferred between disk and main memory
  in blocks
• sizes range from 512 bytes to several kilobytes
• Smaller blocks more transfers from disk
• Larger blocks more space wasted due to
  partially filled blocks
• Typical block sizes today range from 4 to 16
  kilobytes
• Disk-arm-scheduling algorithms order pending
  accesses to tracks so that disk arm movement is
  minimized
• elevator algorithm move disk arm in one
  direction (from outer to inner tracks or vice
  versa), processing next request in that
  direction, till no more requests in that
  direction, then reverse direction and repeat

21
Optimization of Disk Block Access (Cont.)

• File organization optimize block access time by
  organizing the blocks to correspond to how data
  will be accessed
• E.g. Store related information on the same or
  nearby cylinders.
• Files may get fragmented over time
• E.g. if data is inserted to/deleted from the file
• Or free blocks on disk are scattered, and newly
  created file has its blocks scattered over the
  disk
• Sequential access to a fragmented file results in
  increased disk arm movement
• Some systems have utilities to defragment the
  file system, in order to speed up file access

22
FILE SYSTEM STRUCTURE (Cont)

• A database system is organized as several layers
  of software
• Query parser translates a higher level query
  language to an internal representation
• Query optimizer transforms the internal
  representation to an efficient execution paradigm
  
• Concurrency control and crash recovery ensures
  consistency of data in the presence of multiple
  concurrent update operations and
  crash-recoveries.
• Index methods efficient retrieval of records
  for fast retrieval and update operations
• Abstraction of multiple records on a disk page
  implements the concept of multiple records on a
  disk page.

23
BIG PICTURE
SELECT SS FROM emp WHERE sal gt 50K
DBMS
24
Overall Organization
SELECT SS FROM emp WHERE sal gt 50K
Relational Algebra operators ?, ?, ?, ?, ?, ?,
?, ?, ?
25
Overall Organization
SELECT SS FROM emp WHERE sal gt 50K
Query Parser
?SS(?salgt 50K (emp))
Relational Algebra operators ?, ?, ?, ?, ?, ?,
?, ?, ?
26
QUERY TREE

• ?SS(?salgt 50K (emp)) becomes a query tree

Computer Screen
?
TMP File1
? salgt 50K
emp
27
Overall Organization
Query Parser
Query Optimizer
Query Interpretor
Relational Algebra operators ?, ?, ?, ?, ?, ?,
?, ?, ?
Index structures
Abstraction of records
Buffer Pool Manager
File System
28
FILE SYSTEM STRUCTURE (Cont)

• Buffer manager maintains a portion of memory that
  is conceptualized as disk page frames. It
  maintains which disk pages are memory resident.
  It also implements a replacement policy in order
  to swap a page out in favor of another disk page
  that is being referenced. This happens because
  the number of memory page frames is significantly
  smaller than the number of disk pages.
• File manager provides the following services
  create a file, delete a file, read a disk page
  into a specific memory address given the physical
  address of disk page on the secondary storage
  device, write a disk page from a memory address
  on to the appropriate physical disk address,
  insert a page into a file, modify a page, and
  delete a page from a file.

29
FILE SYSTEM STRUCTURE (Cont)

• When a program requests a disk page (by
  specifying its address), the buffer manager takes
  the following steps
• Check if the page is in the buffer.
• If it is then pass its address to the calling
  program.
• Otherwise, read the page from the disk into the
  buffer, possibly replacing some other page, and
  then pass its address to the calling program.
• Pinned blocks Occasionally, the DBMS needs to
  specifically indicate that some blocks have to be
  kept in the buffer until released by unpinning
  them. These blocks are termed pinned.
• Forced writing of blocks to disks To preserve
  the consistency of the database during
  crash-recovery, the DBMS might force the buffer
  manager to flush some blocks to disks.

30
PHYSICAL ORGANIZATION OF RECORDS AND BLOCKS

• Key issues in organizing a file into blocks and
  records
• Formatting fields within a record.
• Formatting records within a block.
• Assigning records into blocks.

31
PHYSICAL ORGANIZATION OF RECORDS AND BLOCKS
(Cont)

• Formatting fields within a record
• Fixed length fields stored in a specific order
• Address of attribute i ß ? Lk
• Fixed length fields stored on an indexed heap
• Fields may be stored in an arbitrary manner
• There is exactly one pointer in the header for
  each field, whether it is present or not.
• The order of pointers is fixed and specifies the
  order of attributes for all records.

ß
Name SS age salary
i-1
k1
Name SS age salary
32
PHYSICAL ORGANIZATION OF RECORDS AND BLOCKS
(Cont)

• Variable length fields delimited by special
  symbols
• Variable length fields delimited by length

32
4
4
4
33
PHYSICAL ORGANIZATION OF RECORDS AND BLOCKS
(Cont)

• Now that once the structure of a record is
  defined, it must get mapped to disk page.
  Consider fixed length records only.
• Fixed-length store records continuously within
  the block.
• record i is located at
• Ri ß (i-1)L

ß
1 2 3

n
34
PHYSICAL ORGANIZATION OF RECORDS AND BLOCKS
(Cont)

• Disadvantage
• Records may span multiple disk page
• Solution dont allow if results in disk
  fragmentation
• Insertion and deletion become complicated
• How do you utilize space that was unallocated?
• Page reorganization affects external pointers

35
PHYSICAL ORGANIZATION OF RECORDS AND BLOCKS
(Cont)

• Indexed Heap
• Each page consists of an array of pointers, each
  pointer points to a record within the block.
• A record is located by providing its block number
  and index in the pointer array. This combination
  is called a TID and an RID.
• Insertion and deletion are easy, accomplished by
  manipulating the pointer array.
• The contents of a block may be reorganized
  without affecting external pointers pointing to
  records. RID does not change when records are
  moved around within a block.