STORAGE STRUCTURE-DBMS, Study notes of Database Management Systems (DBMS)

Download STORAGE STRUCTURE-DBMS and more Study notes Database Management Systems (DBMS) in PDF only on Docsity!

21CSC205P-Database

Management Systems

UNIT-V

TOPICS

• Storage Structure
• Transaction control
• Concurrency control algorithms and Graph
• Issues in Concurrent execution
• Failures and Recovery algorithms
• Case Study: Demonstration of Entire project by applying all the concepts learned with minimum Front-End requirements, NoSQL Database, Document Oriented, Key Value pairs, Column Oriented

Physical Storage Media

Classification of Physical Storage Media

• Speed with which data can be accessed
• Cost per unit of data
• Reliability o data loss on power failure or system crash o physical failure of the storage device
• Can differentiate storage into: o Volatile storage: loses contents when power is switched off o Non-volatile storage: ▪ Contents persist even when power is switched off. ▪ Includes secondary and tertiary storage, as well as batter- backed up main-memory.

Storage Device Hierarchy

3. Flash Memory

• Data survives power failure
• Data can be written at a location only once, but location can be erased and written to again ▪ Can support only a limited number (10K – 1M) of write/erase cycles. ▪ Erasing of memory has to be done to an entire bank of memory
• Reads are roughly as fast as main memory
• But writes are slow (few microseconds), erase is slower
• Cost per unit of storage roughly similar to main memory
  • Widely used in embedded devices such as digital cameras, phones, and USB keys
• Is a type of EEPROM (Electrically Erasable Programmable Read-Only Memory)

4. Magnetic-disk Storage

• 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
  • direct-access – possible to read data on disk in any order, unlike magnetic tape
• Capacities range up to roughly 400 GB currently
  • Much larger capacity and cost/byte than main memory/flash 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 rare

6. Tape Storage - Non-volatile, used primarily for backup (to recover from disk failure), and for archival data - Sequential-access – much slower than disk - Very high capacity (40 to 300 GB tapes available) - Tape can be removed from drive  storage costs much cheaper than disk, but drives are expensive - Tape jukeboxes available for storing massive amounts of data - hundreds of terabytes (1 terabyte = 10 9 bytes) to even multiple petabytes (1 petabyte = 10 12 bytes)

Storage Hierarchy (Cont.)

• Primary Storage: Fastest media but volatile (cache, main memory).
• Secondary Storage: next level in hierarchy, non-volatile, moderately fast access time - also called on-line storage - E.g. flash memory, magnetic disks
• Tertiary Storage: lowest level in hierarchy, non-volatile, slow access time - also called off-line storage - E.g. magnetic tape, optical storage

Magnetic Disks Physical Characteristics of Disks

• Read-write head
  • Positioned very close to the platter surface
  • Reads or writes magnetically encoded information.
• Surface of platter divided into circular tracks
  • Over 50K-100K 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: 500 to 1000 (on inner tracks) to 1000 to 2000 (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 (1 to 5 usually)
  • one head per platter, mounted on a common arm.
• Cylinder i consists of i th^ track of all the platters

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 won’t match recomputed checksum
  • Ensures successful writing by reading back sector after writing it
  • Performs remapping of bad sectors

Disk Subsystem (cont.)

• Disks usually connected directly to computer system
• In Storage Area Networks (SAN) , a large number of disks are connected by a high-speed network to a number of servers
• In Network Attached Storage (NAS) networked storage provides a file system interface using networked file system protocol, instead of providing a disk system interface

Performance Measures of Disks

• Access time – the time it takes from when a read or write request is issued to when data transfer begins. - 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.
  • 25 to 100 MB per second max rate, lower for inner tracks
  • Multiple disks may share a controller, so rate that controller can handle is also important
    • E.g. SATA: 150 MB/sec, SATA-II 3Gb (300 MB/sec)
    • Ultra 320 SCSI: 320 MB/s, SAS (3 to 6 Gb/sec)
    • Fiber Channel (FC2Gb or 4Gb): 256 to 512 MB/s
• Mean time to failure (MTTF) – the average time the disk is expected to run continuously without any failure.

Redundant Array of Independent Disks (RAID)

• RAID is a technology that uses multiple physical disk drives to protect data from a single disk failure.
• The purpose of RAID is to ensure that at the time of failure, there should be one copy of data which should be available for immediate use.
• RAID levels define the use of disk arrays. RAID levels
• RAID 0
• RAID 1
• RAID 2
• RAID 3
• RAID 4
• RAID 5
• RAID 6

RAID 0

• RAID 0 consists of striping, but no mirroring or parity, but no redundancy of data. It offers the best performance, but no fault tolerance.
• In this level, a striped array of disks is implemented. The data is broken down into blocks and the blocks are distributed among disks.
• Block “1, 2” forms a stripe.
• Each disk receives a block of data to write/read in parallel.
• Reliability: there is no duplication of data. Hence, a block once lost cannot be recovered.