DBMS Short Question And Answer Set-5

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1. Define shadow paging.
An alternative to log-based crash recovery technique is shadow paging. This technique needs fewer disk accesses than do the log-based methods.

2. Define page.
The database is partitioned into some number of fixed-length blocks, which are referred to as pages.

3. Explain current page table and shadow page table.
The key idea behind the shadow paging technique is to maintain two page tables during the life of the transaction: the current page table and the shadow p age table. Both the page tables are identical when the transaction starts. The current page table may b e changed when a transaction performs a write operation.

4. What are the drawbacks of shadow-paging technique?

  • Commit Overhead
  • Data fragmentation
  • Garbage collection

5. Define garbage collection.
Garbage may be created also as a side effect of crashes. Periodically, it is necessary to find all the garbage pages and to add them to the list of free pages. This process is called garbage collection.

6. Differentiate strict two phase locking protocol and rigorous two phase locking
protocol.
In strict two phase locking protocol all exclusive mode locks taken by a transaction is held until that transaction commits. Rigorous two phase locking protocol requires that all locks be held until the transaction
commits.

7. How the time stamps are implemented ?

  •  Use the value of the system clock as the time stamp. That is a transaction‟s time stamp is equal to the value of the clock when the transaction enters the system.
  •  Use a logical counter that is incremented after a new timestamp has been assigned; that is the time stamp is equal to the value of the counter.

8. What are the time stamps associated with each data item?

  •  W-timestamp (Q) denotes the largest time stamp if any transaction that executed WRITE (Q) successfully.
  •  R-timestamp (Q) denotes the largest time stamp if any transaction that executed READ (Q) successfully.

9. What are the advantages and disadvantages of indexed sequential file?
The advantage of ordering records in a sequential file according to a key is that you can then search the file more quickly. If you know the key value that you want, you can use one of the relatively fast searches. The disadvantage is that when you insert, you need to rewrite at least everything after the insertion point, which makes inserts very expensive unless they are done at the end of the file. An indexed file approach keeps a (hopefully) small part of each row, and some kind of “pointer” to the row’s location within the data file. This allows a search to use the index, which is ordered by the index and (again hopefully) much smaller and therefore much faster than scanning the entire data file for the indexed data.

10. What is database tuning? 
Database tuning describes a group of activities used to optimize and homogenize the performance of a database. It usually overlaps with query tuning, but refers to design of the database files, selection of the database management system (DBMS), operating system and CPU the DBMS runs on.

11. Give the measures of quality of a disk.

  • Capacity
  • Access time
  • Seek time
  • Data transfer rate
  • Reliability
  • Rotational latency time.

12. Compare sequential access devices versus random access devices with an example
sequential access devices random access devices Must be accessed from the beginning It is possible to read data from any location  Eg:- tape storage Eg:-disk storage Access to data is much slower Access to data is faster Cheaper than disk Expensive when compared with disk

13. What are the types of storage devices?

  • Primary storage
  • Secondary storage
  • Tertiary storage

14. Define average seek time.
The average seek time is the average of the seek times, measured over a sequence of random requests.

15. Define rotational latency time.
The time spent waiting for the sector to be accessed to appear under the head is called the rotational latency time.

16. Define average latency time.
The average latency time of the disk is one-half the time for a full rotation of the disk.

17. What is meant by data-transfer rate?
The data-transfer rate is the rate at which data can be retrieved from or stored to the disk.

18. What is meant by mean time to failure?
The mean time to failure is the amount of time that the system could run continuously without failure.

19. What are a block and a block number?
A block is a contiguous sequence of sectors from a single track of one platter. Each request specifies the address on
the disk to be referenced. That address is in the form of a block number.

20. What are called journaling file systems?
File systems that support log disks are called journaling file systems.