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Wednesday, 23 November 2016

Which of the following is the correct block size in database disk storage?

Which of the following is the correct block size in database disk storage?


2) Consider a hard disk with a sector size of 512 bytes, and 50 sectors per track. A block may be of several KBs in size. Which of the following is the valid block size?

(a) 256 bytes
(b) 786 bytes
(c) 4096 bytes
(d) 26112 bytes


(C) 4096 bytes/4 KB


  • A block (cluster) is a unit of memory used to specify the cluster of one or more sectors (physical disk area). Block is the term used by operating systems to specify the minimal size of memory that can be accessible from hard disk.
  • Sector is the minimal size of physical storage (hard disk) and is the subdivision of a track of the hard disk.
The block size should be a multiple of sector size.

(a) 256 bytes – it is half of a sector. We need at least a sector. WRONG

(b) 786 bytes – it is not a multiple of sector size. WRONG

(c) 4096 bytes – it is a multiple of given sector size. 512 * 8 = 4096. RIGHT

(d) 26112 bytes – it is indeed a multiple of the sector size. 512 * 51 = 26112. But, it exceeds the size of a track (Given, 512 bytes * 50 tracks = 25600 bytes). The size of block (or cluster) cannot exceed the size of a track. Hence, WRONG.

Option (c), 4096 bytes is the valid block size.

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