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Question number: 811

» Basic CS » Operating System

Essay Question▾

Describe in Detail

What is FtDisk?


  • FtDisk is fault tolerant disk driver for NT which provides several ways to combine multiple SCSI disk drives into one logical volume.

  • Its special property is that it can continue operating under failure of some of parts.

    • If operating quality decreases, the decrease is proportional to the failure.

    • Provides high availability for life critical systems.

  • Uses a hardware technique called sector sparing and NTFS uses a software technique called cluster remapping.

Fault tolerance is achieved by many techniques:

  • Fault masking:

    • Process that prevents faults from introducing error.

  • Reconfiguration:

    • Process of isolating and eliminating faulty components from a system and restoring the system.


  • Higher reliability

  • Lower total cost

  • Psychological worry free use.


  • Cost of redundancy

Question number: 812

» Basic CS » Operating System

Essay Question▾

Describe in Detail

Describe the Buddy system of memory allocation.


Buddy system in image

Buddy System in Image

Buddy system in image

  • Free memory is maintained linked list, each of equal sized blocks.

  • Blocks on the kth level is of size .

  • When memory is required by a process, the block size of next higher order is divided into two- the two pieces differ in address only in their bit.

  • This technique allocates memory in power of 2, example , where x is an integer.

  • The programmer has to decide on, or to write code to obtain, the upper limit of x.

  • Unfortunately this system results in making it impossible to allocate a single remaining 976k of memory into smaller block.


  • Every memory block in system has an order, where the order is an integer ranging from 0 to a specified upper limit.

  • The size of a block of order n is proportional to .

  • Blocks are exactly twice the size of blocks that are one order lower.

  • When a larger block is split, it is divided into two smaller blocks- every block becoming unique buddy to other.

  • The size of the smallest possible block determines the smallest memory block that can be allocated.

  • Low limit is desirable to minimize average memory waste per allocation.

  • Smallest block size is that of order-0 block.

  • Higher orders are expressed as powers of two.