Hard drives and RAID technology for IBPS SO

Hi Folks, 

Today we will learn some important concepts related to Data Storage for IBPS SO exam. A lot of questions are asked from this topic as observed in SBI SO and other specialist officer exams.

What is non volatile storage?

Non volatile memory is the long term persistent memory where the information can be retrieved even after turning off and on the device. This is typically used as a secondary storage
Examples of non volatile storage devices are: hard disk drive, floppy disk, magnetic tape, optical disk, paper tapes, punched cards, ferroelectric RAM, EPROM. 



Different types of Hard Drives


A hard drive is a non volatile storage device where digitally encoded data is stored on rotating magnetic platters. Different types of HDD are as follows:

PATA (Parallel Advance Technology Attachment)

  • It is also called IDE (Integrated Drive Electronics)
  • Sends 8 bit data at a time. 
  • Transfer speed up to133 mb/sec
SATA (Serial Advance Technology Attachment)
  • It sends data bit by bit.
  • Transfer speed up to 300 mb/sec
SCSI (Small Computer System Interface)
  • These drives are hot-swappable i.e can be inserted or removed from the running system without affecting performance or causing damage to data. 
  • Transfer speed up to 640 mb/sec

RAID and its levels


RAID stands for Redundant Array of Inexpensive (Independent) Disks. Its a technology used to increase the performance and reliability of data storage. RAID system consists of 2 or more drives working in parallel. Important levels of RAID are discussed below:

RAID level 0 (Striping)
  • Data is split into blocks and is written across all drives in the array. 
  • It offers great performance in read & write operations. 
  • The only disadvantage being that it is not fault tolerant i.e if one drive fails, all data in RAID 0 array will be lost. So it should not be used for critical information storage. 
RAID level 1 (Mirroring)
  • In this level, data is stored twice i.e on both the drives so minimum 2 disks required. 
  • If 1 drive fails, controlled switches to another for data recovery and continues operation. 
  • Because of double storage, effective storage capacity decreases to half.
  • It is used for critical information storage. e.g. - accounting systems. 
RAID level 5 (Parity)
  • It requires minimum 3 drives and can work with up to 16 drives. 
  • Data blocks are stripped across the devices.
  • Parity data for one drive is written to another drive and whole parity checksum data is spread over all drives.
  • Using this parity data, computer can recalculate the data of other data blocks in case that data is no longer available. 
  • So even if a drive fails, you will still have access to all the data which makes it SECURE.
  • Reads are faster than writes due to parity.
  • Ideal for servers that have limited no. of data drives. 
RAID level 6 (Dual Parity)
  • In level 6, parity data is written to two drives. 
  • Minimum 4 drives are required and 2 drives can fail simultaneously (extreme case) without affecting performance.
  • Reads are faster and writes are more slower than RAID 5 due to double parity

What is Write Hole problem?


Write hole problem occurs when power failure happens during a write operation. Though all the RAID array types are affected by this, mostly RAID level 5 is recognized with this problem.
We can avoid this problem by ensuring 'write atomicity' i.e if the operation is interrupted due to external reasons, system remains either in original state or final state, not in between.

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