Wednesday, January 5, 2011

RAID level selection for databases

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RAID, an acronym for Redundant Array of Independent Disks, is a technology that provides increased storage functions and reliability through redundancy, combining multiple disk drives components into a logical unit where all drives in the array are interdependent.

Different levels of RAID:
  • RAID 0 – Block level striping without mirroring
  • RAID 1 - Mirroring without striping
  • RAID 2 - Bit-level striping with dedicated Hamming-code parity. 
  • RAID 3 - Byte striping with dedicated parity drive
  • RAID 4 - Block striping with dedicated parity 
  • RAID 5 - Block striping with distributed parity
  • RAID 6 - Block striping with double distributed parity
Comparison of RAID Levels: 



Level

Advantages

Disadvantages

RAID 0

Fastest I/O

No overhead for parity

Simple design, easily implemented

Not really RAID

One drive failure destroys all data

Not for mission-critical deployment

RAID 1

All drives usable for data reads

Can be implemented w/ 2 drives

Greatest storage overhead - 100%

Highest cost/capacity ratio

RAID 3

High transfer rates

Degraded mode still fast

Requires spindle synchronization

Can't do overlapped I/O

RAID 4

High read transfer rates

Efficient use of capacity

Poor write rates

Parity drive can be bottleneck

RAID 5

Very high read rate

Efficient use of capacity

Slower write rates

Slow rebuild times

RAID 6

Allows failure of multiple drives

Very poor write performance

Proprietary solution, rare

RAID 7

Supposed to be fastest

Proprietary, very expensive

RAID 1+0

Very high reads and writes

Most expensive


RAID for databases:

There are a number of factors to consider when choosing the appropriate RAID level for a specific application’s data storage.
RAID 5 is better suited for mostly read-oriented applications while RAID 0+1 is also better suited for write intensive applications. In addition, stripping component (RAID 0) of RAID 0+1 offers the same read performance as RAID 5.

In our case for CRM application’s database, where we had to ensure good read-write speed with data protection, we chose RAID 1 for 2 drives (500 GB SATA) in Dell PowerEdge 6850 server and for a sports portal we used RAID 10 in  Dell PowerEdge M600 server and for a Bank database I had RAID 10 with remote mirroring in IBM Power System.

References:








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