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Wednesday, August 25, 2010

RAID data recovery

Primarily used with entry-level servers, software-based arrays rely on a standard hostadapter and execute all I/O commands and mathematicallyintensive RAID algorithms in the host server CPU. Thiscan slow system performance by increasing host PCI bustraffic, CPU utilization, and CPU interrupts. Some NOSssuch as NetWare and Windows NT include embedded RAIDsoftware. The chief advantage of this embedded RAIDsoftware has been its lower cost compared tohigher-priced RAID alternatives. However, this advantageis disappearing with the advent of lower-cost, bus-basedarray adapters.
  • Low price
  • Only requires a standard controller

RAID 0 - Data striping without redundancy (no protection). Optimized for Performance
RAID 0 uses striping to write data across multiple drives simultaneously. This means that when you write a 5GB file across 5 drives, 1GB of data is written to each drive. Parallel reading of data from multiple drives can have a significant positive impact on performance.
The trade-off with RAID 0 is that if one of those drives fail, all of your data is lost and you must retore from backup.
RAID 0 is an excellent choice for cache servers, where the actual data being stored is of little value, but performance is very important.
  • Minimum number of drives: 2
  • Strengths: Highest performance.
  • Weaknesses: No data protection; One drive fails, all data is lost.
Data AData A
Data BData B
Data CData C

RAID 1 - Disk mirroring. Optimized for Redundancy
RAID 1 uses mirroring to write data to multiple drives. This means that when you write a file, the file is actually written to two disks. If one of the disks fails, you simply replace it and rebuild the mirror.
The tradeoff with RAID 1 is cost. With RAID 1, you must purchase double the amount of storage space that your data requires.
  • Minimum number of drives: 2
  • Strengths: Very high performance; Very high data protection; Very minimal penalty on write performance.
  • Weaknesses: High redundancy cost overhead; Because all data is duplicated, twice the storage capacity is required.
Standard Host
Data AData A
Data BData B
Data CData C
Original DataMirrored Data
Standard Host
Adapter 1
Standard Host
Adapter 2
Data AData A
Data BData B
Data CData C
Original DataMirrored Data

RAID 5 - Block-level data striping with distributed parity. A Good Compromise
RAID 5 stripes data across multiple disks. RAID 5, however, adds a parity check bit to the data. This slightly reduces available disk capacity, but it also means that the RAID array continues to function if a single disk fails. In the event of a disk failure, you simply replace the failed disk and keep going.
The tradeoffs with RAID 5 are a small performance penalty in write operations and a slight decrease in usabable storage space.
  • Minimum number of drives: 3
  • Strengths: Best cost/performance for transaction-oriented networks; Very high performance, very high data protection; Supports multiple simultaneous reads and writes; Can also be optimized for large, sequential requests.
  • Weaknesses: Write performance is slower than RAID 0 or RAID 1.
Parity AData AData A
Data BParity BData B
Data CData CParity C

RAID 0/1 - RAID 10
RAID 0+1 - Combination of RAID 0 (data striping) and RAID 1 (mirroring). Optimize for Performance and Redundancy
RAID 0+1 combines the performance of RAID 0 with the redundancy of RAID 1.
To build a RAID 0+1 array, you first build a set of RAID 1 mirrored disks and you then combine these disk sets in a RAID 0 striped array.
A RAID 0+1 array can survive the loss of one disk from each mirrored pair. RAID 0+1 cannot survive the loss of two disks in the same mirrored pair.
RAID-10 and RAID-01 are not the same thing and it does matter. RAID-01 is a mirrored configuration of two striped sets. RAID-10 is a stripe across a number of mirrored sets.
RAID-10 provides better fault resilience and "rebuild" performance than RAID-01. Both array types provide very good to excellent overall performance by combining the speed of RAID-0 with the redundancy of RAID-1 without requiring parity calculations.
  • Minimum number of drives: 4
  • Strengths: Highest performance, highest data protection (can tolerate multiple drive failures).
  • Weaknesses: High redundancy cost overhead; Because all data is duplicated, twice the storage capacity is required; Requires minimum of four drives.
Data AData AmAmA
Data BData BmBmB
Data CData CmCmC
Original DataOriginal DataMirrored DataMirrored Data
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