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RAID Level 0: Striped Disk Array without Fault Tolerance

 RAID Level 0 requires a minimum od 2 drives to implement

Characteristics & Advantages

• RAID 0 implements a striped disk array, the data is broken down into blocks and each block is written to a separate disk drive
• I/O performance is greatly improved by spreading the I/O load across many channels and drives
• Best performance is achieved when data is striped across multiple controllers with only one drive per controller
• No parity calculation overhead is involved
• Very simple design
• Easy to implement

Disadvantages

• Not a “True” RAID because it is NOT fault-tolerant
• The failure of just one drive will result in all data in an array being lost
• Should never be used in mission critical environments

Recommended Applications

Video Production and Editing

Image Editing

Pre-Press Applications

Any application requiring high bandwidth

Raid Level 1: Mirroring & Duplexing

 

RAID Level 1 requires a minimum of 2 drives to implement

Characteristics & Advantages

• One Write or two Reads possible per mirrored pair							

• Twice the Read transaction rate of single disks, same Write transaction rate as single disks
• 100% redundancy of data means no rebuild is necessary in case of a disk failure, just a copy to the replacement disk
• Transfer rate per block is equal to that of a single disk
• Under certain circumstances, RAID 1 can sustain multiple simultaneous drive failures
• Simplest RAID storage subsystem design

Disadvantages

• Highest disk overhead of all RAID types (100%) - inefficient
• Typically the RAID function is done by system software, loading the CPU/Server and possibly degrading throughput at high activity levels. Hardware implementation is strongly recommended
• May not support hot swap of failed disk when implemented in “software”

Recommended Applications

Accounting

Payroll

Financial

Any application requiring very high availability

RAID Level 2: Hamming Code ECC

 

Each bit of data word is written to a data disk drive (4 in this example: 0 to 3). Each data word has its Hamming Code ECC word recorded on the ECC disks. On Read, the ECC code verifies correct data or corrects single disk errors.

Characteristics & Advantages

• “On the fly” data error correction
• Extremely high data transfer rates possible
• The higher the data transfer rate required, the better the ratio of data disks to ECC disks
• Relatively simple controller design compared to RAID levels 3,4 & 5

Disadvantages

• Very high ratio of ECC disks to data disks with smaller word sizes - inefficient
• Entry level cost very high - requires very high transfer rate requirement to justify
• Transaction rate is equal to that of a single disk at best (with spindle synchronization)
• No commercial implementations exist / not commercially viable

RAID Level 3: Parallel Transfer with Parity

 

The data block is subdivided (”striped”) and written on the data disks. Stripe parity is generated on Writes, recorded on the parity disk and checked on Reads. RAID Level 3 requires a minimum of 3 drives to implement

Characteristics & Advantages

• Very high Read data transfer rate
• Very high Write data transfer rate
• Disk failure has an insignificant impact on throughput
• Low ratio of ECC (Parity) disks to data disks means high efficiency

Disadvantages

• Transaction rate equal to that of a single disk drive at best (if spindles are synchronized)
• Controller design is fairly complex
• Very difficult and resource intensive to do as a “software” RAID

Recommended Applications

Video Production and live streaming

Image Editing

Video Editing

Prepress Applications

Any application requiring high throughput

RAID Level 4: Independent Data Disks with Shared Parity Disk

 

Each entire block is written onto a data disk. Parity for same rank blocks is generated on Writes, recorded on the parity disk and checked on Reads.

RAID Level 4 requires a minimum of 3 drives to implement

Characteristics & Advantages

• Very high Read data transaction rate
• Low ratio of ECC (Parity) disks to data disks means high efficiency
• High aggregate Read transfer rate

Disadvantages

• Quite complex controller design
• Worst Write transaction rate and Write aggregate transfer rate
• Difficult and inefficient data rebuild in the event of disk failure
• Block Read transfer rate equal to that of a single disk

RAID Level 5: Independent Data Disks with Distributed Parity Blocks

Each entire data block is written on a data disk; parity for blocks in the same rank is generated on Writes, recorded in a distributed location and checked on Reads. RAID Level 5 requires a minimum of 3 drives to implement

Characteristics & Advantages

• Highest Read data transaction rate
• Medium Write data transaction rate
• Low ratio of ECC (Parity) disks to data disks means high efficiency
• Good aggregate transfer rate

Disadvantages

• Disk failure has a medium impact on throughput
• Most complex controller design
• Difficult to rebuild in the event of a disk failure (as compared to RAID level 1)
• Individual block data transfer rate same as single disk

Recommended Applications

File and Application servers

Database servers

Web, E-mail, and News servers

Intranet servers

Most versatile RAID level

RAID Level 6: Independent Data Disks with Two Independent Distributed Parity Schemes

Two independent parity computations must be used in order to provide protection against double disk failure. Two different algorithms are employed to achieve this purpose. RAID Level 6 requires a minimum of 4 drives to implement

Characteristics & Advantages

• RAID 6 is essentially an extension of RAID level 5 which allows for additional fault tolerance by using a second independent distributed parity scheme (dual parity)
• Data is striped on a block level across a set of drives, just like in RAID 5, and a second set of parity is calculated and written across all the drives; RAID 6 provides for an extremely high data fault tolerance and can sustain multiple simultaneous drive failures
• RAID 6 protects against multiple bad block failures while non-degraded
• RAID 6 prodects against a single bad block failure while operating in a degraded mode
• Perfect solution for mission critical applications

Disadvantages

• More complex controller design
• Controller overhead to compute parity addresses is extremely high
• Write performance can be brought on par with RAID Level 5 by using a custom ASIC for computing Reed-Solomon parity
• Requires N+2 drives to implement because of dual parity scheme

Recommended Applications

File and Application servers

Database servers

Web and E-mail servers

Intranet servers

Excellent fault-tolerance with the lowest overhead

RAID Level 10: Very High Reliability combined with High Performance

RAID Level 10 requires a minimum of 4 drives to implement

Characteristics & Advantages

• RAID 10 is implemented as a striped array whose segments are RAID 1 arrays
• RAID 10 has the same fault tolerance as RAID level 1
• RAID 10 has the same overhead for fault-tolerance as mirroring alone
• High I/O rates are achieved by striping RAID 1 segments
• Under certain circumstances, RAID 10 array can sustain multiple simultaneous drive failures
• Excellent solution for sites that would have otherwise gone with RAID 1 but need some additional performance boost

Disadvantages

• Very expensive / High overhead
• All drives must move in parallel to proper track lowering sustained performance
• Very limited scalability at a very high inherent cost

Recommended Applications

• Database server requiring high performance and fault tolerance

RAID Level 50: High I/O Rates & Data Transfer Performance

RAID Level 50 requires a minimum of 6 drives to implement RAID 50 should have been called “RAID 03″ because it was implemented as a striped (RAID level 0) array whose segments were RAID 3 arrays (during mid-90s) Most current RAID 50 implementation is illustrated above RAID 50 is more fault tolerant than RAID 5 but has twice the parity overhead High data transfer rates are achieved thanks to its RAID 5 array segments High I/O rates for small requests are achieved thanks to its RAID 0 striping Maybe a good solution for sites who would have otherwise gone with RAID 5 but need some additional performance boost Very expensive to implement All disk spindles must be synchronized, which limits the choice of drives Failure of two drives in one of the RAID 5 segments renders the whole array unusable

RAID Level 0+1: High Data Transfer Performance

RAID Level 0+1 requires a minimum of 4 drives to implement

Characteristics & Advantages

• RAID 0+1 is implemented as a mirrored array whose segments are RAID 0 arrays
• RAID 0+1 has the same fault tolerance as RAID level 5
• RAID 0+1 has the same overhead for fault-tolerance as mirroring alone
• High I/O rates are achieved thanks to multiple stripe segments
• Excellent solution for sites that need high performance but are not concerned with achieving maximum reliability

RAID 0+1 is NOT to be confused with RAID 10. A single drive failure will cause the whole array to become, in essence, a RAID Level 0 array

Disadvantages

Very expensive / High overhead All drives must move in parallel to proper track lowering sustained performance Very limited scalability at a very high inherent cost

Recommended Applications

Imaging applications

General fileserver

Issue: We have HSG80/HSG60/HSZ70 SANs and the StorageWork agent installed on the different servers as steamd daemon doesn’t inform the system administrator(s) the concrete status of faield disks on the SANs. Every time, to check the status of disks on SA we have to connect to the SAN via serial connection and run the command SHOW FAILED

#!/bin/sh
#
exec expect -f "$0"
spawn /usr/bin/minicom -C temp.txt
expect "hsg80_2>"
send "show failedr"
sleep 2
send "exitr"
expect eof

Script checkhsg80.sh

#!/bin/sh

# created by Vinh Le <vinhlg_at_vcomtech.net> June 2007

# second script

ADMINS=systemadmin@vcomtech.net,vinhlg@vcomtech.net

./checkhsg80.sh

#kill -HUP `ps -ef | grep minicom | awk -F" " ' { print $2 } '`

FAILED=`cat temp.txt | grep FAILEDSET | awk -F" " ' { print $3 }' `

if [ "$FAILED" != "" ]; then

 cat ./temp.txt | mail -s "Warning: Failed disk(s) on HSG80" $ADMINS

else

# echo "HSG80 is OK" |  mail -s "SAN HSG 80 is OK" systemadmin@vcomtech.net

fi

rm -f temp.txt

Script checkhsg80b.sh

Set this script running as cron job. It checks the SAN every hour.

0 * * * * /root/checkhsg80b.sh > /dev/null 2>&1