RAID for Rookies

RAIDing each level

RAIDing for Rookies

Before World of Warcraft fans get excited, this isn't about leading a group of Leeroy Jenkins's.  Instead, without further ado, we will cover the most fundamental levels of RAID schemes - RAID 0, RAID 1, RAID1+0, RAID 5 and RAID 6.

RAID 0, otherwise known as a Striping Array, is easily remembered as an all-or-nothing RAID, built purely for performance.  At its simplest definition, it splits up data into 'blocks' - these are distributed between the hard-disk drives in the array as equally as possible.  This allows larger data files to be processed faster as the individual hard-disk drives operated read/write in unison. Ultimately, this allows the combining of their respective blocks to form the complete data file far quicker than the file could be sought from a standard single hard-disk drive. 

On the flip-side, RAID 0 offers no redundancy.  This means that if one drive errors, loses integrity or fails, the entire RAID setup is virtually impossible to salvage - hence, the data stored on the RAID 0 setup will be forfeit.  Therefore, it could be possible to liken a RAID 0 setup to a fast single hard-disk drive; you have performance but no data parity.

RAID 1, is the antithesis of RAID 0.  It is typically consisting of two hard-disk drives which operate in unison, with the key difference being that they are exact sector-by-sector mirror of each other.  This offers no performance advantage, but in the event of a hard-disk drive failure, you can run off just the one remaining hard-disk drive.  They typically operate by a read request being fulfilled by one drive; yet write requests being fulfilled by both.  Wear-levelling has been known to be employed to share the read requests between drives to lengthen the lifespan of the RAID setup before failure.

RAID1+0 (or otherwise known as RAID10) combines the elements of RAID0 and RAID1.  That is, it consists of a striped RAID 0 setup which is made of a mirrored RAID (1).  This is sometimes deemed a suitable RAID setup for a compromise between speed and redundancy of data, however, in the event of a drive failure, the striped element of the RAID will not be able to operate at original speed – thereby taking a noticeable performance hit over the full array’ speed.  The mirrored element of this array allows a failed drive to be replaced and the array restored to original operational integrity, thereby allowing the array to be rebuilt with minimal difficulty.

Around-about this point, you may think that all possible beneficial setups for RAID have been established; you'd be incorrect however... 
RAID 5 is similar to RAID1+0 in that it has both striping (for performance) and redundancy, however, it differs in the method of its redundancy.  Instead of traditional mirroring of stripes, it will distribute the stripes across all bar-one of the hard-disk drives in the array - the final drive will be used to store the data parity - essentially a copy of the data in its entirety.  This might not sound too amazing if you are thinking that the data parity is retained on just one disk drive in the array, however, the RAID5 setup distributes the parity logically, and equally, across all drives, best displayed in the following diagram:

RAIDing for Rookies
RAID5 - simplified.  Maybe...  


As if RAID5 couldn't be improved upon further, an additional array, RAID6, was created. 

RAID6 caters for an additional parity (essentially a mirrored parity) being incorporated into the array.  This allowed for up to two hard-disk drives in the array to fail before the array would become 'at risk'; thereby offering the greatest array stability and recovery whilst still offering performance through striping.


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Most Recent Comments

06-02-2013, 15:26:37

Josh Weston
A great write-up.
Welcome to the team, Matt.Quote

06-02-2013, 15:41:27

Impressive write-up there

However, as an ex-storage consultant, please permit me to point out a few niggles

Page 2 - Raid 5 : You mention "Instead of the traditional mirroring of stripes"... I believe this should read "striping of mirrors" as you aptly described in the Raid 1+0 section above. Remember that striping of mirrors offers greater redundancy than mirroring of stripes (raid 1+0 is better than 0+1).

Page 2 - Raid 6 : The parity is not mirrored, it should be (in any decent implementation) a completely separate parity generation - this offers extra protection against bad blocks that store Raid-5 parity data during rebuild operations.

Page 3 - Mirroring : This attribute offers substantial performance gains for read operations, but none for write operations.

Page 3 - Raid FAQ 1 : Performance improvements are in-line with hard drives, eg: Raid-0 across 2 SSDs will double the throughput in both read and write operations. While real-life improvements may barely be noticeable, the performance increase is real. Also TRIM is now supported in Raid-1, so I argue that wear-rates are not substantially increased over a single SSD. I do agree it rarely makes financial sense though

Raid FAQ 3 : It is very possible to partition your different size drives and then use software RAID across like-size partitions. Leaving you with free partitions you can use for data storage, with the proviso they have no redundancy. Eg: 2TB + 500gb - partition the 2TB into 1.5TB + .5TB, then RAID across the 500gb partition and the 500gb drive. You can subsequently use the 1.5TB partition as normal.

I fully approve of the work you've put in to this and it is overall very good
I am however anally retentive having done this for a living for a while so I hope you'll take my comments as "constructive criticism" rather than any kind of dig

Keep up the good work!Quote

06-02-2013, 22:35:08

Thank you kindly folks for the welcome!

Spaceboy - i'm honestly impossible to offend, so don't worry about that. The article was aimed at beginners, so my intention was lay some basic phrases and general understanding in this article, and then 'polish it up' in future advanced articles.
I do bow to your knowledge on this subject though, I am but a Jack of All, Master of None, based on the comparison between our job roles! Quote

07-02-2013, 00:26:55

Nice wright upQuote

07-02-2013, 10:17:09

Good work, I will agree with the RAID 5 comments - I tried a software/onboard method, and got a 5-6MBps throughput, when I upgraded to a dedicated RAID card (adaptec), that went up to over 200MBps.Quote

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