Free RAID guide for dummies?

Posted on 2008-11-17
Last Modified: 2012-05-05
Does anyone have a good easy to understand guide on raid?

i can't get through dry documentation, i am more of a hands on learner, though if i have enough pictures i can get through it

(make all the fun you want, at least i know my learning style :) )
Question by:funnymanmike
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    Here is for Raid 1 instruction for step by step:
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    by:Mark Wills
    what are you after ? Explanations, descriptions, pro's + con's or how to's
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    my current understanding of raid: multiple HD's acting as one, where information is copied to multiple, where in case of a faulty HD, it can be replaced and rebuilt without losing information

    what i want to learn: how to understand the menu system, how to set this up, and more of a understanding on it. so explanations, descriptions, and how to's. i feel with enough how to's i will have a good understanding of RAID

    RAID 3 is standard for servers is it not?

    Thanks for the link for RAID 1, i will review.
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    by:Mark Wills
    Ahhhh... I see...

    There are a number of RAID configurations... 4 are popular, and a couple of new ones coming.

    RAID 0 = striping - needs at least a pair of disks and not often used - performance only, 1 disk goes, so does the array.
    RAID 1 = mirror - needs a pair and by pairs (e.g. 2, 4, 6) of disks - mirrors one disk on another
    RAID 5 = striped parity - a defacto standard RAID when you don't have a real RAID requirement - trying to get performance of R0 with some fault tolerance - minimum three disks - good general purpose file storage, excellant read, ordinary write.
    RAID 10 = mirror sets within a striped set used mainly in database environments where speed of striping is required with security of mirror minimum 4 and then in pairs.

    then there are others (and will not discuss - in the bah-humbug category) :
    RAID 2 = Hamming code parity very small stripes across 3 disks trying
    RAID 3 = Striped set with dedicated parity or bit interleaved parity or byte level parity needs 3 disks - parity disk can go down and arrary survives
    RAID 4 = Block level parity - like 3 but parity on blocks of data not bytes - needs 3 disks
    RAID 6 = Striped set with dual distributed parity - more fault tolerant than 5 (and better I think)

    striping is good because data is spread over a number of disks to achieve a level of concurrency in terms of multiple spindles (disks) being accessed at the same time. in fact the more spindles the better.
    mirroring is good because of security / redundancy - is a disk fails, the other disk kicks in and business as usual.
    parity is meant to be an improvement on striping, because it does offer a level of fault tolerence (not much) and has good read performance, lousy write performance (because of the parity check, and CPU hit in doing so)

    hot swap disks are preferred so that if a disk does go down, then it can be swapped dynamically.

    rebuilding an array depends on the type of RAID. and can assure you the fastest and easiest are the mirrored arrays. the most painful are parity or striped arrays.

    RAID stands for redundant array of independant disks and was invented when 25meg was considered a big disk (with a dozen or so dinner plate platters), and the drive about the size of a washing machine - which was considered compact at the time. So, to get large data elements, had to some how get these things linked together - hence RAID was invented.

    Nowadays, the size and speed of an individual disk is really quite extraordinary, so, the reason for raid has kind of gone by the wayside. higher speed and smaller diameters has decreased latency and increased throughput to the point where sometimes wonder if RAID is still such a priority. But fear not, the disk people invented SAN just to make sure large arrays of disks are still required.

    But then there are the real factors to make RAID work best :
    Stripe = spread the load and gain concurrency over several spindles
    Cache = keep in memory - or onbaord chaching (in the controlelr) so pysical disk access is minimised
    Chunks = size of data elements being transferred - normally matching the nature and type of data to be held on disk (smaller chunks can improve speed)

    and the Controller - biggest factor by far - does it have multiple channels where disks or arrays can have their own channel for transferring data to / from disk, on board caching so a truckload of data can be accessed without going to disk - leaving disk activity for real IO tasks.

    then there is the disks themselves - they are bigger, faster and far more reliable than ever before, so we often see a RAID configuration giving plenty of capacity... The big difference is SAS versus SATA and we can now see huge performance (especially under load) of say a 15kRPM 140gig SAS Disk for that same price as a huge capacity 1TB SATA (that's terabyte thankyou very much).

    In RAID - it probably isn't so much the overwhelming capacity that SATA has to offer, but the speed that SAS promises, and given the speed, then it is the reliability or fault tolerance. For those reasons, probably best to spend the money on the best multi-channel IO controller with huge on-board cache with groups of arrays for system, database, logs, transient workspace all running off their own (fibre) channels as much as possible, with all disks on all arrays being the same - possibly 70gig or 140 gig or 300 gig, and then multiples according to capacity requirements. Now if you are talking about terabytes of data, then there is not much choice - you really have to go to the terabyte capacity of sata - the cost would simply be far too prohibitive otherwise - then get many more than you need (given the considerably lower cost) and use striping technology to help with speed.

    Soon we will have solid state drives where capacities and performance will yield other technologies a bit antiquated...

    Wiki has some good articles :
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