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About RAID (was Data Security)

Discussion in 'Hardware' started by Bluephoenix, 15 Apr 2007.

  1. Bluephoenix

    Bluephoenix Spoon? What spoon?

    3 Dec 2006
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    I seem to see a lot of people asking the question: "how can I help secure/backup my data?" lately, so I thought I'd write this little guide.

    First we'll deal with redundancy of data which is what a lot of people often incorrectly call 'security'

    1) How paranoid are you about your data?
    This is the first thing to address when looking at data security measures, and the answer generally falls into a few narrow categories.
    a) I need to always have multiple copies and a redundant master copy.
    b) I must always have redundant data because my data is super-important.
    c) constant redundancy is not quite necessary and a weekly backup is fine.

    2) How much am I willing to spend?
    Data redundancy is not cheap by any means, and requires a little more expense to setup/maintain. whether you feel the expense is justified is up to you. Again the cost differs by the answer of the question above.
    a) get a hardware RAID 6 card and magnetic tape backup. Expect to shell out about $2000 on storage and related devices alone.
    b) just get a hardware RAID card, lower end models are fine, but make sure to use RAID levels 1,5 or 6
    c) an external drive backup should do you fine, get one that is at least 1.5x as large as your machine's maximum capacity. (for multiple file versions, and quick backup dumps)

    3) What is this RAID you speak of?
    RAID is an acronym that means Redundant Array of Independent Disks.
    RAID has many flavors, and these functions are represented by numbers. All RAID levels require at least 2 drives of the same type to operate, some require more.

    RAID 0: Striping.
    Striping means that for a given file, a part of it is put on each disk. This increases access times, but means your data is 2 times as vulnerable.

    RAID 1: Mirroring.
    Mirroring means that two disks of exactly the same capacity are used and the data is duplicated on both 1:1 copy. The downside to this method is that the cost per gigabyte of usable storage goes up by 2x.

    RAID 5: Striped with parity.
    RAID 5 is an animal that is harder to understand. It has the qualities of striped disks, but can usually be rebuilt in the case of a single harddrive failure. the way it works is by striping a file across disks and adding a parity sector to the last disk. this parity sector provides information about the other sectors. RAID 5 has the advantages of fast access with some data redundancy, but loses 1 drive's worth of space to parity. This type requires at least 3 drives to implement.

    RAID 6: Striping with Double parity
    RAID 6 is an improved version of RAID 5 that uses two parity blocks instead of one. the advantages to RAID 6 is that it can suffer at least 2 concurrent harddrive failures, and still be rebuilt, this is the most 'secure' type of RAID. the bad part is that it loses more drives to parity than RAID 5. however the loss to parity goes down as the size of the array increases, and it will scale well in comparison to RAID 5. This level requires at least 4 drives to implement.

    4) my motherboard has RAID, will this work like the card you spoke of?
    short answer, NO.
    Motherboard RAID is software driven RAID that is commonly referred to as 'fakeraid' because it does not offer the ability to plug the drives into a new controller and recover the data, unlike a hardware card where if the card dies it can be replaced by a copy of that card and the data retrieved.

    5) Can I expand RAID storage later?
    For RAID 0, 5 and 6, the answer is yes, most of the time; and if the card you are using supports it.

    6) Will the OS be able to boot from the RAID card?
    This also depends on the card used.

    7) will the RAID * on one card work with any other RAID * card available?
    No, because each manufacturer has a different implementation. you may have some luck using cards from the same manufacturer, but this is a crapshoot.

    8) I see cards listing 'hotspare' and 'immediate rebuild'; what are these?
    Hotspare means that an unused drive is connected to the card and in the event of a failure it will rebuild the array using that card.
    Immediate rebuild means that when a failed drive is replaced, the system will automatically rebuild the array.
    in a card with both items it means it will do either or, but if the hotspare is used the replacement drive becomes the new hotspare.

    9) what RAID cards do you recommend?
    If you're running a 680i board then the best idea is to use an Areca RAID 6 PCI-E x8 card in the middle slot. But be prepared to shell out around $750-$800US for it.

    other than that, any 3ware card.

    Hope this helps you all. :thumb:
    Last edited: 18 May 2007
  2. Bluephoenix

    Bluephoenix Spoon? What spoon?

    3 Dec 2006
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    I've been asked since the last article to put up a simple list of Pros/Cons for RAID, so here it is.

    Much less likely to loose data (with the exception of Lv0)
    Read/write speed can be increased (0,5,6)
    More total storage per system (array appears as 1 drive letter)
    Easier to troubleshoot (the controller will tell if a drive fails, if no warning then controller failure)

    Can be more expensive (sometimes it isn't if you know what to look for)
    More complex to set up

    again, feel free to post any comments/additions/corrections :dremel:
  3. Mister_Tad

    Mister_Tad Super Moderator Super Moderator

    27 Dec 2002
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    keep an eye on the bit-tech front page for an in-depth look at RAID very soon :thumb:

    a couple bits in your first post are a little off the mark...
    "RAID 5 doesn't scale well because the larger the number of disks, the more compression of the parity sector"
    "and it will scale well in comparison to RAID 5"

    RAID5 scales very well, regardless of the amount of drives. as a general rule, the more drives, the better. there is no "compression" of parity information

    there is the same amount of parity information on a 10 disk array as a 3 disk array. the only factor to consider when using large numbers of disks with RAID5 is how likely you are to lose a second drive when rebuilding after a first failure.

    RAID6 just as RAID5 does, as there isn't a great deal of difference between them. In general, its a bit slower than RAID6 where writes are concerned because theres double the amount of parity written.
    Last edited: 18 May 2007
  4. Bluephoenix

    Bluephoenix Spoon? What spoon?

    3 Dec 2006
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    I don't know what I was thinking about then I wrote that particular line, but its right now. :thumb:

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