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Raid 3?

Discussion in 'Storage' started by hat, May 23, 2010.

  1. hat

    hat Maximum Overclocker

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    I've just read that RAID 3 is basically two drives in a RAID 0 config, plus one drive for backup, but the data redundancy is only 66%. What if I had say two 300GB drives and a 600GB drive? Would only a certian amount of the data be copied over to the 600GB drive?
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  2. slyfox2151

    slyfox2151

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    the total space avalible to use is 66% on raid 3,

    the data redundency is 100%.

    if you lose 1 disk you wont lose any data.




    if you combine disks of a different size in a raid, the smallest disk will determin the size of the raid.

    (smallest disk size * number of disks) any extra space cannot be use'd
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  3. hat

    hat Maximum Overclocker

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    Well, you wouldn't RAID 0 two different drives togeather anyway, so I suspect the size of the backup drive would determine the size of the RAID 0 array.

    This looks like a pretty cool solution, maybe I'll look into it in the future. It would be pretty cool to get like a RAID 0 array between two 1TB drives and have a 2TB for backup.

    What if I got say, two 1TB WD Blacks in RAID 0 for uber performance, and got a WD Green 2TB drive to back it up. Would the speed of the entire array be limited by the 2TB Green drive? I understand it would be relatively slow when writing new files as it would have to be copied over to the 2TB Green drive, but I'm talking about reading from files that are already there.

    Quick RAID 1 question: if you have a 50MB file, does it evenly place 25MB on each drive, or does it arbitrarially dump files wherever it feels like?
    Last edited: May 24, 2010
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  4. slyfox2151

    slyfox2151

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    raid 3 has no backup drive.


    it is raid 0 with a parity drive.

    lets say its configued this way


    abcdef is data
    -P is parity infomation

    A C E files goes to disk 1

    B D F files goto Disk 2


    -p goes to disk 3





    if disk 3 dies you lose all parity infomation. however you still have all the data compleatly intact on disks 1 and 2.


    if disk 1 or disk 2 dies, you lose all the data on that disk.
    you must rebuild the infomation using the parity disk to get the data.



    there is no backup of the infomation, parity is simply added so you can use mathmatics to guess what infomation is missing.





    Raid 1 does not split the files, its an exact copy on each disk. both disks get the exact same infomation written on them the 50MB file is written to them both at the same time.

    Raid 0 is the level that splits infomation. depending on what you set the Stripe size at (4 8 16 32 64 128 or even 256kb) thats how the file is split. if you set it to 128kb,
    the 1st 128kb of the 50 MB file will be written to disk 1
    the 2nd 128kb of the 50MB file will be written to disk 2
    the 3rd 128kb of the 50 MB file will be written to disk 1
    and so on untill the end of the file,

    if at the end of the file there is only 50kb left to be written, it will still write 128kb worth of data to the disk.

    EDIT:
    the raid level your talking about is not Raid 3 but Raid 1E
    http://en.wikipedia.org/wiki/File:RAID_1E.png

    that is the speed of raid 0 with the redundency of raid 1 With ODD number of total disks.
    Last edited: May 24, 2010
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  5. hat

    hat Maximum Overclocker

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    I meant to say RAID 0, lol.

    So.. what exactly is this "parity"? If disk 3 is the "parity" drive, and if disk 1 or 2 dies, than what exactly is disk 3 being used for? You say parity is there to "guess what information is missing". What good is that if the data wasn't actually backed up?

    Is there a way to do what I thought RAID 3 was doing?
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  6. slyfox2151

    slyfox2151

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    Raid 1E is the closed raid level i can think of to what you want. Raid 1E combines raid 0 and 1. you get the performance of 2 drives and Full redundency if 1 drive dies, you can use 50 % of the total array size. so 3x 250gb disk means the volume size = 375gb (250 x 3 Devided by 2)

    Raid is not a backup at all. Raid is not to be use'd as a backup.

    when parity is use'd it uses XOR to compare the Data on disk 2 with the parity on disk 3 to recover the missing data on the DEAD disk 1.




    to do what you asked, the simplest way would be to create a raid 0 and do 6 hourly backups to another disk
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  7. slyfox2151

    slyfox2151

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    EDIT to my post.

    XOR is 100 % accurate, there is no guessing with it.
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  8. hat

    hat Maximum Overclocker

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    RAID stands for "redundant array of indepentant discs". As far as I've read, RAID is for backup, and RAID 0 is actually a misnomer, since there is no actual backup going on.

    So, with RAID 3, if disc 1 or 2 blows up, the "parity" disc 3 will replace the files on disc 1 when I get a new one?

    It looks like the best soluiton is to go RAID 0+1
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  9. slyfox2151

    slyfox2151

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    Raid is not backup. full stop.

    if a virus deletes all ur data its gone, if 2 disks or more die at the same time (surge or anything could do that) your data is gone. that is why its not a backup solution.

    if raid 0 is too risky for you then raid 3 is no better.


    Raid 3 question.
    Yes when u install a fresh HDD and rebuild the raid it will restore the lost data to the disk.



    Raid 0+1 is Raid 1E

    Raid 0+1 use's an EVEN amount of disks. 4 is the lowest number of usable disks.

    Raid 1E uses an ODD number is disks. 3 is the lowest number of usable disks.

    Note there is a difference in how they run exacly.
    raid 1E will mix the data between all of the disks. Raid 1+0 will only mix data between 50% of the disks.
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  10. hat

    hat Maximum Overclocker

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    Hm, then what happens in the meantime?

    Yeah, I meant backup in the sense of drive failure, not a virus or an epic surge that kills everything.
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  11. slyfox2151

    slyfox2151

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    raid is used if u cant stand the down time due to a failed disk.
    or to gain performance. or even both of those resons

    i have a large amount of knowledge on Raids, if u have anymore questions ask.



    the easyist option if you want speed + redundency would infact be raid 0+1 due to the lack of support with raid 3 and 1E
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  12. hat

    hat Maximum Overclocker

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    Thanks, I'll keep you in mind in case I need to know anything else, but as it stands now it looks like anything other than RAID 0, RAID 1 and RAID 0+1 looks pretty useless, at least in my eyes.

    I am curious as to the differences between RAID 0+1 and RAID 10 though.
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  13. slyfox2151

    slyfox2151

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    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.


    Notice how the Raid Levels are swaped, that is the difference between Raid 10 and Raid 01
    Raid 10 and 0+1 requier 50 % of the total disk size to be use'd for Mirrored data.
    Raid 5 will give you anywhere from 67% to 90%+. Raid 5 total size is Disk =N minus 1


    [​IMG]
    [​IMG]
    Last edited: May 24, 2010
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  14. Hockster

    Hockster

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  15. hat

    hat Maximum Overclocker

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    So RAID 10 is a RAID 0 array of two RAID 1 arrays, and RAID 0+1 is a RAID 1 array of two RAID 0 arrays. Which one is best?
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  16. slyfox2151

    slyfox2151

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    Raid 10 would be the better option due to support from most Raid controllers.

    performance wise there is no difference.
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  17. Hockster

    Hockster

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    Six of one, half a dozen of the other. :D

    I don't think there's really a difference. Arguments could be made for either.
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  18. Steevo

    Steevo

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    I use RAID 5 for distributed parity and lower overhead, disk space is cheap, data is very valuable.
    hat says thanks.
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