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Thread: Mdadm .VS hardware RAID

  1. #21
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    Quote Originally Posted by energyman View Post
    not with raid6 where you can have 2 devices fail and still recover.
    Sure, but you need a minimum of five disks before that's more efficient than mirroring.

    And mirroring - what do you do when one disk reports the contents of block X as FOO but the other one as BAR. Which one has the correct block?
    Generally speaking, if a disk fails then either the sector will be readable or it won't: with mirroring you can keep the readable sector and ignore the unreadable one. You could get the disks out of sync if they were writing during a power-down such that one disk was written to and the other wasn't, but that's a problem with any form of RAID which doesn't have battery backup.

    Admittedly much of this could be worked around by smarter RAID implementations: i.e. don't fail the entire disk if one block is unreadable, but continue attempting to read the rest of the data as you rebuild the parity onto a new disk. Doesn't ZFS do something like that?

  2. #22

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    Quote Originally Posted by energyman View Post
    why are you using raid1 and not 5 or 6? Just asking.
    most Sun boxes don't come with a gaggle of disks. I have the SAN for any big arrays.

    my v20zs, for instance, have two disk drives. my v40zs can hold six, but I really don't need six local disks. The SAN can outperform anything locally that I could install. The v40zs where I have more than two disks, I usually just use the extra for liveupdate or RAID10.

  3. #23
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    Quote Originally Posted by lbcoder View Post
    Software raid can NEVER be *faster* (it is at best AS fast for stupid raid types, like striping) than hardware raid unless those who implemented the hardware raid were a bunch of rabid chimps on acid. Any kind of parity-raid (raid 5, 6, etc.) will ALWAYS be slower in software raid than in hardware raid, especially when the host CPU is under load.
    I think I may have to disagree with you on this one. I know for a fact that my CPU is a hell of a lot faster than pretty much every areca board I've used.

  4. #24
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    Quote Originally Posted by movieman View Post
    Sure, but you need a minimum of five disks before that's more efficient than mirroring.



    Generally speaking, if a disk fails then either the sector will be readable or it won't: with mirroring you can keep the readable sector and ignore the unreadable one. You could get the disks out of sync if they were writing during a power-down such that one disk was written to and the other wasn't, but that's a problem with any form of RAID which doesn't have battery backup
    that is not about 'dying disk'. A unclean shutdown, a glitch and suddenly you have two different blocks. With raid5/6 you can recover. With Raid1 you only can hope that the kernel makes the right guess.

  5. #25
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    Quote Originally Posted by energyman View Post
    With raid5/6 you can recover. With Raid1 you only can hope that the kernel makes the right guess.
    I'm not sure I understand the difference: if you corrupt random blocks on a RAID5, how do you tell which are the correct ones? Obviously if you can tell that one disk didn't complete all its writes you can recreate the RAID from the other disks by completely rewriting that one (assuming the other disks don't have bad blocks in files you last updated months ago which cause the whole array to die while rebuilding), but the same applies to a RAID1. Maybe you could do some magic by looking at the output from recovering a block from different combinations of disks to try to figure out which is corrupt.

    Personally the more I read about RAID failures, the more I'm coming to the conclusion that I need to install a Solaris server at home and switch to ZFS... it seems to be the only RAID system that doesn't risk losing all your data when there's a disk error.

  6. #26
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    Quote Originally Posted by movieman View Post
    I'm not sure I understand the difference: if you corrupt random blocks on a RAID5, how do you tell which are the correct ones?
    You compute the correct data using the relevant ECC algorithm, which works as long as you have enough non-corrupt pieces for that block.

  7. #27
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    Quote Originally Posted by Ex-Cyber View Post
    You compute the correct data using the relevant ECC algorithm, which works as long as you have enough non-corrupt pieces for that block.
    That doesn't really help me to understand what RAID5 is supposed to be doing. From what I do understand, RAID5 writes multiple data blocks and then a parity block; we're talking about a situation where all those blocks are read from the disk with no errors, but one of them has corrupt data. How can it recover from that? You can tell that one of the blocks is wrong because the parity is wrong, but I don't see how you can tell _which_ block is wrong.

    ZFS, for example, stores a checksum which allows it to determine which of the blocks is corrupt (if it gets the correct checksum by merging data from disks 1, 2 and 3 then the data on disk 4 must be bad), but RAID5 doesn't seem to store any extra data which would allow it to do something similar.
    Last edited by movieman; 11-06-2009 at 01:33 PM.

  8. #28
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    Quote Originally Posted by movieman View Post
    That doesn't really help me to understand what RAID5 is supposed to be doing. From what I do understand, RAID5 writes multiple data blocks and then a parity block; we're talking about a situation where all those blocks are read from the disk with no errors, but one of them has corrupt data. How can it recover from that? You can tell that one of the blocks is wrong because the parity is wrong, but I don't see how you can tell _which_ block is wrong.
    I might be getting my schemes mixed up; I thought RAID 5 used something more complicated than parity (e.g. Reed-Solomon).

  9. #29
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    Quote Originally Posted by Ex-Cyber View Post
    I might be getting my schemes mixed up; I thought RAID 5 used something more complicated than parity (e.g. Reed-Solomon).
    I may be confused too, but all I could find on RAID5 implied that it typically just XORs the data together and stores that as parity, so you can verify the block by XORing all the parts and checking the result is zero.

    Obviously if you go to two parity disks then you can figure out which block is correct if only one of them is wrong.

  10. #30
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    Quote Originally Posted by lbcoder View Post
    The advantage of software raid is portability (i.e., you can plug your disks into ANY controller and access your array rather than being stuck buying a new card if the card burns out), and cost (i.e., you don't need to buy a raid card for software raid).
    ..and flexibility. At least with the hardware RAIDs I've played with (though it's been a long time since my Adaptec-worshipping days ;-), you could only work with whole disks. /sda+/sdb+/sdc combined to only one of RAID0|RAID1|RAID5 and that was that. Software RAID lets you have /sda1+/sdb1+/sdc1 be a RAID1 root, with /sda2+/sdb2+/sbc2 being a RAID0 /tmp and /sda3+/sdb3+/sdc3 as RAID5 /home, etc.

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