On 11/09/2020 21:14, Brian Allen Vanderburg II wrote:
That's right, I get the various combinations confused. So does raid61 allow for losing 4 disks in any order and still recovering? or would some order of disks make it where just 3 disks lost and be bad? Iinteresting non-the-less and I'll have to look into it. Obviously it's not intended to as a replacement for backing up important data, but, for me any way, just away to minimize loss of any trivial bulk data/files.
Yup. Raid 6 has two parity disks, and that's mirrored to give four parity disks. So as an *absolute* *minimum*, raid-61 could lose four disks with no data loss.
Throw in the guarantee that, with a mirror, you can lose an entire mirror with no data-loss, that means - with luck and a following wind - you could lose half your disks, PLUS the two parities in the remaining disks, and still recover your data. So with a raid-6+1, if I had twelve disks, I could lose EIGHT disks and still have a *chance* of recovering my array. I'm not quite sure what difference raid-61 would make.
(That says to me, if I have a raid-61, I need as a minimum a complete set of data disks. That also says to me, if I've splatted an 8+2 raid-61 across 11 disks, I only need 7 for a full recovery despite needing a minimum of 8, so something isn't quite right here... I suspect the 7 would be enough but I did say my mind goes Whooaaa!!!!)
It would be nice if the raid modules had support for methods that could support a total of more disks in any order lost without loosing data. Snapraid source states that it uses some Cauchy Matrix algorithm which in theory could loose up to 6 disks if using 6 parity disks, in any order, and still be able to restore the data. I'm not familiar with the math behind it so can't speak to the accuracy of that claim.
That's easy, it's just whether it's worth it. Look at the maths behind raid-6. The "one parity disk" methods, 4 or 5, just use XOR. But that only works once, a second XOR parity disk adds no new redundancy and is worthless. I'm guessing raid-6 uses that Cauchy method you talk about - certainly it can generate as many parity disks as you like ... so that claim is good, even if raid-6 doesn't use that particular technique.
If someone wants to, mod'ing raid-6 to use 3 parity disks shouldn't be that hard ...
But going back to your original idea, I've been thinking about it. And it struck me - you NEED to regenerate parity EVERY TIME you write data to disk! Otherwise, writing one file on one disk instantly trashes your ability to recover all the other files in the same position on the other disks. WHOOPS! But if you think it's a good idea, by all means try and do it.
The other thing I'd suggest here, is try and make it more like raid-5 than raid-4. You have X disks, let's say 5. So one disk each is numbered 0, 1, 2, 3, 4. As part of formatting the disk ready for raid, you create a file containing every block where LBA mod 5 equals disk number. So as you recalculate your parities, that's where they go.
Cheers, Wol
