Wol, I guess so that this code is possibly hammering the disk. But the implementation of encoding/decoding part is not a big deal. The codes used in RAID5 and RAID6 (and also in Global Parity) are in the category of Maximum Distance Separable (MDS) codes (some famous examples of these codes are Hamming and ReedSolomon). If you have interest you can take a look at this paper: https://arxiv.org/pdf/1205.0997 But my main challenge now is what parts of mdadm should be modified, and how can I enable debug mode in mdadm. I wonder if anyone can give me a clue about the debug stuff. Bests, Mostafa On Sat, Jan 27, 2018 at 6:43 PM, Wols Lists <antlists@xxxxxxxxxxxxxxx> wrote: > On 27/01/18 14:29, mostafa kishani wrote: >> Thanks for your response Wol >> Well, maybe I failed to illustrate what I'm going to implement. I try >> to better clarify using your terminology: >> In the normal RAID5 and RAID6 codes we have one/two parities per >> stripe. Now consider sharing a redundant sector between say, 4 >> stripes, and assume that the redundant sector is saved in stripe4. >> Assume the redundant sector is the parity of all sectors in stripe1, >> stripe2, stripe3, and stripe4. Using this redundant sector you can >> tolerate one sector failure across stripe1 to stripe4. We already have >> the parity sectors of RAID5 and RAID6 and this redundant sector is >> added to tolerate an extra sector failure. I call this redundant >> sector "Global Parity". >> I try to demonstrate this as follows, assuming each RAID5 stripe has 3 >> data sectors and one parity sector. >> stripe1: DATA1 | DATA2 | DATA3 | PARITY1 >> stripe2: PARITY2 | DATA4 | DATA5 | DATA6 >> stripe3: DATA7 | PARITY3 | DATA8 | DATA9 >> stripe4: DATA10 | DATA11 | PARITY4 | GLOBAL PARITY >> >> and the Global Parity is taken across all data and parity as follows: >> GLOBAL PARITY = DATA1 X DATA2 X DATA3 X DATA4 X DATA5 X DATA6 X DATA7 >> X DATA8 X DATA9 X DATA10 X DATA11 X PARITY1 X PARITY2 X PARITY3 >> >> Where "X" stands for XOR operation. >> I hope it was clear. > > OWWW!!!! > > Have you done and understood the maths!!!??? > > You may have noticed I said that while raid-6 was similar in principle > to raid-5, it was very different in implementation. Because of the maths! > > Going back to high-school algebra, if we have E *unique* equations, and > U unknowns, then we can only solve the equations if E > U (I think I've > got that right, it might be >=). > > With raid-5, parity1 = data1 xor data2 xor data3. Now let's assume > somebody thinks "let's add parity2" and defines parity2 = data1 xor > data2 xor data3 xor parity1. THAT WON'T WORK. Raid-5 relies on the > equation "if N is even, then parity2 is all ones, else parity2 is all > zeroes", where N is the number of disks, so if we calculate parity2 we > add absolutely nothing to our pre-existing E. > > If you are planning to use XOR, I think you are falling into *exactly* > that trap! Plus, it looks to me as if calculating your global parity is > going to be a disk-hammering nightmare ... > > That's why raid-6 uses a *completely* *different* algorithm to calculate > its parity1 and parity2. > > I've updated a page on the wiki, because it's come up in other > discussions as well, but it seems to me if you need extra parity, you > really ought to be going for raid-60. Take a look ... > > https://raid.wiki.kernel.org/index.php/What_is_RAID_and_why_should_you_want_it%3F#Which_raid_is_for_me.3F > > and if anyone else wants to comment, too? ... > > Cheers, > Wol -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
