Michael Evans <mjevans1983@xxxxxxxxx> writes: > On Sun, Mar 7, 2010 at 4:52 AM, Goswin von Brederlow <goswin-v-b@xxxxxx> wrote: >> Neil Brown <neilb@xxxxxxx> writes: >> >>> On Sat, 06 Mar 2010 18:17:44 -0500 >>> "Guy Watkins" <linux-raid@xxxxxxxxxxxxxxxx> wrote: >>> >>>> } >>>> } At a minimum I would build a 3-disk raid 6. raid 6 does a lot of i/o >>>> } which may be a problem. >>>> >>>> If he only needs 3 drives I would recommend RAID1. Can still loose 2 drives >>>> and you don't have the RAID6 I/O overhead. >>>> >>> >>> and as md/raid6 requires at least 4 drives, RAID1 is not just the best >>> solution to survive two failures on a 3-device array, it is the only solution. >>> >>> NeilBrown >> >> Except that there also is raid10 with 3 mirrors. :) >> >> MfG >> Goswin >> >> PS: Why doesn't raid6 still not allow 3 drives for the special case of >> converting raid1 -> raid6? >> >> >> -- >> 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 >> > > That should be obvious: > > Possible stripes: > > Start: > 1, 1, 1; > 2, 2, 2; Start: 1, 1, 1; 2, 2, 2; 3, 3, 3; ... > 'raid6' overtake... > 1, q, Q; > 2, q, Q; Middle: 1, P, Q; P, Q, 2; Q, 3, P; ... End: 1, 2, P, Q; 4, P, Q, 3; P, Q, 5, 6; ... > 'raid6' overtake with missing; > 1, (missing 2), q, Q; > 3, (missing 4), q, Q; > > In the first overtake case you have the requirement of generating 200% > parity, which probably won't work for the algorithm and is a silly > idea in general since it's computationally far less expensive to store > another copy of either form of data instead. The sick 3 disk raid6 case should have both the P and Q identical to the data block. It is indeed computational a waste to go through the expensive P/Q parity algorithm for the same result as mirroring but this is only ment as a transitional state. > In the second you're gaining the space of a second disk at the cost of > being already degraded; why not just go for raid 5 instead? > > You can overtake raid5 later with raid6 if you add more devices. Because then you are going from 2 mirror disks to 1 parity disk even if only temporary. You are reducing the number of disks failures you can survive from 2 to 1 and the high load during a reshape makes a failure more likely than normal operations. Or can you go from 3 way raid1 to 4 disk raid6 in a single step? MfG Goswin -- 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
