On Wed, Mar 10, 2010 at 9:47 AM, Goswin von Brederlow <goswin-v-b@xxxxxx> wrote: > 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 > You are not planning on staying with 3 devices though. Just stick with 2 redundancy raid 1 until you have four devices. Then overtaking from raid 1 + hotspares at 4 devices total to raid 6 with 2 data devices and 2 parity devices per stripe makes sense. -- 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
