From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michael Evans Subject: Re: What RAID type and why? Date: Thu, 11 Mar 2010 02:44:07 -0800 Message-ID: <4877c76c1003110244m1c5e1500n3c4c7a63dccf35de@mail.gmail.com> References: <5bdc1c8b1003061402n1281b64es9fa597b8bc714bd5@mail.gmail.com> <87f94c371003061433x404a8c2fgcb61f817af6ecb1@mail.gmail.com> <9089562724D84B3C858E337F202FF550@m5> <20100307132113.7e2c95b6@notabene.brown> <877hpoqoob.fsf@frosties.localdomain> <4877c76c1003071240t6814248eqd63f391bc278aee8@mail.gmail.com> <87pr3cnk5j.fsf@frosties.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <87pr3cnk5j.fsf@frosties.localdomain> Sender: linux-raid-owner@vger.kernel.org To: Goswin von Brederlow Cc: Neil Brown , Guy Watkins , Greg Freemyer , Mark Knecht , Linux-RAID List-Id: linux-raid.ids On Wed, Mar 10, 2010 at 9:47 AM, Goswin von Brederlow wrote: > Michael Evans writes: > >> On Sun, Mar 7, 2010 at 4:52 AM, Goswin von Brederlow wrote: >>> Neil Brown writes: >>> >>>> On Sat, 06 Mar 2010 18:17:44 -0500 >>>> "Guy Watkins" wrote: >>>> >>>>> } >>>>> } At a minimum I would build a 3-disk raid 6. =A0raid 6 does a lo= t of i/o >>>>> } which may be a problem. >>>>> >>>>> If he only needs 3 drives I would recommend RAID1. =A0Can still l= oose 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 on= ly solution. >>>> >>>> NeilBrown >>> >>> Except that there also is raid10 with 3 mirrors. :) >>> >>> MfG >>> =A0 =A0 =A0 =A0Goswin >>> >>> 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-rai= d" in >>> the body of a message to majordomo@vger.kernel.org >>> More majordomo info at =A0http://vger.kernel.org/majordomo-info.htm= l >>> >> >> 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 20= 0% >> parity, which probably won't work for the algorithm and is a silly >> idea in general since it's computationally far less expensive to sto= re >> 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 t= his > 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 failur= e > more likely than normal operations. > > Or can you go from 3 way raid1 to 4 disk raid6 in a single step? > > MfG > =A0 =A0 =A0 =A0Goswin > 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" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html