From mboxrd@z Thu Jan 1 00:00:00 1970 From: NeilBrown Subject: Re: RAID5 -> RAID6 conversion, please help Date: Wed, 11 May 2011 13:29:35 +1000 Message-ID: <20110511132935.0438143a@notabene.brown> References: <002a01cc0f68$1c851180$558f3480$@priv.hu> <20110511093155.5b1a203e@notabene.brown> <4DC9CCAF.9010709@crc.id.au> <20110511102116.494bf0fd@notabene.brown> <20110511104730.175372fe@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Dylan Distasio Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On Tue, 10 May 2011 21:04:29 -0400 Dylan Distasio = wrote: > It's possible I did use different versions but I thought I had > upgraded both of them right before the reshapes. Sorry if this is an > elementary question, but does writing the 2nd parity block always to > the last drive instead of rotating it increase the odds of a total > loss of the array since the one specific drive always has the 2nd > parity block? No. The only possibly impact is a performance impact, and even that wo= uld be hard to quantify. The reason that parity is rotated is to avoid a 'hot disk'. Every upda= te has to write the parity block and if they are all on one disk then every wr= ite will generate a write to that one disk. Because of the way md implements RAID6, every write involves either a r= ead or a write to every device, so there is no real saving in rotating parity. I think (but am open to being corrected) that rotating parity is only important for RAID6 if the code implements 'subtraction' as well as 'addition' for the Q syndrome (which md doesn't) and if you have at lea= st 5 drives, and you probably wouldn't notice until you get to 7 or more dri= ves. =2E.. so it might make sense to make mdadm default to converting to the= -6 layout... You can request it with "--layout=3Dpreserve". >=20 > If so, do you think normalizing would be worth the risk of something > going wrong with that operation? I'm just trying to get a feel for > how much of a difference this makes. Not worth the risk. NeilBrown >=20 >=20 > > mdadm first converts the RAID5 to RAID6 in an instant atomic operat= ion which > > results in the "-6" layout. =A0It then starts a restriping process = which > > converts the layout. > > > > If you end up with a -6 layout then something when wrong starting t= he > > restriping process. > > > > Maybe you used different version of mdadm? =A0There have probably b= een bugs in > > some versions.. > > > > NeilBrown > > > > > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid"= in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- 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