From mboxrd@z Thu Jan 1 00:00:00 1970 From: "=?iso-8859-1?Q?Michael=20Sallaway?=" Subject: =?iso-8859-1?B?UmU6IHJhaWQ2IGFuZCBwYXJpdHkgY2FsY3VsYXRpb25z?= Date: Wed, 15 Sep 2010 15:55:24 +0000 Message-ID: <20100915155524.31380.qmail@s217.sureserver.com> Reply-To: "=?iso-8859-1?Q?Michael=20Sallaway?=" Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Sender: linux-raid-owner@vger.kernel.org To: Neil Brown Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids > -------Original Message------- > From: Neil Brown > To: Michael Sallaway > Cc: linux-raid@vger.kernel.org > Subject: Re: raid6 and parity calculations > Sent: 15 Sep '10 10:26 > It is best not to think to think to much about chunks.=A0=A0Think ab= out strips > (not stripes). > A strip is a set of blocks, one per device each at the same offset. > Think of page sizes blocks/strips. > Each strip has a P block and a Q block and a bunch of data blocks.=A0= =A0Which > is P and which is Q and which each data block is a function of the o= ffset, > the layout and the chunk size.=A0=A0Once you have used the chunksize= to perform > that calculation, don't think about chunks any more - just blocks an= d strips. > =20 Aah, perfect -- that makes sense, thanks for that. As a sort-of follow up question, would anyone know if the data size of = a Q calculation affects the result at all? eg. if I do a 64kb Q calcula= tion on 10 drives of data, would that be the same as doing 16x 4kb Q ca= lculations on sequential blocks of the same data, then concatenating it= together? (I can't remember what that operation property is called....= ?) I've been reading the maths of RAID6 PDF (http://kernel.org/pub/linux/k= ernel/people/hpa/raid6.pdf), but I'm a bit too rusty to understand Galo= is fields, and if the data size matters. I presume the data ordering is= also critical for a Q calculation, correct? (eg. drives have to be d0 = -> d10 in order, not just random). And, in contrast, for the P calculations, data size and input order mak= es no difference, correct? (since it's just a simple bitwise XOR of all= the inputs). > =20 > Ouch... good luck. Thanks! I'm the only one to blame, though -- it happened in the month b= etween "getting the new system set up" and "setting up backups for the = new system". So it's the only copy of the data.... whoops. :-) Thanks for the help/advice! Cheers, Michael -- 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