> -------Original Message------- > From: Neil Brown <neilb@xxxxxxx> > To: Michael Sallaway <michael@xxxxxxxxxxxx> > Cc: linux-raid@xxxxxxxxxxxxxxx > Subject: Re: raid6 and parity calculations > Sent: 15 Sep '10 10:26 > It is best not to think to think to much about chunks. Think about 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. Which > is P and which is Q and which each data block is a function of the offset, > the layout and the chunk size. Once you have used the chunksize to perform > that calculation, don't think about chunks any more - just blocks and strips. > 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 calculation on 10 drives of data, would that be the same as doing 16x 4kb Q calculations 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/kernel/people/hpa/raid6.pdf), but I'm a bit too rusty to understand Galois 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 makes no difference, correct? (since it's just a simple bitwise XOR of all the inputs). > > Ouch... good luck. Thanks! I'm the only one to blame, though -- it happened in the month between "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" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
