On Wednesday April 26, jlewis@xxxxxxxxx wrote: > > I suspect I should have just kept out of this, and waited for someone like > Neil to answer authoratatively. > > So...Neil, what's the right answer to Tuomas's 2 disk RAID5 question? :) > .. and a deep resounding voice from on-high spoke and in it's infinite wisdom it said.... "yeh, whatever" The data layout on a 2disk raid5 and a 2 disk raid1 is identical (if you ignore chunksize issues (raid1 doesn't need one) and the superblock (which isn't part of the data)). Each drive contains identical data(*). Write throughput to a the r5 would be a bit slower because data is always copied in memory first, then written. Read through put would be largely the same if the r5 chunk size was fairly large, but much poorer for r5 if the chunksize was small. Converting a raid1 to a raid5 while offline would be quite straight forward except for the chunksize issue. If the r1 wasn't a multiple of the chunksize you chose for r5, then you would lose the last fraction of a chunk. So if you are planning to do this, set the size of your r1 to something that is nice and round (e.g. a multiple of 128k). Converting a raid1 to a raid5 while online is something I have been thinking about, but it is not likely to happen any time soon. I think that answers all the issues. NeilBrown (*) The term 'mirror' for raid1 has always bothered me because a mirror presents a reflected image, while raid1 copies the data without any transformation. With a 2drive raid5, one drive gets the original data, and the other drive gets the data after it has been 'reflected' through an XOR operation, so maybe a 2drive raid5 is really a 'mirrored' pair.... Except that the data is still the same as XOR with 0 produces no change. So, if we made a tiny change to raid5 and got the xor operation to start with 0xff in every byte, then the XOR would "reflect" each byte in a reasonable meaningful way, and we might actually get a "mirrored pair"!!! But I don't think that would provide any real value :-) - 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
