On 17/04/13 09:56, Mikael Abrahamsson wrote: > On Tue, 16 Apr 2013, David Brown wrote: > >> you are going to have terrible RMW performance for small writes. >> However, as > > As I said, I don't have problem with lower performance. My workload is > write once and few, read many. If the performance is approximately the > approximately the same as a 10 drive RAID-6, but with double the > storage, I'm fine. I would expect read performance for triple-parity raid to be similar to Raid5 or Raid6 - i.e., you get good striped performance, especially for large files as they are spread over many spindles. Of course, since triple-parity md raid does not yet exist, that's just theoretical... > >> I am not sure there is much real-world need of triple parity raid for >> normal arrays - even with better cpu scaling, it would still be a lot >> slower than two raid6 arrays LVM'ed together. I foresee it's main use >> as a temporary measure during array maintenance. For example, if you >> have a raid6 and you want to swap out the drives for bigger ones, then >> you could temporarily add an extra drive for a third parity using a >> non-symmetrical layout. Once this extra drive is synced, then you can >> step through the other drives doing a replace-and-resync, knowing that >> you still have the double parity safety. Then at the end of the >> process you drop the third parity again. > > Well, I run RAID6+spare. I'd rather run a triple parity drive unless the > write performance penalty is huge. > It's encouraging to hear people are interested in this. But before it can be implemented, there has to be someone with an understanding of Linux md raid who can implement it. I know the maths involved, but I have no experience with Linux kernel work (I work with embedded systems - while I use the same programming language as the kernel, it's a very different style of programming). -- 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
