Hello, 2009/2/28 H. Peter Anvin <hpa@xxxxxxxxx>: > Peter Rabbitson wrote: >> Hi, >> >> I am experimenting with raid6 on 4 drives on 2.6.27.11. The problem I am >> having is that no matter what chunk size I use, the write benchmark >> always comes out at single drive speed, although I should be seeing >> double drive speed (read speed is at near 4x as expected). > > I have no idea why you "should" be seeing double drive speed. All > drives have to be written, so you'd logically see single drive speed. I'm afraid that might be incorrect. Let's assume we want to write 100MB of data onto a 4 drive raid6. Let's divide 100MB of data into two parts, say A and B, each 50MB big. Writing the data on the raid, would mean writing: * A on disk1 * B on disk2 * XOR(A,B) on disk3 * Q(A,B) on disk4 That is actually assuming 50MB chunk, and whole chunk writes, etc. Each of written portions would have been 50MB in size. That sounds reasonable to me, as with 2 data disks, only half of data has to be written on each. The fact that disks are really striped with data, XOR and Q doesn't change the image in terms of amount written. I do hope I had understood the situation correctly, but I'll be ever happy to be proved wrong. Kind regards, Mike -- 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
