mark delfman wrote:
Thank you Bill... maybe a silly question but doesn’t the fact we can achieve 1.6GBsec on MD RAID0 mean we have drive bandwidth available to MD to use? To the block device (sas drives in this case) it does not know if the data is been xor’d or just striped, its just blocks of data. So wouldn’t this mean that we know that MD RAID6 ‘could’ achieve up to 1.6GB if the software could send it out quicker? Hence my logic of ‘deal with MD software aspects faster = faster writes’
Are your tests and assumptions valid... remember that raid6 will put 1.6GB of data on the drives for each 800MB of data from the application in minimum 4 stripe mode (2 data stripes, 1 P stripe, 1 Q stripe). So the proper comparison of raid6 software overhead is to run the same application writing a fixed large amount of data to 4 drives at raid0 and the same 4 drives at raid6. overhead = raid0_time * 2 - raid6_time; Of course if you add more data stripes, the ratio of user data to disk data gets better than 1/2. jim -- 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
