Chan Chung Hang Christopher wrote:
We were talking about RAID1; RAID5/6 is a different area. Linux software RAID1 is a safeguard against disk failure; it's not designed for speed increase. There is a number of things that could be improved in Linux software RAID; read performance of RAID1 is one of them - this _is_ why some hardware RAID1 adapters indeed are faster than software. Read http://kernelnewbies.org/KernelProjects/Raid1ReadBalancing - since the 2.6.25 kernel a simple alternating read is implemented, but that does not take the access pattern into account.
I have not read that yet but that is odd since I have been blasted by others before for doubting md raid1 doing multiple disk reads.
I know current centos alternates because I had a box with bad memory corrupt a raid1 filesystem and after fixing it and fsck'ing the disk, errors would slowly re-appear as they were hit on the alternate disk that previous fsck runs had not seen.
To really speed things up, you would want to try to avoid seeks. A simple alternating pattern may let the heads be in different places on small reads, but for a large file you will end up doing the same head motions on both drives (wasting the same time) as the reads alternate.