Lennart Sorensen wrote:
Linux software raid (at least for raid 0 and 1 and combinations) is often faster than what a hardware raid card can do, and almost certainly better than what any fakeraid pulls off (since their drivers are often crap at doing the raid in software). raid 5 on the other hand can often be done faster by a dedicated xor engine, and since you don't have to do as much reading to write you reduce bus bandwidth. Probably the main advantage of hardware raid is reduced us usage since only the actual desired data goes over the bus, while with software raid, raid 1 involves twice as much bus traffic to write to both devices.
I guess the reason the YouTube guys used hardware RAID 1 and then put software RAID 0 on top of that is that this means only sending the data once, but with the kernel doing the striping you get improved parallelization by seeing multiple SCSI devices rather than just one.
Certainly many people run 3ware cards as just large driver controllers and use linux software raid on them to get the best performance.
One thing has occurred to me: The Adaptec 2015S is a zero-channel SCSI RAID controller, which (I think) means it has no SCSI controller onboard but rather uses the one on the motherboard. Apparently this saves space:
http://www.avadirect.com/product_details_parts.asp?PRID=1740So... now I'm wondering if I could actually just remove the Adaptec altogether and simply attach the SCSI drives directly to the motherboard? Has anybody done that? Is it possible, or even a good idea? I'm only thinking about this because maybe it would enable cutting one more possible point of failure out of the equation, plus the fact that the dpt_i2o drivers on AMD64 are still a pain in the ass. Of course I wouldn't have hot-swap capability any more (I think?) because that's one of the things that the Adaptec gives you... but, honestly, I really have little knowledge of this stuff. I am pretty much poking around in the dark with a flashlight here. Anybody got any real experience, please let me know...
If you have a fast cpu and you aren't using it for anything else (like is often the case on a file server) and if you don't cause your pci bus to be saturated, then software raid will probably be faster than hardware raid in general.
I am liking the idea of software RAID. But the niggling thing in the back of my mind is that for RAID 1, it might be better to use the Adaptec, since it means only sending the data once. However... if the Adaptec is using the motherboard's SCSI controller, then surely it too must be sending the data twice over the SCSI bus. It's just doing it for us, rather than the kernel doing it. I mean, all the four drives are still on the SCSI, and if the Adaptec doesn't have its own SCSI controller, then it must just be a middleman doing some additional work that could just as well be done by the kernel. Or is that too naive?
Thanks again, /Neil