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=1740
So... 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
--
To UNSUBSCRIBE, email to [EMAIL PROTECTED]
with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]