Re: Software vs Hardware RAID 10?
On Sun, Aug 26, 2007 at 09:46:26AM -0500, Neil Gunton wrote:
> 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.
>
> 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
Yeah I am not quite sure how the 'zero channel' controllers communicate
with the actual scsi controller. Perhaps it actually ends up having to
do all the pci bus transfers that software raid does, in which case I
hardly see any benefit. Real raid cards have the drives connected
directly so that you know they won't be wasting anyone elses bandwidth
to do their work.
> 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...
SCSI has no purpose to me anymore. SATA or SAS is the only type of
drive I will consider for use. SAS is really quite impresive, although
I tend to just use SATA for what I do. I don't run servers in my
current job (someone elses job, and they do use SAS drives).
> 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?
Don't forget the scsi bus is a serious bottle neck. If all your drives
at 320MB/s scsi drives, then you have a total of 320MB/s available for
all transfers on that bus. That's about the same as a single SAS or
SATA drive has for itself. Parallel scsi is seriously outdated and
fortunately going away quickly now that SAS is available.
--
Len Sorensen
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