OT: Torn between SCSI and SATA for RAID

[Ian Jefferson]

Hi Chris,

I have many of the same questions.  SATA is plenty fast for home  
systems and modern drives are smoking stuff that was enterprise class  
just a few years ago.  'twas ever thus.

Cables are a nightmare IMHO.  This was by far the reason I've been a  
big fan of SCSI for a long time.  You can make a pretty effective and  
tidy Raid system by custom making a short length daisy chain scsi  
cable. I have not explored this recently but used to do this ~5+  
years ago for non-raid applications.  We used to run into device  
compatibility problems on the SCSI bus though so same drive mfg might  
be a good idea.  Perhaps things have improved.

You can buy old 80 pin 16 bit SCSI controllers quite reasonably on  
EBay.  Even though the bus speeds might be 40 or 80 MB/sec (that's  
bytes) this still exceeds what I get on single disk SATA benchmarks.   
My impression is that modern drives are backward compatible with  
older SCSI but I've not tested this extensively, just a couple of  
anecdotes.

You can do quite well in the used Enterprise market.  You might have  
a look at pricewatch.com for some low cost SCSI disks.  My experience  
has been that S/P-ATA drives seem to be easily available in large  
sizes, > 300 GB whereas SCSI seems to be available in volume only for  
smaller drives ~100-200GB.

Above is mostly supposition.

I have been experimenting with SATA to see what's possible.  There  
are gizmo's, "Backplanes", out there that make the cabling issue easier:

I have one of these:
http://www.mwave.com/mwave/viewspec.hmx?scriteria=BA20689

And I'm considering one of these:
http://www.mwave.com/mwave/viewspec.hmx?scriteria=BA20690

Similar devices are available for SCSI and PATA drives they are a  
little difficult to find.  You can google for backplane, 3X5 and 2X3  
that type of thing.

I finally got gvinum to work for me under 6.1 i386 RELEASE for Raid  
5.  The volume manager concept appeals to me because you can work  
with smaller chunks pieces of storage than whole disks. So with the  
same set of physical disks you can contemplate different RAID  
strategies depending on how much performance you want, all at the  
same time.  So far my benchmarks indicate that a 3 partition raid 5  
vinum disk performs fine for me.  Minimum write performance is around  
7MB/s and Minimum read is around 14MB/s.  Usually however writes came  
in on  the low side of 15 MB/s and reads around 50 MB/s.  This is all  
just a first attempt though without any attempt to tune the raid  
set.  With two 5X3 backplanes and software Raid 5 you could build PDQ  
a 4TB system and your drives would not have to be identical.

Even with a backplane device though you end up with quite a cable issue.

The last option I've considered is to look at some of the SATA to  
SCSI backplanes.  There are commercial solutions that allow you to  
put SATA or PATA drives up to 12 in an enclosure then connect to your  
host computer via SCSI.  I haven't found anything cheap though.   
Cheap = < 20% of the drive cost.  Apple sells such a device as do  
numerous other manufacturers.  Search for SATA Raid.

IJ


On May 11, 2006, at 7:51 PM, cknipe at savage.za.org wrote:

>
>
> My questions that I'm posting is not really related towards the  
> performance of
> the system, it's more towards the capacity of the system... I guess  
> it boils
> down to the physical hardware... How does everything connect, how  
> to expand
> systems, and how to run arrays bigger than what one single  
> controller can
> provide...
>
> --
> C