RAID controller question...

[Joel Jaeggli]

On Wed, 9 Sep 1998, Mark Hahn wrote:

> > Thanks for the lead...  Insight had the DPT PM3334UDW for $1275 which
> > puts it in the same price range as the ICP GDT6518RD.  As for Mylex,
> 
> phew!  I'm not trying to belittle those of you who believe in RAID
> hardware, but these prices are enough to stop and make you think.
> $1275 is 50G of fast, cool UDMA disk.  yes, such disks will only sustain
> 12-14 MB/s apiece, but I've heard people report good results using the MD 
> driver under Linux.

Up to a point. You don't increase your perfromance when you have more than
one ide disk per channel. since only one can have the bus at a time and
the controler can't have more than one operation pending a time. scsi
controllers can regularly have as many as 24 pending read write operations
per channel.
 
> remember, the touchstone of Beowulf is off-the-shelf, commodity hardware.
> $1275 might not blow the budget of a large cluster project, but it's 
> worth thinking about whether you'd be better off just using comodity
> hardware and mirroring, etc.

Compared to dec storage-works arrays this stuff is pretty commidity. Just
about every decent (and I use that term loosely) netware server you'll
ever see will have a raid controller of some variety. given that you can
compare controllers from 1/2 dozen vendors in a reasonably
straight-forward fashion it's pretty common hardware.

> (and yes, before you ask, I _have_ measured 12-14 MB/s, sustained,
> Ext2 with 4K blocks, on run-of-the-mill Triton/Promise/ViaMVP3-type
> UDMA controllers.  depending on the processor, the bandwidth incurrs
> 7-20% CPU overhead, most probably due to the filesystem, not the disk
> interface.)

I've measured about 11, but the difference isn't worth arguing about...
;) That being as it may, a 4 channel raid controller has a lot going for
it that aren't necessarily built into the os, such as hot plug disk
replacement, automatic rebuilding of mirrors or raid 5 stripe sets, and
more sophisticated diagnostics/error reporting (if the controller has a
serial port).

What's really impressive about all this in terms of commidity computers is
that you could put a  .5 terabyte raid0 (mirrored) array in one pc(56 18gb
disks) instead of having to buy an alpha-server 1000/2000 and a gigantic
storage-works array or similar piece of hardware (name your vendor) to
acomplish the same thing.

> regards, mark hahn.

joelja 

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Joel Jaeggli				       joelja@darkwing.uoregon.edu    
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