another newbie question

[Tony Hammitt]

This is getting kind of silly.  You can't buy an RS/6000 server
(or whatever they are calling them now) with just one PCI bus.
You can get 32 independant PCI buses on an IBM S80.  It is quite
possible to double the throughput of an RS/6000 just by
rearranging the disks, I've done it myself.  I'm sure that Sun,
SGI, HP and Compaq have the same thing.

My $180 PC motherboard has two PCI buses.  Any new mid-range
server has a better motherboard than that, I'd hope.  As I
mentioned, you can get a 4-bus 8-way Xeon box pretty cheaply
(compared to a commercial Unix box) nowadays.  Penguin
Computing sells these, so does VALinux.

The state of the art has moved on from just one bus per computer.
It did so a long time ago.

There is no point in discussing this further.

Regards,

Tony Hammitt

Jeffrey Watts wrote:
> 
> On Fri, 20 Oct 2000, Tony Hammitt wrote:
> 
> > You have to put up with the fact that your Ultra160 card has to hook
> > up to the computer through one bus.  PCI32 (at 33MHz) is limited to
> > 132 MB/s roughly divided by the number of cards on that bus.  Even if
> > you use PCI64 at 66MHz, it's still slower than the bandwidth that the
> > processors are capable of doing.
> 
> This was what bugged me about your math last time.  Regardless of which
> processor generates the parity stripe (main CPU or the RAID controller
> processor), the data STILL has to be sent through the PCI bus to get to
> the disk.  Doesn't matter if the parity checksum is generated by a slow
> 100MHz processor on the RAID card, or an ultra-fast GHz PIII, the PCI
> bandwidth to the disks is the same.  I'm willing to bet that most RAID
> cards can generate a parity stripe faster than the PCI bus can get data to
> them (probably by an order of magnitude).
> 
> The onboard IDE and SCSI controllers prevalent in the industry today all
> hang off of the PCI controller.  Same bandwidth as the RAID card.  In
> fact, I'd argue that the RAID card would do writes FASTER, since in most
> situations it will be able to do DMA access to get the data, and leave the
> processor alone.  There is no way that a software solution can somehow use
> less CPU time or get more PCI bandwidth, which is the bottleneck as you've
> identified.  Your point is that the CPU has much higher bandwidth to the
> memory than the PCI bus does, but what your argument fails do address is
> that the CPU must read all of the data from main memory, compute the
> parity checksum, then WRITE all of it to the PCI bus.  No way that is
> faster than simply writing it to the bus.
> 
> Some more comments below:
> 
> >  128MB of RAM is considerably less than the 2GB of main memory.
> 
> Considering the size of disk caches, and the danger of huge disk buffers,
> 128MB is more than adequate for mid-range applications.  I think my
> $600,000 CLARiiON FibreChannel RAIDs (which have GOBS more bandwidth) have
> 2GB of cache.  But they have built-in UPSes and dual power feeds.  A $2000
> server doesn't need that much.
> 
> > The operating system doesn't have to have the data transferred into it
> > before it can do the checksum calculations, and it can use more than
> > one bus to transfer the data to the disks.
> 
> No, one bus.  There is a single PCI controller on most motherboards.
> Both IDE controllers on the typical board hang off of it.  Most IDE RAID
> implementations have multiple controllers as well, that hang off the same
> controller.  No difference.
> 
> Jeffrey.
> 
> o-----------------------------------o
> | Jeffrey Watts                     |
> | watts at jayhawks.net         o-----------------------------------------o
> | Systems Programmer         | "If this thread is annoying, please     |
> | Network Systems Management |  imagine what it is like to see an      |
> | Sprint Communications      |  idealistic project stymied and made    |
> o----------------------------|  ineffective, because people don't      |
>                              |  usually give it the credit for what it |
>                              |  has done.  If you're an idealist like  |
>                              |  me, that can ruin your whole decade."  |
>                              |  -- Richard Stallman                    |
>                              |  Regarding the GNU/Linux debate         |
>                              o-----------------------------------------o

-- 
Doubt is not a pleasant condition, but certainty is absurd. -- Voltaire