On 8/1/23 22:42, Grant Taylor wrote:
On 8/1/23 7:20 PM, David Crayford wrote:
What’s the difference between between channelized I/O and a rack of
x86 servers connected to a SAN using fibre channel driven by high
speed HBAs?
I don't know.
My understanding is that Fibre Channel is an evolution of SCSI which
is supposedly a somewhat intelligent controller wherein the OS asks
said controller to fetch / store some data for it. As I understand
it, the OS & main CPU aren't involved in the transfer beyond asking
the controller to do the transfer on it's behalf.
SCSI originally had much more limited scale ... by design. The acronym
expands to "small computer system interface".
I haven't read-up on the details of FCP, but I do suspect it follows
SCSI yet with dramatically relaxed limits. Operationally, FCP appears to
be a lot like FICON, and that's channelz.
I'd have to reference documentation to see if / how much Direct Memory
Access comes into play vs the CPU's involvement in the transfer to /
from the controller.
DMA is significant.
True: PCs have had DMA in corner cases for a long time.
DMA is part of the equation.
But between the controller and the back end drive, as I understand it,
the CPU ins't involved.
Right.
A channel processor for an IBM-class "mainframe" operates independently
of the CPU(s) other than being triggered when a CPU says "go run this
channel program" and effectively "don't bother me until you're done".
So I can't say that "a rack of x86 servers connected to a SAN using
fibre channel" isn't using channelized I/O. I think in many ways they
are.
This is a place where minutia matters.
If the CPUs are truly free to continue their own work until SAN fibre
channel independently completes its work, that sounds like a mainframe
class channel.
Grnat. . . .
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These things can be hard to pin down. Labeling is sometimes
counter-productive.
In the automotive industry, is the vehicle a sedan or a van or a
mini-van or an SUV or a truck?
So in the auto industry, we hear "cross over". [sigh]
I think Jon Perryman first asked us to define mainframe. And I bit!
[voice of Leonard Bones McCoy] "Dammit Jon! I'm a software developer,
not a field service engineer!"
But it really started with Andrew Hudson at Ars Technica getting a
number of facts wrong.
-- R; <><
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