>But its not clear from the mobo docs, or from just staring at the mobo
>or from adaptecs web site, whether the U2/LVD connector on the mobo
>really represents its own "chain" or just a point to branch a U2/LVD
>cable from the one single chain thats connecting everything.
>
>Adaptec do mention a second chip which is specifically for providing
>a non U2/LVD bus in conjunction with the 7890, but I have no idea if
>this mandatory if you want to put, say, a standard 50 pin wide scsi
YM narrow. Wide is 68 pins.
>connector onto the mobo, or if its a special feature that my mobo does
>not have.
The second chip is not mandatory for doing this. The chip's function is
unrelated to bus width.
LVD stands for "Low Voltage Differential". Differential signaling means
that a bit is represented by two wires; transmission of data is done by
forcing the wires to have a voltage differential (they are normally
pulled to roughly the same voltage by the bus termination if nothing is
driving them). The "Low Voltage" part refers to the fact that the newer
LVD stuff uses a very small voltage swing, something like 1V, while the
older plain "Differential SCSI" uses a much higher voltage swing.
SE or "Single Ended" SCSI signaling uses 1 wire and a ground reference
per bit. The wire is pulled to ground to signal '0' and to 5V to signal
'1'.
The advantage of differential is that it has superior noise immunity and
is better suited to long transmission lines than single-ended. This
allows cable length to go up dramatically. SE Ultra1 is limited to
something like 1.5m total cable length with 3 or more devices, while LVD
Ultra2 is about 12m (if I recall correctly).
The chip Adaptec is talking about (their "SpeedFlex" chip) is
essentially a buffer chip. If SE devices are connected to a LVD bus,
the SCSI spec requires the whole LVD bus to fall back to SE mode for
backwards compatability. This also implies a fallback to Ultra1 (20 MHz
clock), since Ultra2 (40 MHz clock) is only allowed in conjunction with
LVD.
Adaptec's chip is designed to sit between LVD and SE devices; it
translates between the two signalling types while keeping the two bus
segments electrically isolated. This allows SE devices to be connected
to a LVD controller without forcing the whole bus to drop down to SE
mode.
>Supermicro make another board that is specifically
>advertised as having 2 SCSI busses.
That sounds like 2 SCSI controllers. With the buffering chip, you still
have one bus, just cut into two isolated segments.
You should check your motherboard to see if it has Adaptec's buffering
chip. If it doesn't have one, you should try to force aic7xxx to use
only Ultra1, since the higher clock rate is probably not going to be
reliable.
>understood. its pretty dismal. some people have claimed to have gotten
>twice this performance from udma drives that cost probably 50% less. phooey.
Sustained transfer rate is like many other peak rates: it doesn't mean
as much as people think it does. Random access performance is usually
more important. And right now, the way the HD market works is that SCSI
drives are optimized for random access performance, while IDE drives are
optimized for space per dollar (which has the side effect of tending to
increase transfer rate). In fact, most 10K RPM SCSI disks sacrifice
some transfer rate to get better random access; they have smaller
diameter platters, which eliminates the part of the disk where you get
the best transfer rate, but also reduces seek times.
That said, SCSI drives of a given generation are usually faster in both
areas than IDE drives of the same generation. You have a
second-generation Cheetah model; Seagate is beginning to sell the
fourth-generation version as we speak, and it has been benchmarked at
around 35 MB/s sustained in its best zone.
It depends on what you're doing, but there is a very good chance you
would not see better real world performance if you substituted a 30 MB/s
IDE drive for the Cheetah.
Also note that the Quantum Viking II isn't a 10K RPM drive as stated in
your first mail; it's a 7200 RPM drive, and a fairly old one at that.
Quantum's only 10K drives are the Atlas 10K and the Atlas 10K II (the
latter is just arriving on the market). Quantum took quite a while to
introduce a 10K drive.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.rutgers.edu
Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Fri Apr 07 2000 - 21:00:16 EST