Re: Daisy Chain vs. FW 800-to-400 Cable

In article 1imv1uh.mil3dm16spsowN%dempson@xxxxxxxxxxxxx, David Empson at
dempson@xxxxxxxxxxxxx wrote on 9/6/08 10:27 AM:

Nick Naym <nicknaym@xxxxxxxxx> wrote:

In article 1imub44.1l2zi27fpfkwN%dempson@xxxxxxxxxxxxx, David Empson at
dempson@xxxxxxxxxxxxx wrote on 9/5/08 7:11 PM:

In terms of speed, it makes no difference whether the drives are daisy
chained or connected to separate Firewire ports on the computer. The
FW400 and FW800 ports in an iMac are on the same Firewire bus.

That's what I thought (though, not initially). However, when I put the
question to a LaCie tech, I was told:

(Quick summary of my detailed explanation below: the LaCie tech is
mistaken, or their Firewire devices have a really bad design.)

"The disadvantage to daisy-chain is that you are spitting up your bandwidth

Exactly the same as if you plug into separate ports on the computer, as
they are on the same Firewire bus.

Some Mac models (probably only high-end ones like a Mac Pro) might have
separate Firewire buses for different ports, but an iMac certainly
doesn't (I checked the developer note for the latest model).

You are only "splitting up your bandwidth" in the sense that if you were
trying to transfer data to/from/between both Firewire drives at the same
time, each one would only be able to use about half the Firewire
bandwidth on average.

If only one drive is active, the inactive one has no impact on Firewire
throughput.

Firewire daisy chaining also has almost no delay on data travelling
through to the daisy chained port.

This is completely different to USB, where the addition of a hub will
slow down the communication due to the hub having to buffer and
retransmit entire USB packets.

The only real disadvantage of daisy chaining Firewire is if the device
in the middle is switched off and it doesn't power its Firewire
interface hardware from the bus. In this case, switching off the "mid
chain" device will cut off the signal to the "end chain" device.

- in addition, if you happen to add a FW400 then your entire chain drops to
FW400 speeds."

I haven't seen any evidence of this, and I've just done a test with two
of my Firewire devices to disprove it.

1. An Ice Cube hard drive enclosure, which has 2xFW800 and 1xFW400
ports. It contains a 500 GB Seagate Barracuda.

2. A Hot Buttered 5.25" enclosure, which has 2xFW400 ports. It contains
a Pioneer DVD writer.

Both devices have an Oxford chipset.

My computer is a MacBook Pro (August 2007 model), which has FW800 and
FW400 ports which are on the same bus (same architecture as an iMac).

My test consisted of copying a large file from the hard drive (using the
'dd' command line tool and directing output to /dev/null, so it isn't
influenced by the speed of the computer's internal drive). dd reports
the transfer rate, which is a handy way of doing an accurate
measurement.

If I connect the hard drive to a FW800 port it reads at 60 MB/s.

If I connect the hard drive to a FW400 port it reads at 40 MB/s.

With the hard drive on the FW800 port, if I also connect the DVD writer
at the same time, it has no effect on the speed of file transfers from
the hard drive, whether I plug it into the FW400 port or daisy chain it
from the hard drive.

The only situation where the hard drive speed is affected is if I
connect the DVD writer (FW400) to the computer and daisy chain the hard
drive from the DVD writer. This limits the speed of the hard drive to
FW400.

Summary: mixing FW400 and FW800 devices only limits the speed of the
FW800 devices if the data has to travel through a device or cable which
uses FW400, at least for well designed Firewire devices.

In your situation: if you connect the LaCie first (via FW800), and the
Icy Dock second (via FW400 using a FW400 to FW800 cable), your LaCie
should operate at FW800 speeds, but your Icy Dock will operate at FW400
speeds.

If you connect the Icy Dock first (via FW800) and the LaCie second (also
via FW800) then both devices should operate at FW800 speeds.

You can check in System Profiler under the Firewire section to see the
speed for each device. If the device supports FW800 then the "Maximum
Speed" should be 800 Mb/sec, but if the connection arrangement is
limiting its speed, then the "Current Speed" will be 400 Mb/sec.

Does the Icy Dock have two FW800 ports?

Yes it does.

If so, you might be able to reverse the order of the drives: iMac to Icy
Dock (FW800), then Icy Dock to LaCie (FW800). That would allow best
possible speed to both drives.

Now I'm again confused: If daisy chaining doesn't introduce any speed
penalty, why should the connection order make a difference?

Because if you put the LaCie first, then you have to use its FW400 port
to connect the Icy Dock. This limits the Icy Dock to FW400 speed.

If you put the Icy Dock first, you can connect everything with FW800
cables and both drives should run at FW800 speeds.

(I'm assuming the LaCie doesn't have a bad design such that it is always
operating at FW400 because one of its ports is FW400, or it slows down
to FW400 if anything is connected to the FW400 port.)


There must¹ve been some sort of miscommunication between the tech-support
guy and me. He is someone I¹ve dealt with before, and certainly appears to
know his stuff (he¹s not the kind of guy who likes to hear himself talk, nor
to offer incorrect advice when, in fact, he doesn¹t know the answer).

My original question was:


³... is there any disadvantage to daisy-chaining the Quadra with another, FW
800-only external HD?²


To which he responded:


³The disadvantage to daisy-chain is that you are spitting up your bandwidth
­ in addition, if you happen to add a FW400 then your entire chain drops to
FW400 speeds.²


Your more-detailed explanation makes a lot of sense. It certainly clarifies
what he probably meant re: splitting the bandwidth. But the part of his
statement regarding the slowdown to FW 400 speeds still puzzles me,
notwithstanding your very clear explanation (further verified by your actual
testing). The reason I¹m still puzzled is that his claim is (seemingly)
consistent with a LaCie ³white paper² I found last night, entitled ³FireWire
800 ­ Technology Brief²
(www.lacie.com/download/more/WhitePaper_FireWire_800.pdf), which includes
(on the second page) the following paragraph:


³Legacy and Beta Devices Working Together

The new standard was designed to be backwards compatible, meaning that
FireWire 800 devices will still operate via the original FireWire 400 port.
To connect a FireWire 800 device to a FireWire 400 port, a specific adapter
cable must be used. There are two types of FireWire 400 ports: 6-pin and
4-pin. For FireWire 800 devices to work, they must be connected by placing
the 9-pin end of the FireWire cable into the FireWire 800 port of the
device, and the opposite 6-pin or 4-pin end into the FireWire 400 port.

The same holds true for FireWire 400 devices being connected to a FireWire
800 host port. The 4-pin or 6-pin end of the FireWire cable must be
connected to the FireWire 400 port of the device, and the 9-pin end must be
connected to the FireWire 800 port.

When FireWire 400 and FireWire 800 devices are mixed, all transfer rates
revert to the original FireWire 400 speed.²


This last sentence is the source of my remaining ³puzzlement,² as it does
not temper the claim with any qualifiers regarding the location of the FW
800 devices along the chain.














--
iMac (24", 2.8 GHz Intel Core 2 Duo, 2GB RAM, 320 GB HDD) ? OS X (10.5.4)

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