Chain or not?

I understand that chaining many Drobos will cause instability, connection, and speed problems. I don’t use Firewire a lot, so I’m curious whether or not this is a per-port problem or if it’s per bus? That is, would there be any difference between four Drobos chained together versus two Drobos chained and connected to one port and the other two Drobos chained and connected to a second port, but both of those ports using the same hub? (I ask, because I believe the Mac Pro 3.1 only uses one Firewire800 bus).

Additionally, what are the performance hits or improvements one might expect with two Drobos chained together versus two Drobos plugged directly into the computer via Firewire800?

I would surmise that copying between two chained Drobos would be much faster and copying to the Drobos from the computer or the other way around would be the same and that performing activities from the computer to both Drobos simultaneously would be poor (I/O contention).

Using two ports on the same Firewire bus, would the Drobo<–>Drobo or Drobo<–>Computer performance be any different in any circumstance?

I’ve only been able to do limited testing as I’m stuck in the middle of a significant data transfer that is going to take some time. I’ve researched what I could on this subject, with no conclusive answers so far.

Any insight is surely appreciated.


I have not yet done the test myself, but I am very interested by the answer since I will have several TBytes to transfer soon from one Drobo to an other.
Basically, FW800 is a daisy chained bus, so the bandwidth used by one connected appliance can and will slow down the other appliances transfers.
But considering the Drobo transfer rates (around 30MBytes/s best case) and the FW800 theoretical limit (98MBytes/s), the slow down should not be more than 20-30% with only 2 Drobos connected.

Beware however not to chain any FW400 appliance on a FW800 chain, even if they remain idle, since it would divide by 2 the available bandwidth.
And having 2 FW800 connectors on a host does not guarantee 2 maximum speed independent transfers, since it could very well be that those 2 connectors share a common FW800 chip and driver.

Obviously, this slow down can be made much worse by sub-optimal FW800 HW/SW implementation, both on the Drobo and PC/Mac side…

i dont see why it should cause instability or connection problems?

and copying between two drobos will not be fast - the data doesnt go direct from one to the other, it will go to your computer, then back out to the second drobo

Also older Macs and Mac Minis cannot support multiple chained devices, specifically the Drobos. It actually does cause connectivity issues or performance issues.

Also some FW buses may be FW800 but when multiple devices are connected to the same bus, its shares the speeds between the 2 devices so the 2 FW800 devices are not getting true FW800 speeds.

Here is our KB on daisy chaining drobos: http://support.datarobotics.com/app/answers/detail/a_id/201

In our office, we daisy chained 3 Drobos off our mac-mini and the drobo’s started to go into standby or reboot randomly.

Like Jennifer said, Firewire bandwidth is shared by all devices (same like SCSI). Thus if you have 4 DroboPros on the same Firewire 800 bus, they share the same 800 Mbps of bandwidth. Two might get 400 Mbps while the other get get 0 Mbps, all four might get 100 Mbps each, or likely it will be some mix based on access. No matter what, the total bandwidth usage of all devices will never exceed 800 Mbps.

Also, the Firewire bus will reduce its speed to the highest speed of the slowest device. If you connect an S100 device to an S800 bus, the entire bus will run at S100.