FireWire bus bandwidth for multiple chained drives

[fluxtransistor]

I'm acquiring a 2012 Mac Mini and a bunch of LaCie 2big RAID enclosures, with the intention of making a FreeBSD/ZFS file server.
Some of these enclosures would connect over FireWire 800, but the Mac Mini only has one port. They would be daisy-chained using FW800 cables.

In a situation where I'm, say, writing to two RAID enclosures at once, what would the transfer rate be? Is it still 800Mb/s (100MB/s), or perhaps half of that (as the bandwidth is shared)?

 

[joevt]

I don't think anyone would make a bus that halves the max transfer rate every time you double the number of connected devices. That would be ridiculous. If a drive is not reading or writing, then the other drive can use all the bandwidth.

However, if you write to multiple disks on the same bus at the same time then of course they need to share the total bandwidth.

FireWire 800 is superior to FireWire 400 not only because it is faster, but also because it is full duplex instead of half duplex, which means it should be able to transmit and receive at the same time, so you could be writing 800 Mb/s to one drive while reading 800 Mb/s from another.
https://en.wikipedia.org/wiki/IEEE_1394
I've never tested that.

The 2012 Mac mini is an Intel Mac. All Intel Macs use PCIe. PCIe is full duplex. However, the FireWire controller might be a PCI device behind a PCIe-PCI bridge, and PCI is half duplex. I would check the device tree in IORegistryExplorer.app or the ioreg command or use pcitree.sh with pciutils to verify what kind of FireWire controller is being used and to see what bridges are between it and the CPU.

I would use ATTODiskBenchmark.app to test multiple disks in macOS since it is the only benchmark I know of that can test multiple disks at once without making them a RAID.

 

[fluxtransistor]

Thanks for the write up! What you describe makes a lot of sense. I will test things out and let you know.