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Dirty Secrets of Thunderbolt Daisy Chaining: Never Reliable, Probably Never Will Be, Kernel Panics with Sustained I/O eg Backups

re: Apple Core Rot
re: Understanding Thunderbolt 3 Bandwidth

Terminology: Thunderbolt 4 is a renaming of USB-C + Thunderbolt 3 along with some minor specification changes. As used here, Thunderbolt 3 means any/all of these.

Thunderbolt 3 daisy chaining is a much hyped feature which allows one Thunderbolt device to be attached to the previous one, up to 6 devices per Thunderbolt bus.

Not new: this is a long-running issue (many years), but I am writing it up once again, as a reader recently wrote about similar issues. You are not alone!

For two days, I was trying to make several backups at once on my 2019 Mac Pro, to various multi-bay enclosures. After a dozen or so kernel panics involving an “unresponsive CPU” or some such, I was feeling rather frustrated.

I could not get my #$#*$$* backups done, due to these repeated kernel panics. Worse, the reboot cycle is quite long, and checking for file system damage another chore.

Maybe you have only 2 or 3 devices, if so count your blessings. I have six multi-bay hard drive enclosures and two Thunderbolt SSDs and more. It is a solid argument for one larger enclosure like the OWC Thunderbay 8 instead of two OWC Thunderbay 4 units, other considerations aside. Ditto for a large internal SSD—anything to reduce the daisy chaining reliability mess and data/power cable rat’s nest.

Yes, you can daisy chain six devices on one bus and it will work for some period of time. Great science fair demo. But 10 or 38 or 72 or 127 minutes into that backup, the system will kernel panic. It is guaranteed in my experience. The higher the device count and the greater the I/O throughput, the quicker the crash will come. Perhaps I should say on Intel Macs—I am unsure if M1/M2 Macs have similarly screwed Thunderbolt—likely so given the same specs for devices/cables.

Quick tests won’t show the problem; it’s about sustained high-throughput I/O eg when doing hundreds of gigabytes or more of backup—precisely when you need to start things going and go do something else with your time.

Thunderbolt daisy chaining beyond 2 devices is unreliable garbage, has been for years, goes unacknowledged by Apple and Intel (which also legally silence licensees on the matter), and will probably never be fixed. Reliability will vary, depending on device count and throughput across the bus.

With rare exceptions, it’s not your device, it’s not your computer, it’s not your cables. That falsifiable bullshit claim can be proven by swapping components and seeing the same problems. Do not accept that explanation from tech support without exceptional evidence.

Working around unreliable Thunderbolt Daisy Chaining

To work around the issue, a machine with a bare minimum of 3 Thunderbolt busses is needed. Otherwise fantastic machines like the 2019 iMac 5K and all its siblings suck because they have a single Thunderbolt bus.

Minimize device count on each bus

Minimize the number of devices on each bus, especially actively used devices eg devices doing the I/O (reading or writing). Take into account bandwidth losses from displays, or that fast SSD or RAID box might run sluggishly with writes.

For example, if you have 6 Thunderbolt devices, put 2 of them on each bus, and when making backups, try to have only one device on each bus actively doing stuff.

Minimize simultaneous I/O

I ended up having to restrict my backups to one source and two destinations, all three on separate Thunderbolt busses. I could only do so by temporarily disconnecting other things. Once I did this, no more kernel panics. But it took me 24 hours to get it all done, rather than ~6 hours that it could have taken if things could have run simultaneously.

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