USB 3 and a 3 line aggregated links?

Hello All!

If I install three lines for each LAG, between an iMac, switches, and another iMac; will I then be able to achieve close to true USB 3.0 speeds, when transferring files among these networked machines?

In other words, will transfer speeds through the network roughly equal what I can achieve when moving files between an iMac and an external drive that’s connected to the iMac via a USB 3.0 connection?


Bill Taylor

How close are the iMacs? You can directly connect two Macs with a Thunderbolt cable, configure them for networking and transfer data over them. Networks always have extra overhead so the speed won’t be as fast as if you put one Mac in Target Disk Mode and mounted it like a drive but the network transfers can be faster than USB 3.0’s speed (5Gb/s).

USB is not bidirectional so you can’t directly connect two Macs to do networking. USB needs Ethernet adapters for networking, if you had three of the cheap ones, they’re only 1Gb/s each. There are standards for 2.5Gb/s and 5Gb/s Ethernet but I don’t know if anyone makes USB adapters for Macs that operate at those speeds. Even if there are, you’d be better off getting a single 10Gb/s Ethernet adapter for each Mac.

Optical Thunderbolt cables are expensive but so are 10Gb adapters and switches (you wouldn’t need a network switch to connect just two Macs, they can be connected directly); if you’re only ever going to need a high-speed connection between two Macs, an optical thunderbolt cable could be a better investment than Ethernet adapters and cabling (and possibly a switch).

Anyway, in practice Link Aggregation does not provide bandwidth as if the three connections were really one. If you’re trying to move one big file, the computers are not going to load balance the network packets across all three connections, they’ll all travel over one. If you 're moving a folder of files, each file might go over a separate connection so you could have three files transferring in parallel.

Do you actually have an SSD attached to that USB3 so that you can truly exploit those 5 Gbps? Regular unraided HDDs do not exceed Gigabit by much, let alone faster Ethernet standards.

This is critical.

Link aggregation software never randomly distributes packets across the links because that tends to kill overall throughput. It results in some amount of out-of-order packet delivery, which the TCP protocol interprets as packet loss/congestion, and responds by slowing down the data transfer.

So there will be algorithms to ensure that the packets for a single stream (typically TCP, but could be other protocols as well) will all use a single link.

Good aggregation software will generate a hash of various connection-defining parameters (source address, destination address, protocol, port numbers, maybe other values) so that when there are multiple streams, they will be distributed across all your links, but no single stream will ever use more than one link.

If Apple uses multiple TCP (or other protocol) connections when transferring multiple files, then they might be able to take advantage of the aggregation. If, however, it uses only one connection for all the files, then the link aggregation system will only use one link in the LAG.

To speed up a single connection, you will need faster links. You can look into getting 10G interfaces and switches (and cables that can handle the bandwidth), but that may cost more than you want to spend.

Depending on the quantity of data involved and the number of times you need to perform a transfer like this, Sneakernet might be more appropriate. Connect a Thunderbolt SSD to one computer, copy all your files to it, then move the SSD to the other computer and copy them off.

I’m not joking about this either. Sometimes physical delivery is the most efficient. Which is why off-site backup companies usually provide a way to mail hard drives (at least for the initial backup and for whole-network restores).

At the extreme end, Amazon AWS has their Snowball and Snowmobile products, where they mail you a server (or deliver a 45’ shipping container full of servers), to which you transfer all your data and then send back. On receipt, they merge the data with your cloud storage. Far far faster and more economical than the fastest Internet connections, if you have that much data to transfer.

As the old joke says: “Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway.”


I believe Plugable makes a very reasonably priced 2.5Gb Ethernet-USB 3 network adaptor. Whether you add a 2.5Gb switch or directly connect the machines this is likely to be the most cost efficient means of speeding bandwidth between machines. It uses cat5e cabling, so it won’t require any new cables if you upgrade your switch. And some of the Multigig switches have a range of port speeds so you can get one with some 1Gb and a few higher speed ports (I’ve used Netgear switches with no issue).