Temporary loss of network connections during backup

Great, that eliminates Ethernet limited to 100 M bits/sec as the issue.

To see if an Ethernet link is throwing more than a handful of initial errors, using the en number you derived, you can use Terminal command:

netstat -I en2

This is the resulting output. Counters are In-packets, In-errors, Out-packets, Out-Errors, Collisions. There should never be more than handful of errors from starting up, and in most cases, NONE.

Name       Mtu   Network       Address            Ipkts Ierrs    Opkts Oerrs  Coll

en2   8163  <Link#4>    00:01:d2:1a:00:dd   696697     0   484301     0     0

en2   8163  grantsmacpr fe80:4::461:ea0d:   696697     -   484301     -     -

en2   8163  192.168.0/23  192.168.0.204     696697     -   484301     -     -

If the link were running beyond its ability to run and be stable, for example it auto-speeded to 10Gb but the cabling could only reliably support 2.5Gb, we would see non-zero errors counts, and errors increasing over time. (and possibly, disconnecting)

when you run into problems like this, you should completely remove (not just disable) any User-installed VPN software.

User-added VPN software is known to meddle in MANY things, far beyond its stated scope. In addition, it may be sending your local traffic out to the Internet and back, and that can't possibly be as fast a local traffic.

can you find a spare large, external drive? it does not have to be fast.

add it an additional LOCAL Time machine drive. Time machine will create a new stand-alone backup set on the newly attached drive, starting with today. After that, every-other backup goes to every-other drive.

Then you will have the setup to compare local vs NAS backups, to see what that relationship might be. If the symptoms change, that might be the clue we have been looking for.

lets try something completely different to continue to make progress:

Download and run this little "discovery" Utility, EtreCheck, developed by collecting LOTS of standard System Calls available inside macOS to create a report of what is happening inside. the report is pre-laundered fo any personally-identifiable information, so can be freely posted back to the forums for readers to analyze. the app does not install ANYTHING, and you are free to Trash it the moment you done with it, leaving NOTHING behind.

Using EtreCheck to Troubleshoot Potential… - Apple Community

.

Having a VPN active can direct ALL network traffic out to the remote node, rather than allowing local transfer to your NAS.

By far the easiest way to cause poor performance, instability, overheating and crashing is to install ANY third-party speeder-uppers, Cleaners, Optimizers, or Virus scanners. or a VPN that you installed yourself.

The idea that a third party, with no special knowledge of the inner workings of MacOS, can somehow find a simple way to protect your computer — that is not already being done by MacOS itself — suggests that the MacOS developers are somehow "holding out on you". That is absurd.

You should remove any and all (other than Apple built-in) virus scanners, speeder uppers, optimizers, cleaners, App deleters or VPN packages you installed yourself, or anything of that ilk.

Third-party file Sync-ers such as DropBox, BackBlaze, OneDrive, or GoogleDrive can ruin performance, but are not inherently dangerous.

Are you still using an Ethernet connection to your Router? What speed is the link ACTUALLY running at?

Actual Speed:

The good way to check the actual connection speed USED to be Network Utility, But in Catalina and later, Apple has deprecated network Utility and now you have to use a Terminal command to see your actual connection speed. First, you need to know what en number the link is. then you use a command like this one, substituting the actual en number.

my main Ethernet connection uses BSD name en2 (as shown in) :

 menu > about this Mac > (system report) > network:

Aquantia AQC107-B0:

Name: ethernet

Type: Ethernet Controller

Bus: PCI

Slot: Slot-3

Vendor ID: 0x1d6a

Device ID: 0x87b1

Subsystem Vendor ID: 0x1d6a

Subsystem ID: 0x0001

Revision ID: 0x0002

Link Width: x4

BSD name: en2

Kext name: AppleEthernetAquantiaAqtion.kext

Location: /System/Library/Extensions/IONetworkingFamily.kext/Contents/PlugIns/AppleEthernetAquantiaAqtion.kext

Version: 1.0.64

Terminal command:

ifconfig en2 | grep media

with this as my output for 10 Gigabit Ethernet:

	media: 10Gbase-T <full-duplex,flow-control>	

For ‘regular’ Gigabit Ethernet, you should get this instead:

    media: 1000baseT <full-duplex,flow-control>

In my opinion, the last steps of your backup ACROSS YOUR NETWORK to your NAS are overwhelming your network connection, especially if your only network connection is over Wi-Fi.

If you hold down the Option key, you get a snapshot of your Wi-Fi operating conditions, like this older one. Transcribe or screenshot your results for assistance:

toward the end of the backup, two special things might be happening:

one is deciding what backup dates are expendable, and eliminating the ones that are no longer needed. This is not just a matter of deleting them, because they may contain unique files that are referenced by other backup dates.

another is checking/re-computation of links between files referenced by multiple backup dates. This could involve reading many links and making sure all pointers are accurate.

it seems to me that if space on the backup drive is getting tight, there may be a lot of computation going on, so a lot of I/O to get the links to be manipulated.

Ideal backup drive size is said to be more than 2.5 times larger, for long-term trouble-free operation.

¿How does the backup drive size compare with the amount of "stuff' to be backed up?

While there have been a few reports of ENTIRE home networks being overcome with traffic from one computer, I think such instances are likely rare.

from your description, it still sounds like that same computer's connection to things around it on your home network are being swamped by that data during the late stages of backup.

Tell me about all the internet connections to your Mac, and in particular, which connection is TOPMOST in

System Preferences > network

.

great, that eliminates broken wires and under-performing cables as a possible source of problems.

Have you adjusted the MTU/ block size on the Gigabit Ethernet connection? is it still the default 1500?

You might get more speed by increasing the block size modestly, provided your connected equipment, especially the central Switching device (like your Router in this case) can handle the larger size packets. To go near 8K byte packet size, you will need to increase an internal buffer size in the Mac using a Terminal command.

I use an Ethernet Switch device, rather than the Router, as the center of my Ethernet network. The switch allows larger packet sizes when transferring locally, but my Router does not support larger than 1500 byte packets, so it is on a 'spur' instead of in the middle of every local and remote transfer.

'More than one' makes it miraculous that your computer operates at all. I recommend you completely remove them all while debugging, and consider never re-installing.

User-installed VPN are both wildly over-sold as security software (which is not needed) and they tend to actually accomplish the reverse, making your Mac slow, unstable, and far less secure. Some packages get inspired to break into your secure connections and insert themselves so they can read your data FIRST. Most are completely incompetent at that, and compromise your security in the process. They can cause slowdowns and crashing.

because the Mac buffer size is limited, you may need to make this adjustment to run around 8K frames or higher:

sudo sysctl -w net.inet.raw.maxdgram=16384

I think the default is 8192, and up to twenty plus 8 bytes of overhead may limit the actual size used to around 8164 (without changing it).

great discussion about large frames with Mac and other equipment here:

Mac Pro NIC not close to 1 Gbps connectio… - Apple Community

.