Why is write speed worse when IPV6 is activated?

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Why is write speed worse when IPV6 is activated?

DS620slim, DS415+
Operating system
  1. Linux
  2. macOS
  3. Windows
Mobile operating system
  1. Android
  2. iOS
I made some tests on my DS620slim (all SSD), as I was not 100% happy with the difference between Read speed (115MB/s=ok), and Write speed (78MB/s=low).
Both speeds from windows 10, with samsung M2 SSD, CAT6 cables, Netgear GS109E switches (2x) between pc and nas. reading and writing a file 400MB, and 4000MB.
Checked on second pc (laptop, one switch): similar speed difference. Used two different speed test tools.

Resource monitoring does not show many clues, CPU, memory etc <25% used, nas is actually doing almost nothing. IO speed is given similar to the data from the nas speed test, although there are spikes in the write speeds, not in the read, that one is more flat.

I switched off IPV6 in the NAS network settings and write speed went up to 88MB/s. Still not 100% happy, but pretty acceptable.
Rest of te settings, I think are standard, SMB3, MTU 1500, nothing out of the standard, except some firewall rules.

Also changed: disabled IPV6 on W10 pc NIC, and on the Router, but this did not change the write speed in either way.
So it seems to be the NAS that struggles with IPV6, and only on the write, read is fine.

Any thoughts?
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Yep, in network settings, disabled in tab IPv6 and that is what I did.
IPv4 was active as well, and still is.
My provider offers IPv6, it is activated by default on the router, but did not use it anyway on the LAN.

Actually, I made some more test-combinations: If either end of the IPv6 is switched off, write speed is higher. So leaving Synology IPv6 activated and disable IPv6 on the W10 machine NIC, write speed is also higher.
So I am not anymore sure it is the Synology, W10 or both :-( will come back after test on the macbook, first have to find a proper test tool for mac.
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Ok, My macbook reads and writes >105MB/s, regardless of IPv6 activated or not.
So definitely this is a W10 network issue/driver issue for the realtec onboard GB nic.
Let's close the topic, let you know if I find a solution.
-- post merged: --

The Realtek ethernet NIC driver that is standard in W10 was clearly not Ok.
After a quick Realtek update to latest driver:

Running a 400MB file write on H: twice...
Iteration 1: 113,78 MB/sec
Iteration 2: 118,30 MB/sec
Average (W): 116,04 MB/sec
Running a 400MB file read on H: twice...
Iteration 1: 108,17 MB/sec

Pfff, from just below 80MB/sec to 116, quite an improvement. :), proves that relying on W10 stock drivers is not always ok...

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