Holy Canoli You were right.

Checked the Synology’s network interface page and it was negotiating at 100 Mbps to my Orbi router. Your comment about 11.4 MB/s lining up with a 100 Mbps ceiling is exactly what got me to actually check the link speed instead of just assuming gigabit.

Bought a cheap unmanaged switch, plugged the NAS and server into it directly, came up at 1000 Mbps, and now I’m getting 107-115 MB/s on the same tests. Updated the post and credited you. Appreciate you pushing back on it.

You could 1000% percent be right here. I don’t have an education on networking or system ops.

The NAS can sustain gigabit writes without complaint. My premis is the kernel isn’t flushing at wire speed in a steady stream. It hoards 2+ GB in page cache then fires a burst of concurrent NFS RPCs all at once. The NAS can’t ack them fast enough and the client declares it dead.

I ran three dd tests, same 2GB file, same NAS, only changed client config:

• 128K buffers: 101 MB/s, dies at 2GB
• 32K buffers: 2.1 GB/s (all page cache, nothing hitting the wire), dies even harder
• 64MB dirty page cap: steady 11.4 MB/s, zero timeouts, clean finish

The NAS handled all 2GB in test 3 because it arrived as a drip instead of a wall.

At least thats the working theory :)