c8d759c claimed the ~125 Mbit/s seed rate was ks4's OVH uplink. That was
wrong, and measuring it says so: ks4 uploads at 609 Mbit/s, downloads at
900, and nas pulls 670 from OVH's network. WireGuard is not it either —
zero UdpRcvbufErrors, wg-crypt kworkers at ~2%, both hosts ~85% idle.
The limit is `data` sitting on sdb5, one 7200 rpm HGST 6 TB disk: during
the send it does 109 r/s at 14 MB/s with ~131 KB requests and a queue
depth of ~1.0, which is the random-IOPS ceiling of a single HDD reading a
fragmented 1.42 TiB dataset. 14 MB/s is ~112 Mbit/s on the wire, exactly
what we see, while the network sits idle.
Parallelism is the only lever and it works by disk queue depth, not
bandwidth: a second concurrent copy takes sdb from 14 to 21 MB/s and the
request size from 131 KB to 514 KB, lifting the tunnel from 113 to 153
Mbit/s. Recorded, along with the decision to keep the seed sequential —
one-off pass, incremental refreshes after, and incus-copy.sh is shared by
all three legs.
Also corrects the set size to the actual 1.42 TiB (~29 h, not ~31 h).
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Julien LutranandClaude Opus 5
ceff4ec0d0