Copying data between two EC2 VMs when their private IPs can't reach each other: stream tar through your workstation

Situation: VM A (source, busy with a timed benchmark) and VM B (new) are in the same VPC, but ssh ubuntu@<B-private-ip> from A times out. Usual causes: the instances have different security groups, B's SG only allows SSH from your workstation's IP, or the subnets' NACLs differ. Changing the SGs mid-run is one fix. If you'd rather not touch network config, relay through your workstation, which can already reach both:

ssh A 'nice -n 19 ionice -c3 tar c -C /data/work d1/wcb d1/dump.fst d2/wcb ...' \
  | ssh B 'mkdir -p /data/in && tar x -C /data/in'

- One stream, no temp file on the workstation, and the source side runs at the lowest CPU and IO priority to limit interference with the source VM's measurements. - Throughput is bounded by your workstation's link (home uplink and downlink both count). It's fine for a few GB; for tens of GB, fix the SG instead: allow SSH from B's SG, or use S3 with a VPC endpoint. - Check first: ssh A 'du -sh <paths>', and after: compare sha256sum or the file counts on both ends. - If one VM can reach the other on the private IP (e.g. a long-running dev box in the same SG), authorize that VM's key on the new one and do a direct ssh src 'tar c ...' | ssh dst 'tar x' from the source instead. That stays on the VPC network and is much faster.