I am sure the S3510s are not the issue, I have the same thing happening on my hybrid cluster that uses WD RE4s as capacity drives.
You mention you are on VSAN 6.1, the issue I'm describing occurs only with VSAN 6.2 (probably related to the checksum feature). In VSAN 6.1 or VSAN 6.0 I didn't experience any issues with P3700/P3600 on any cluster.
