Best practices for SSD + RAID?

Hello

What should the parity size be when running 2x Samsung 850 Pro SSDs connected to an H710P in a PE R720xd in a RAID 1 array?

RAID 1 is not a parity array type and you do not set a parity size. I suspect that you are talking about the stripe size.

The stripe size does not make a big difference on a RAID 1. The stripe size is most important when you are spreading the data across multiple disks. The size of the stripe can have an impact on the performance of the array by spreading data across the drives or limiting the data access to a single drive. By changing the stripe size you can make your arrays seek time or sequential access times improve. Smaller stripe sizes are typically better for sequential reads/writes as they typically spread data across more drives, and larger stripe segment sizes are typically better for fast access times as the data you are accessing is typically on a single drive.

In short, unless you are going to hire a consultant to analyze your data usage to recommend settings then I would leave it all on default. Changing the stripe segment size from the default would likely only be useful in a database environment with consistent file size usage and a RAID level spread across multiple HDDs.

Thanks