Can the number of parity shards more than the number of data shards? about reedsolomon

[darrenldl]

Why "the number of parity shards to be below the number of data shards."?

The recommendation was likely made in relation to performance, as the time required grows quadratically with the number of parity shards used. And most people only want a percentage of overhead that is less than 100% if it's for error recovery purposes.

Can the number of parity shards more than the number of data shards?

The only restriction is that the data + parity shard count must not exceed 256 in total, as reedsolomon uses Galois field (2^8).

I see sia.tech can split data in to 30 segments, and recover from any 10 of 30 segments , can reedsolomon do this ?

You can emulate the behaviour of the Sia software by picking data shard count of 10 and parity shard count of 20, and distribute each shard to a unique host. Picking parity shard count of 20 means you can lose up to 20 shards of the 30 shards, which is same as being able to recover from any 10 of the shards.

Note that Sia could be using a more complex scheme to achieve the same behaviour, but the above arrangement is the most straightforward way of doing so.