How to configure number of threads writting to Disk

[rbotzer]

It’s not configurable, but you can partition drives to increase the number of devices (each partition is a device in Linux). There will be a write-q and associated write thread for each device.

Horizontal scaling (v3.16.0.6)

No problem. You can scale your writes by adding drives to the node (vertically), or by adding nodes with the same number of drives (horizontally). There’s also the impact of partitioning. To be super clear, there’s a write queue per-device, and in Linux a device is a partition, not a drive. To mitigate temporary peaks the streaming write buffers go into the write queue, whose size is controlled by max-write-cache. Partitioning the drive and assigning those devices gives you more of that cache …

Just be aware that it’s unlikely your problem is write threads, but rather the overall performance of your drive.