server: wr_file: Request up to 8 iofd write buffers if available
Writing to disk is usually slower than handling network traffic, so it's a desirable to increase write buffers to try to submit our write queue to the kernel/VFS as fast as possible, to avoid it growing too much. This should also improve CPU use at the expense of some dozen KBs of extra use, which in the server should be plenty available.
Using up to 8 buffer sounds like a good start, being it a good compromise between memory requirements and batching of msgbs. It also turns outs it's the maximum amount of buffers allowed by osmo_io current implementation ;)
