Re: WAL prefetch

Right. My point is that while spawning bgworkers probably helps, I don't
expect it to be enough to fill the I/O queues on modern storage systems.
Even if you start say 16 prefetch bgworkers, that's not going to be
enough for large arrays or SSDs. Those typically need way more than 16
requests in the queue.

Consider for example [1] from 2014 where Merlin reported how S3500
(Intel SATA SSD) behaves with different effective_io_concurrency values:

[1]
https://www.postgresql.org/message-id/CAHyXU0yiVvfQAnR9cyH=HWh1WbLRsioe=mzRJTHwtr=2azsTdQ@mail.gmail.com

Clearly, you need to prefetch 32/64 blocks or so. Consider you may have
multiple such devices in a single RAID array, and that this device is
from 2014 (and newer flash devices likely need even deeper queues).'