* Jeff Janes:
> Does the kernel really read a data block from disk into memory in
> order to immediately overwrite it? I would have thought it would
> optimize that away, at least if the writes are sized and aligned to
> 512 or 1024 bytes blocks (which WAL should be).
With Linux, you'd have to use O_DIRECT to get that effect (but don't
do that), otherwise writes happen in page size granularity, writing in
512 or 1024 byte blocks should really trigger a read-modify-write
cycle.