> I do not intend to undertake raw disk tablespaces for 7.5. I'd be
> interested if anyone could provide some real world benchmarking of file
> system vs. raw disk. Postgres benefits a lot from kernel file system cache
> at the moment.
Yes, and don't forget that pg also relys on the OS for grouping and
sorting the physical writes and doing readahead where appropriate.
The use of raw disks is usually paired with the use of kernel aio.
The difference is said to be up to 30% on Solaris. I can assert, that
it made the difference between a bogged down system and a much better behaved
DB on Sun here.
My experience with kaio on AIX Informix is, that kaio is faster as long as IO
is not the bottleneck (disk 100% busy is the metric to watch, not Mb/s), while
for an IO bound system the Informix builtin IO threads that can be used instead
win. (Since they obviously do better at grouping, sorting and readahead
than the AIX kernel does for kaio)
Overall I think the price and komplexity is too high, especially since there are
enough platforms where the kernel does a pretty good job at grouping, sorting and
readahead. Additionally the kernel takes non PostgreSQL IO into account.
Andreas