On Thu, 26 Jun 2008, Peter T. Breuer wrote:
> Double buffering is a killer.
No, it isn't; it's a completely trivial bit of overhead. It only exists
during the time when blocks are queued to write but haven't been written
yet. On any database system, in those cases I/O congestion at the disk
level (probably things backed up behind seeks) is going to block writes
way before the memory used or the bit of CPU time making the extra copy
becomes a factor on anything but minimal platforms.
You seem to know quite a bit about the RAID implementation, but you are a)
extrapolating from that knowledge into areas of database performance you
need to spend some more time researching first and b) extrapolating based
on results from trivial hardware, relative to what the average person on
this list is running a database server on in 2008. The weakest platform I
deploy PostgreSQL on and consider relevant today has two cores and 2GB of
RAM, for a single-user development system that only has to handle a small
amount of data relative to what the real servers handle. If you note the
kind of hardware people ask about here that's pretty typical.
You have some theories here, Merlin and I have positions that come from
running benchmarks, and watching theories suffer a brutal smack-down from
the real world is one of those things that happens every day. There is
absolutely some overhead from paths through the Linux software RAID that
consume resources. But you can't even measure that in database-oriented
comparisions against hardware setups that don't use those resources, which
means that for practical purposes the overhead doesn't exist in this
context.
--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD