Re: BBU Cache vs. spindles

Tom Lane wrote:
> Uh, no, it is not.  The difference is that we can update a byte in a
> shared buffer, and know that it *isn't* getting written out before we
>
Well, I don't know where yu got the idea I was refering to that sort of
thing - its
the same as writing to a buffer before copying to the mmap'd area.
> It's true that we don't know whether write() causes an immediate or
> delayed disk write, but we generally don't care that much.  What we do
>
Which is what I was refering to.
> care about is being able to ensure that a WAL write happens before the
> data write, and with mmap we don't have control over that.
>
>
I think you have just the same control either way, because you can only
force ordering
with an appropriate explicit sync, and in the absence of such a sync all
bets are off for
whether/when each disk page is written out, and if you can't ensure that
the controller
and disk are write through you'd better do a hardware cache flush.too,
right?

A shame that so many systems have relatively poor handling of that
hardware flush.