Re: BBU Cache vs. spindles

Tom Lane wrote:
> The other and probably worse problem is that there's no application
> control over how soon changes to mmap'd pages get to disk. An msync
> will flush them out, but the kernel is free to write dirty pages sooner.
> So if they're depending for consistency on writes not happening until
> msync, it's broken by design. (This is one of the big reasons we don't
> use mmap'd space for Postgres disk buffers.)
>
Well, I agree that it sucks for the reason you give - but you use write
and that's *exactly* the
same in terms of when it gets written, as when you update a byte on an
mmap'd page.

And you're quite happy to use write.

The only difference is that its a lot more explicit where the point of
'maybe its written and maybe
it isn't' occurs.

There need be no real difference in the architecture for one over the
other: there does seem to be
evidence that write and read can have better forward-read and
write-behind behaviour, because
read/write does allow you to initiate an IO with a hint to a size that
exceeds a hardware page.

And yes, after getting into the details while starting to port TC to
Windows, I decided to bin
it. Especially handy that SQLite3 has WAL now. (And one last dig - TC
didn't even
have a checksum that would let you tell when it had been broken: but it
might all be fixed now
of course, I don't have time to check.)

James