Re: BBU Cache vs. spindles

Greg Smith wrote:
> Tom Lane wrote:
> > You've got entirely too simplistic a view of what the "delta" might be,
> > I fear.  In particular there are various sorts of changes that involve
> > inserting the data carried in the WAL record and shifting pre-existing
> > data around to make room, or removing an item and moving remaining data
> > around.  If you try to replay that type of action against a torn page,
> > you'll get corrupted results.
> >
>
> I wasn't sure exactly how those were encoded, thanks for the
> clarification.  Given that, it seems to me there are only two situations
> where full_page_writes is safe to turn off:
>
> 1) The operating system block size is exactly the same database block
> size, and all writes are guaranteed to be atomic to that block size.

Is that true?  I have no idea.  I thought everything was done at the
512-byte block level.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +