Re: Improving replay of XLOG_BTREE_VACUUM records

On 05/02/2015 02:10 AM, Jim Nasby wrote:
> This looks like a good way to address this until the more significant
> work can be done.
>
> I'm not a fan of "RBM_ZERO_NO_BM_VALID"; how about RBM_ZERO_BM_INVALID?
> or BM_NOT_VALID? Or maybe I'm just trying to impose too much English on
> the code; I see the logic to NO_BM_VALID...
>
> + * RBM_ZERO_NO_BM_VALID is the same as RBM_ZERO_AND_LOCK, but does not set
> + * BM_VALID bit before returning buffer so that noone could pin it.
>
> It would be better to explain why we want that mode. How about:
>
> RBM_ZERO_NO_BM_VALID is the same as RBM_ZERO_AND_LOCK but does not set
> BM_VALID before returning the buffer. This is done to ensure that no one
> can pin the buffer without actually reading the buffer contents in. This
> is necessary while replying XLOG_BTREE_VACUUM records in hot standby.

That's a rather strange mode. The BM_VALID flag is internal to the
buffer manager - if you don't know how the buffer manager works, you
cannot understand that description. I couldn't quite understand what
that means myself. What can or can you not do with a buffer that's not
marked as BM_VALID? I'd suggest a mode like this instead:

In RBM_IF_IN_CACHE mode, the page is returned if it is in the buffer
cache already. If it's not, it is not read from disk, and InvalidBuffer
is returned instead.
- Heikki