Re: Review for GetWALAvailability()

On 2020-Jun-16, Kyotaro Horiguchi wrote:

> I saw that the "reserved" is the state where slots are working to
> retain segments, and "normal" is the state to indicate that "WAL
> segments are within max_wal_size", which is orthogonal to the notion
> of "reserved".  So it seems to me useless when the retained WAL
> segments cannot exceeds max_wal_size.
> 
> With longer description they would be:
> 
> "reserved under max_wal_size"
> "reserved over max_wal_size"
> "lost some segements"

> Come to think of that, I realized that my trouble was just the
> wording.  Are the following wordings make sense to you?
> 
> "reserved"  - retained within max_wal_size
> "extended"  - retained over max_wal_size
> "lost"      - lost some segments

So let's add Unreserved to denote the state that it's over the slot size
but no segments have been removed yet:

* Reserved    under max_wal_size
* Extended    past max_wal_size, but still within wal_keep_segments or
          maximum slot size.
* Unreserved    Past wal_keep_segments and the maximum slot size, but
        not yet removed.  Recoverable condition.
* Lost        lost segments.  Unrecoverable condition.


It seems better to me to save the invalidation LSN in the persistent
data rather than the in-memory data that's lost on restart.  As is, we
would lose the status in a restart, which doesn't seem good to me.  It's
just eight extra bytes to write ... should be pretty much free.

This version I propose is based on the one you posted earlier today and
is what I propose for commit.

Thanks!

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
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