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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