Re: finding changed blocks using WAL scanning

On Mon, Apr 22, 2019 at 7:04 PM Tomas Vondra
<tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> Some time ago there was a discussion about prefetching blocks during
> recovery on a standby, and that's a great example of a use case that
> benefit from this - look which blocks where modified in the next chunk
> of WAL, prefetch them. But that requires fairly detailed information
> about which blocks were modified in the next few megabytes of WAL.
>
> So just doing it once per checkpoint (or even anything above a single
> WAL segment) and removing all the detailed LSN location makes it useless
> for this use case.

For this particular use case, wouldn't you want to read the WAL itself
and use that to issue prefetch requests? Because if you use the
.modblock files, the data file blocks will end up in memory but the
WAL blocks won't, and you'll still be waiting for I/O.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company