Re: WAL compression setting after PostgreSQL LZ4 default change

Hi Michael


>
> > If I were switch the default, for me zstd goes first, lz4 is a close
> > second, and pglz is the last of its class.  The only reason why we
> > have not chosen zstd for TOAST is the fact that we don't support it
> > (trickier to add support for it as we have to preserve on-disk
> > compatibility for inline compressed TOAST entries).
>
> > In short, lz4 is available in many environments, but I'm also ready to
> > bet that zstd is equally available in these environments.
> Thanks for the feedback.
>
> Yes, I did consider zstd as well. I agree that zstd can provide a better
> compression ratio than lz4, and if we only look at compression ratio, zstd
> is
> a very attractive choice.
>
> The reason I started with lz4 was mainly to keep this change aligned with
> the
> recent TOAST compression change. Since lz4 has already been accepted there
> as
> the preferred default when available, I thought using the same choice for
> wal_compression=on would make the proposal smaller, more consistent, and
> easier
> to evaluate. so
>
> Second, WAL compression happens on the WAL insertion path for full-page
> images,
> so compression speed and CPU overhead matter a lot. lz4 is generally a
> good fit
> for that trade-off: it still reduces WAL volume, while keeping compression
> latency and CPU cost lower than zstd in many workloads.
>
> That said, I understand the argument for preferring zstd when it is
> available.
> I have prepared an alternative version where wal_compression=on selects
> zstd
> first, then lz4 if zstd is not available, and finally pglz as the fallback
>

Thanks