Re: GUC-ify walsender MAX_SEND_SIZE constant

Hi,

On 2024-01-19 23:04:50 +0330, Majid Garoosi wrote:
> Following is the description which is also written in the commit message:
> MAX_SEND_SIZE parameter was used in WALSender to limit maximum size of
> a WAL data packet sent to a WALReceiver during replication. Although
> its default value (128kB) was a reasonable value, it was written in
> 2010. Since the hardwares have changed a lot since then, a PostgreSQL
> user might need to customize this value.
> For example, if a database's disk has a high bandwidth and a high
> latency at the same time, it makes more sense to read bigger chunks of
> data from disk in each iteration. One example of such disks is a remote
> disk. (e.g. an RBD volume)

The way we read the WAL data is perfectly prefetchable by the the OS, so I
wouldn't really expect gains here.  Have you actually been able to see a
performance benefit by increasing MAX_SEND_SIZE?

I don't think we should add configuration parameters without at least some
demonstration of practical gains, otherwise we'll end up with hundreds of
never-useful config options.

Greetings,

Andres Freund