I see the patch also does throttling by calling vacuum_delay_point().
Being able to throttle the checksum workers not to affect user activity
definitely seems like a useful feature, no complaints here.

But perhaps binding it to vacuum_cost_limit/vacuum_cost_delay is not the
best idea? I mean, enabling checksums seems rather unrelated to vacuum,
so it seems a bit strange to configure it by vacuum_* options.

Also, how am I supposed to set the cost limit? Perhaps I'm missing
something, but the vacuum_delay_point call happens in the bgworker, so
setting the cost limit before running pg_enable_data_checksums() will
get there, right? I really don't want to be setting it in the config
file, because then it will suddenly affect all user VACUUM commands.

And if this patch gets improved to use multiple parallel workers, we'll
need a global limit (something like we have for autovacuum workers).

In any case, I suggest mentioning this in the docs.


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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