Re: WAL Rate Limiting

Heikki Linnakangas <hlinnakangas(at)vmware(dot)com> writes:
> On 01/16/2014 05:39 PM, Andres Freund wrote:
>> Do you see a reasonable way to implement this generically for all
>> commands? I don't.

> In suitable safe places, check if you've written too much WAL, and sleep
> if so. Call it CHECK_FOR_WAL_BUDGET(), along the lines of
> CHECK_FOR_INTERRUPTS(), but called less frequently. Or maybe
> vacuum_delay_point() is a better analogy. Hopefully you don't need to
> sprinkle them in too many places to be useful.

We probably don't really need to implement it for "all" commands; I think
everyone can agree that, say, ALTER TABLE RENAME COLUMN isn't going to
emit enough WAL to really matter. My point was that we should try to hit
everything that potentially *could* generate lots of WAL, rather than
arbitrarily deciding that some are utility commands and some are not.

For INSERT/UPDATE/DELETE, likely you could do this with a single
CHECK_FOR_WAL_BUDGET() added at a strategic spot in nodeModifyTable.c.

regards, tom lane