Re: Out of space situation and WAL log pre-allocation (was Tablespaces)

"Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:
> Tom Lane wrote:
>> Right. This is in fact not a fatal situation, as long as you don't
>> run out of preallocated WAL space.

> Clearly running out of pre-allocated WAL space is likely to be the next
> issue. Running out of space in the first place is likely to be because
> of an intense workload, which is exactly the thing which also makes you
> run out of pre-allocated WAL space. Does that make sense?

I think one of the first things people would do with tablespaces is
stick the data files onto a separate partition from the WAL and clog
files. (Actually you can do this today with a simple symlink hack, but
tablespaces will make it easier and clearer.) The space usage for WAL
is really pretty predictable, because of the checkpoint-at-least-
every-N-segments setting. clog is not exactly a space hog either.
Once you have that separation established, out-of-disk-space can kill
individual transactions but never the database as a whole.

One of the things that bothers me about the present PITR design is that
it presumes that individual WAL log segments can be kept until the
external archiver process feels like writing them somewhere. If there's
no guarantee that that happens within X amount of time, then you can't
bound the amount of space needed on the WAL drive, and so you are back
facing the possibility of an out-of-WAL-space panic. I suspect that we
cannot really do anything about that, but it's annoying. Any bright
ideas out there?

regards, tom lane