Allow WAL inter-flush delay to be configurable

[otoolep]

This increases the inter-flush delay to a default of 2 seconds, and also makes it configurable.

#3215

[pauldix]

need comments for each of the following lines describing what they are and what the value is

[otoolep]

Will cherry-pick to 0.9.1 after merge to master.

[dgnorton]

+1

[toddboom]

+1

[pauldix]

+1

[ecables]

Is it possible to also configure the WAL flush interval timer? What if a WAL flush cannot complete in <10 minutes? Can it default to 10 minutes, and be configurable to accommodate less capable hardware, without falling behind?

[pauldix]

@ecables: It's configurable: https://github.com/influxdb/influxdb/blob/master/etc/config.sample.toml#L40

The 10 minutes is more about making sure that shards that are cold for writes eventually get flushed. It's also about making sure that if you have series that have a 10s sampling interval, enough points are grouped into each series to make the flush efficient.

It doesn't improve performance at all if you're writing to many series and you flush the wall so frequently that when it happens you only have a single point per series.

[ecables]

Perfect; thanks for the explanation.