Re: [HACKERS] "distributed checkpoint"

On Fri, 7 Dec 2007, Simon Riggs wrote:

> "Smoothed" makes a lot of sense for me. We used to have a checkpoint
> spike, now we don't.

To be accurate, there used to be a huge and unavoidable spike, now there's
a control that aims to make it smaller. The problem hasn't completely
gone away yet.

With that in mind, let me start over. Here's what's in the release notes
right now:

"Distributed checkpoints prevent I/O spikes during checkpoints"

It's good this came up, because that is factually wrong; while the average
case is much better some OS-dependant aspects of the spike (what happens
at fsync) are certainly still there. I think it's easier to rewrite this
whole thing so it's technically accurate rather than a simple fix of the
wording, something like this:

"Checkpoint writes can be spread over a longer time period to smooth the
I/O spike during each checkpoint"

It's got spread, it's got smooth, and if I could have worked "silky" in
there too I would have.

--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD