Re: Controlling Load Distributed Checkpoints

On Thu, 7 Jun 2007, Heikki Linnakangas wrote:

> So there's two extreme ways you can use LDC:
> 1. Finish the checkpoint as soon as possible, without disturbing other
> activity too much
> 2. Disturb other activity as little as possible, as long as the
> checkpoint finishes in a reasonable time.
> Are both interesting use cases, or is it enough to cater for just one of
> them? I think 2 is easier to tune.

The motivation for the (1) case is that you've got a system that's
dirtying the buffer cache very fast in normal use, where even the
background writer is hard pressed to keep the buffer pool clean. The
checkpoint is the most powerful and efficient way to clean up many dirty
buffers out of such a buffer cache in a short period of time so that
you're back to having room to work in again. In that situation, since
there are many buffers to write out, you'll also be suffering greatly from
fsync pauses. Being able to synchronize writes a little better with the
underlying OS to smooth those out is a huge help.

I'm completely biased because of the workloads I've been dealing with
recently, but I consider (2) so much easier to tune for that it's barely
worth worrying about. If your system is so underloaded that you can let
the checkpoints take their own sweet time, I'd ask if you have enough
going on that you're suffering very much from checkpoint performance
issues anyway. I'm used to being in a situation where if you don't push
out checkpoint data as fast as physically possible, you end up fighting
with the client backends for write bandwidth once the LRU point moves past
where the checkpoint has written out to already. I'm not sure how much
always running the LRU background writer will improve that situation.

> On a Linux system, one way to model it is that the OS flushes dirty buffers
> to disk at the same rate as we write them, but delayed by
> dirty_expire_centisecs. That should hold if the writes are spread out enough.

If they're really spread out, sure. There is congestion avoidance code
inside the Linux kernel that makes dirty_expire_centisecs not quite work
the way it is described under load. All you can say in the general case
is that when dirty_expire_centisecs has passed, the kernel badly wants to
write the buffers out as quickly as possible; that could still be many
seconds after the expiration time on a busy system, or on one with slow
I/O.

On every system I've ever played with Postgres write performance on, I
discovered that the memory-based parameters like dirty_background_ratio
were really driving write behavior, and I almost ignore the expire timeout
now. Plotting the "Dirty:" value in /proc/meminfo as you're running tests
is extremely informative for figuring out what Linux is really doing
underneath the database writes.

The influence of the congestion code is why I made the comment about
watching how long writes are taking to gauge how fast you can dump data
onto the disks. When you're suffering from one of the congestion
mechanisms, the initial writes start blocking, even before the fsync.
That behavior is almost undocumented outside of the relevant kernel source
code.

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