Re: Controlling Load Distributed Checkpoints

Tom Lane wrote:
> Heikki Linnakangas <heikki@enterprisedb.com> writes:
>> Tom Lane wrote:
>>> I don't think it's a historical artifact at all: it's a valid reflection
>>> of the fact that we don't know enough about disk layout to do low-level
>>> I/O scheduling.  Issuing more fsyncs than necessary will do little
>>> except guarantee a less-than-optimal scheduling of the writes.
> 
>> I'm not proposing to issue any more fsyncs. I'm proposing to change the 
>> ordering so that instead of first writing all dirty buffers and then 
>> fsyncing all files, we'd write all buffers belonging to a file, fsync 
>> that file only, then write all buffers belonging to next file, fsync, 
>> and so forth.
> 
> But that means that the I/O to different files cannot be overlapped by
> the kernel, even if it would be more efficient to do so.

True. On the other hand, if we issue writes in essentially random order, 
we might fill the kernel buffers with random blocks and the kernel needs 
to flush them to disk as almost random I/O. If we did the writes in 
groups, the kernel has better chance at coalescing them.

I tend to agree that if the goal is to finish the checkpoint as quickly 
as possible, the current approach is better. In the context of load 
distributed checkpoints, however, it's unlikely the kernel can do any 
significant overlapping since we're trickling the writes anyway.

Do we need both strategies?

I'm starting to feel we should give up on smoothing the fsyncs and 
distribute the writes only, for 8.3. As we get more experience with that 
and it's shortcomings, we can enhance our checkpoints further in 8.4.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com