Re: Spread checkpoint sync

On 01.12.2010 23:30, Greg Smith wrote:
> Heikki Linnakangas wrote:
>> Do you have any idea how to autotune the delay between fsyncs?
>
> I'm thinking to start by counting the number of relations that need them
> at the beginning of the checkpoint. Then use the same basic math that
> drives the spread writes, where you assess whether you're on schedule or
> not based on segment/time progress relative to how many have been sync'd
> out of that total. At a high level I think that idea translates over
> almost directly into the existing write spread code. Was hoping for a
> sanity check from you in particular about whether that seems reasonable
> or not before diving into the coding.

Sounds reasonable to me. fsync()s are a lot less uniform than write()s, 
though. If you fsync() a file with one dirty page in it, it's going to 
return very quickly, but a 1GB file will take a while. That could be 
problematic if you have a thousand small files and a couple of big ones, 
as you would want to reserve more time for the big ones. I'm not sure 
what to do about it, maybe it's not a problem in practice.

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