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.
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