Re: Re: Faster CREATE DATABASE by delaying fsync (was 8.4.1 ubuntu karmic slow createdb)

On Sun, Feb 14, 2010 at 10:31 AM, Greg Stark <gsstark(at)mit(dot)edu> wrote:
> On Sun, Feb 14, 2010 at 2:03 PM, Greg Stark <gsstark(at)mit(dot)edu> wrote:
>> On Fri, Feb 12, 2010 at 3:49 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>> Greg Stark, have you managed to get your access issues sorted out?  If
>>
>> Yep, will look at this today.
>
> So I think we have a bigger problem than just copydir.c. It seems to
> me we should be fsyncing the table space data directories on every
> checkpoint. Otherwise any newly created relations or removed relations
> could disappear even though the data in them was fsynced. I'm thinking
> I should add an _mdfd_opentblspc(reln) call which returns a file
> descriptor for the tablespace and have mdsync() use that to sync the
> directory whenever it fsyncs a relation. It would be nice to remember
> which tablespaces have been fsynced and only fsync them once though,
> that would need another hash table just for tablespaces.
>
> We probably also need to fsync the pg_xlog directory every time we
> create or rename an xlog segment.
>
> Are there any other places we do directory operations which we need to
> be permanent?

I agree with Tom that we need to see some actual reproducible test
cases where this is an issue before we go too crazy with it. In
theory what you're talking about could also happen when extending a
relation, if we extend into a new file; but I think we need to
convince ourselves that it really happens before we make any more
changes.

On a pragmatic note, if this does turn out to be a problem, it's a
bug: and we can and do fix bugs whenever we discover them. But the
other part of this patch - to speed up createdb - is a feature - and
we are very rapidly running out of time for 9.0 features. So I'd like
to vote for getting the feature part of this committed (assuming it's
in good shape, of course) and we can continue to investigate the other
issues but without quite as much urgency.

...Robert