Re: Moving pgstat.stat and pgstat.tmp

On Dec 5, 2007, at 7:50 AM, Robert Treat wrote:

> On Wednesday 05 December 2007 07:22, Alvaro Herrera wrote:
>> Robert Treat wrote:
>>> On Monday 03 December 2007 20:22, Erik Jones wrote:
>>>> Interesting. If this is anything you'd like to look into I can
>>>> provide whatever diagnostic output you need (iostat, vmstat, dtrace
>>>> script outputs, etc...) but I do have to reiterate that we are an
>>>> extreme corner case due to out schema size. For now, is
>>>> renaming the
>>>> #define'd paths for the stats file and temp file sufficient for
>>>> moving them? Basically, we'd like to move them onto a RAM disk to
>>>> give our disks a break.
>>>
>>> Yeah, we've noticed the same problem (pgstat is the most active
>>> file on
>>> the system... uncovered in much the same way... go solaris).
>>> Actually I
>>> was wondering if it could be done with symlinks, a la moving xlogs.
>>
>> Not really, because a new file is created and renamed in place
>> each time
>> it's going to be rewritten. So the symlink would be lost in the
>> first
>> file rewrite.
>>
>
> Ah yeah, thats what I concluded back then.
>
>> The first idea that comes to mind is to make the path configurable
>> via
>> GUC, so the user could set it to be written to an in-memory
>> filesystem
>> (/tmp in Solaris?).
>
> Yep, thought of that to, though it was after feature freeze so I
> didn't
> propose it. Course if someone wants to sneak that in it would be
> cool :-)
>
>> But then I thought, why do we need it to be a file
>> at all? Why not use a mmap'ed memory area or something like that,
>> and
>> only write it to a file on postmaster shutdown? (Losing the file on
>> unclean shutdown is not a problem, because the file is removed
>> anyway.)
>
> I suppose you need some facility to spill to disk, so maybe being
> in a file is
> better? Seems it might not be in most cases... I wonder how big a
> memory
> space we (or Erik) need.

We made the swapfs 300MB which is actually way more than we need as I
don't think I've seen our pgstat.stat file crack 10MB using the
entirely scientific method of spot-checking :)

Erik Jones

Software Developer | Emma®
erik(at)myemma(dot)com
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