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Re: Moving pgstat.stat and pgstat.tmp

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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@xxxxxxxxxx
800.595.4401 or 615.292.5888
615.292.0777 (fax)

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