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Re: Performance of COPY for Archive operations

From: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "Bruce Momjian" <pgman(at)candle(dot)pha(dot)pa(dot)us>,<pgsql-hackers-win32(at)postgresql(dot)org>
Subject: Re: Performance of COPY for Archive operations
Date: 2004-09-20 20:59:20
Message-ID: NOEFLCFHBPDAFHEIPGBOIEMJCEAA.simon@2ndquadrant.com (view raw or flat)
Thread:
Lists: pgsql-hackers-win32
>Tom Lane wrote
> "Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:
> > Nothing that surprising there, though I think I would like to
> put a WARNING
> > message into the Archiver that triggers if more than
> CHECKPOINT_SEGMENTS WAL
> > files are ready to archive at any one time. Though maybe that
> would cause
> > more problems than it would solve: "Archiving of transaction logs cannot
> > keep up with system activity. If this occurs regularly, you should
> > reconsider your database-disk layout"
>
> Can't see the value of this.  The problem will be readily apparent from
> growth of the pg_xlog directory --- anyone who doesn't notice that
> probably isn't perusing the postmaster log either.

Hmmm, message levels were a point we differed on previously, IIRC.

Certainly, if the growth happened over a long period, then I'd agree - the
admin should have spotted it.

If the behaviour were more volatile, then the admin might not spot it - the
effects are only shown when the system becomes I/O bound, which might be
regularly at peak loading, but never long enough to notice. I had considered
just such volatility in the design, though with regard to operator induced
behaviour like tape changes or deliberate batching of log files.

The issue is that by falling behind the archiver is increasing the
transaction loss window, possibly undermining somewhat the purpose of PITR.

The message shows in the log long after the situation occurred and the space
increase has disipated. The admin may never look at the logs, agreed, but if
the message isn't there they certainly will never notice. You and I will
know, because when the crash occurs, we'll get a pattern of error messages
we'll recognise, but thats not much help to the admin.

Do we wait for such a crash before we add the hint?

> Also, once it starts
> to bleat, what's going to make it stop?  Filling the disk with warning
> messages won't be a constructive improvement on the situation :-(

Filling the disk with log messages would be pointless, agreed.

If the message appeared as part of the normal archiver cycle, then the
message would only appear once per 2*CHECKPOINT_SEGMENTS "transaction log
archived" and "transaction log recycled" messages. Thus no more likely to
fill up the disk.

Of course, the archiver could always report less frequently, since it keeps
state between cycles.

I'm not in a rush to add this, just think its needed, based upon my
observations on Windows.

Best Regards, Simon Riggs


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Subject: Re: Performance of COPY for Archive operations
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