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Re: Forcing WAL switch

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "Walker, Jed S" <Jed_Walker(at)cable(dot)comcast(dot)com>,pgsql-novice(at)postgresql(dot)org
Subject: Re: Forcing WAL switch
Date: 2005-08-12 00:24:54
Message-ID: 200508120024.j7C0Osg25905@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-novice
Tom Lane wrote:
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> >> How is "switch a WAL" an essential component of that scheme?  You can
> >> archive the latest active segment just as well.
> 
> > Ah, but that will be over-written later, so you have to store it
> > somewhere safe, rather than just forcing closure of the current
> > WAL file and forcing an archive of it.
> 
> So?  I still don't see the operational benefit.
> 
> If you are running a true PITR operation, that is you are archiving off
> the complete WAL sequence, then forced WAL switches aren't buying you
> anything except wasted archive space.  You're still going to want to
> archive the active segment when it's done.
> 
> If you're not really doing PITR but just want to use a filesystem-level
> backup, then you can copy the last WAL segment when you're done whether
> it's still active or not.
> 
> I honestly think that WAL-switching is a solution in search of a
> problem.

The issue for me is that you really have to store the partially-filled
WAL file some other than the default archive location because when it is
later archived, you don't want it to be overwritten and perhaps fail in
the middle of the write. Making the WAL file switch solves that issue.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

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