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AW: AW: AW: WAL-based allocation of XIDs is insecure

From: Zeugswetter Andreas SB <ZeugswetterA(at)wien(dot)spardat(dot)at>
To: "'Tom Lane'" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: AW: AW: AW: WAL-based allocation of XIDs is insecure
Date: 2001-03-06 16:38:50
Message-ID: 11C1E6749A55D411A9670001FA687963368228@sdexcsrv1.f000.d0188.sd.spardat.at (view raw or flat)
Thread:
Lists: pgsql-hackers
> >> Hmm.  Actually, what is written to the log is the *modified* page not
> >> its original contents.
> > Well, that sure is not what was discussed on the list for implementation !!
> > I thus really doubt above statement.
> 
> Read the code.

Ok, sad.

> 
> > Each page about to be modified should be written to the txlog once,
> > and only once before the first modification after each checkpoint.
> 
> Yes, there's only one page dump per page per checkpoint.  But the
> sequence is (1) make the modification in shmem buffers then (2) make
> the XLOG entry. 
> 
> I believe this is OK since the XLOG entry is flushed before any of
> the pages it affects are written out from shmem.  Since we have not
> changed the storage management policy, it's OK if heap pages contain
> changes from uncommitted transactions

Sure, but the other way would be a lot less complex.
 
> --- all we must avoid is
> inconsistencies (eg not all three pages of a btree split written out),
> and redo of the XLOG entry will ensure that for us.

Is it so hard to swap ? First write page to log then modify in shmem. 
Then those pages would have additional value, because
then utilities could do all sorts of things with those pages.

1. Create a consistent state of the db by only applying "physical log" pages
	after checkpoint (in case a complete WAL rollforward bails out)
2. Create a consistent online backup snapshot, by first doing something like an 
	ordinary tar, and after that save all "physical log" pages.

Andreas

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Subject: Re: How to shoot yourself in the foot: kill -9 postmaster
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