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Re: beta testing version

From: ncm(at)zembu(dot)com (Nathan Myers)
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: beta testing version
Date: 2000-12-01 08:55:21
Message-ID: 20001201005521.A29208@store.zembu.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, Nov 30, 2000 at 11:06:31PM -0800, Vadim Mikheev wrote:
> > As for replaying logs against a restored snapshot dump... AIUI, a 
> > dump records tuples by OID, but the WAL refers to TIDs.  Therefore, 
> > the WAL won't work as a re-do log to recover your transactions 
> > because the TIDs of the restored tables are all different.   
> 
> True for current way of backing up - ie saving data in "external"
> (sql) format. But there is another way - saving data files in their
> natural (binary) format. WAL records may be applyed to
> such dump, right?

But (AIUI) you can only safely/usefully copy those files when the 
database is shut down.

Many people hope to run PostgreSQL 24x7x365.  With vacuuming, you 
might just as well shut down afterward; but when that goes away 
(in 7.2?), when will you get the chance to take your backups?  
Clearly we need either another form of snapshot backup that can 
be taken with the database running, and compatible with the 
current WAL (or some variation on it); or, we need another kind 
of log, in addition to the WAL.

> > To get replaying we need an "update log", something that might be
> > in 7.2 if somebody does a lot of work.
> 
> What did you mean by "update log"?
> Are you sure that WAL is not "update log" ? -:)

No, I'm not sure.  I think it's possible that a new backup utility 
could be written to make a hot backup which could be restored and 
then replayed using the current WAL format.  It might be easier to
add another log which could be replayed against the existing form
of backups.  That last is what I called the "update log".

The point is, WAL now does one job superbly: maintain a consistent
on-disk database image.  Asking it to do something else, such as 
supporting hot BAR, could interfere with it doing its main job.  
Of course, only the person who implements hot BAR can say.

Nathan Myers
ncm(at)zembu(dot)com

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