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Re: beta testing version

From: Alex Pilosov <alex(at)pilosoft(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: beta testing version
Date: 2000-12-01 06:54:23
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Thu, 30 Nov 2000, Nathan Myers wrote:

> After a power outage on an active database, you may have corruption
> at low levels of the system, and unless you have enormous redundancy
> (and actually use it to verify everything) the corruption may go 
> undetected and result in (subtly) wrong answers at any future time.
Nathan, why are you so hostile against postgres? Is there an ax to grind?

The conditions under which WAL will completely recover your database:
1) OS guarantees complete ordering of fsync()'d writes. (i.e. having two
blocks A and B, A is fsync'd before B, it could NOT happen that B is on
disk but A is not).
2) on boot recovery, OS must not corrupt anything that was fsync'd.

Rule 1) is met by all unixish OSes in existance. Rule 2 is met by some
filesystems, such as reiserfs, tux2, and softupdates. 

> The logging in 7.1 protects transactions against many sources of 
> database crash, but not necessarily against OS crash, and certainly
> not against power failure.  (You might get lucky, or you might just 
> think you were lucky.)  This is the same as for most databases; an
> embedded database that talks directly to the hardware might be able
> to do better.  

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