Re: data loss after vacuum

Tom Lane wrote:
> Allan Tong <actong@www.quateams.com> writes:
> > I'm not sure if this is the right list to send this, but any help
> > would be appreciated.  We recently encountered a problem running
> > postgres where, after a vacuum, all the data in one of our tables
> > was gone.  Now, I guess technically we don't know for sure if it
> > was indeed vacuum that caused the data loss, but it seems likely.
>
> The vacuum output shows that it thought it was removing only 27 out
> of the nearly 700K rows.  So I don't think vacuum is directly to
> blame.  However, it would very possibly have rewritten many of the
> pages in your table, as a byproduct of moving rows, updating tuple
> commit bits, etc.
>
> > ... when I looked at the file contents, it was almost
> > completely null'ed, so it looks like the data is really gone (though
> > shouldn't a full vacuum reclaim the space?).
>
> You mean the pages were all-zero?  It sounds to me like a serious
> hardware failure, or possibly kernel/filesystem misfeasance.  Postgres
> would certainly not have written zeroes, but apparently what got dropped
> onto the disk platter was zeroes.  Such failures are uncommon, but
> by no means un-heard-of.

Isn't that what IDE gives you when it maps a new block to replace a
corrupted one?

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