Re: amcheck (B-Tree integrity checking tool)

Hi,

On Fri, 2016-03-11 at 17:24 +0100, Michael Paquier wrote:
> On Fri, Mar 11, 2016 at 5:19 PM, Anastasia Lubennikova wrote:
> > 
> > BTW, if you know a good way to corrupt index (and do it
> > reproducible) I'd be
> > very glad to see it.
> You can use for example dd in non-truncate mode to corrupt on-disk
> page data, say that for example:
> dd if=/dev/random bs=8192 count=1 \
>     seek=$BLOCK_ID of=base/$DBOID/$RELFILENODE \
>     conv=notrunc

I guess /dev/random is not very compatible with the "reproducible"
requirement. I mean, it will reproducibly break the page, but pretty
much completely, which is mostly what checksums are for.

I think the main goal of the amcheck module is to identify issues that
are much more subtle - perhaps a coding error or unhandled cornercase
which results in page with a valid structure and checksum, but broken
page. Or perhaps issues coming from outside of PostgreSQL, like a
subtle change in the locales.

Simulating those issues requires a tool that allows minor tweaks to the
page, dd is a bit too dull for that.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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