Re: question about checksum in 9.3

On Tue, Jan 14, 2014 at 1:50 AM, Mike Broers <mbroers@gmail.com> wrote:
> Hello, I am in the process of planning a 9.3 migration of postgres and I am
> curious about the checksum features available.  In my test 9.3 instance it
> seemed like this feature provides a log entry of the exact database/oid of
> the corrupt object when it is accessed, but not much else.  I can't find
> much documentation on anything else this feature provides.

Few things:
- The only way to know if a server is using data checksums is to use
pg_controldata.
- Be aware as well of the potential performance impact on your CPU,
checksums are checked each time a page is read, and recalculated each
time a page is updated.
- ignore_checksum_failure can be used to ignore failures. Don't use
that on a production system.

> Is there a built-in method of scanning the server to check for corruption or
> will I have to wait for a corrupt object to be accessed to see the log
> entry?

You can as well access manually tables with some for example
sequential scan to check if blocks are broken or not.

> Is there a relation that stores last checksum status or anyway of
> reporting on what objects are identified by postgres as corrupt or not
> corrupt?

No, you could build one though with a background worker that scans
relation pages and registers that failing blocks.

> Are there any other features of the checksum I am missing besides the log
> entry?

9.4 has a new GUC parameter called data_checksums that allow you to
check with a psql client if checksums are used on a server.

Regards,
--
Michael