Re: Enabling Checksums

On Mon, Mar 4, 2013 at 05:04:27PM -0800, Daniel Farina wrote:
> Putting aside the not-so-rosy predictions seen elsewhere in this
> thread about the availability of a high performance, reliable
> checksumming file system available on common platforms, I'd like to
> express what benefit this feature will have to me:
>
> Corruption has easily occupied more than one person-month of time last
> year for us. This year to date I've burned two weeks, although
> admittedly this was probably the result of statistical clustering.
> Other colleagues of mine have probably put in a week or two in
> aggregate in this year to date. The ability to quickly, accurately,
> and maybe at some later date proactively finding good backups to run
> WAL recovery from is one of the biggest strides we can make in the
> operation of Postgres. The especially ugly cases are where the page
> header is not corrupt, so full page images can carry along malformed
> tuples...basically, when the corruption works its way into the WAL,
> we're in much worse shape. Checksums would hopefully prevent this
> case, converting them into corrupt pages that will not be modified.
>
> It would be better yet if I could write tools to find the last-good
> version of pages, and so I think tight integration with Postgres will
> see a lot of benefits that would be quite difficult and non-portable
> when relying on file system checksumming.

I see Heroku has corruption experience, and I know Jim Nasby has
struggled with corruption in the past.

I also see the checksum patch is taking a beating. I wanted to step
back and ask what percentage of known corruptions cases will this
checksum patch detect? What percentage of these corruptions would
filesystem checksums have detected?

Also, don't all modern storage drives have built-in checksums, and
report problems to the system administrator? Does smartctl help report
storage corruption?

Let me take a guess at answering this --- we have several layers in a
database server:

1 storage
2 storage controller
3 file system
4 RAM
5 CPU

My guess is that storage checksums only cover layer 1, while our patch
covers layers 1-3, and probably not 4-5 because we only compute the
checksum on write.

If that is correct, the open question is what percentage of corruption
happens in layers 1-3?

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +