Re: Block-level CRC checks

On Mon, Nov 30, 2009 at 9:27 PM, Heikki Linnakangas
<heikki.linnakangas@enterprisedb.com> wrote:
> Simon Riggs wrote:
>> Proposal
>>
>> * We reserve enough space on a disk block for a CRC check. When a dirty
>> block is written to disk we calculate and annotate the CRC value, though
>> this is *not* WAL logged.
>
> Imagine this:
> 1. A hint bit is set. It is not WAL-logged, but the page is dirtied.
> 2. The buffer is flushed out of the buffer cache to the OS. A new CRC is
> calculated and stored on the page.
> 3. Half of the page is flushed to disk (aka torn page problem). The CRC
> made it to disk but the flipped hint bit didn't.
>
> You now have a page with incorrect CRC on disk.
>

What if we treated the hint bits as all-zeros for the purpose of CRC
calculation? This would exclude them from the checksum.


Greetings
Marcin Mańk