Re: Block-level CRC checks

On Oct 2, 2008, at 3:18 PM, Alvaro Herrera wrote:
> I have to admit I don't remember exactly how it worked :-)  I think  
> the
> idea was avoiding setting the page dirty until a certain number of  
> hint
> bit setting operations had been done (which I think means it's not
> useful for the present purpose).


Well, it would be useful if whenever we magically decided it was time  
to write out a page that had only hint-bit updates we generated WAL,  
right? Even if it was just a no-op WAL record to ensure we had the  
page image in the WAL.

BTW, speaking of torn pages... I've heard that there's some serious  
gains to be had by turning full_page_writes to off, but I've never  
even dreamed of doing that because I've never seen any real sure-fire  
way to check that your hardware can't write torn pages. But if we  
have checksums enabled and checked the checksums on a block the first  
time we touched it during recovery, we'd be able to detect torn  
pages, yet still recover. That would help show that torn pages aren't  
possible in a particular environment (though unfortunately I don't  
think there's any way to actually prove that they're not).
-- 
Decibel!, aka Jim C. Nasby, Database Architect  decibel@decibel.org
Give your computer some brain candy! www.distributed.net Team #1828