Re: Block-level CRC checks

Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

> Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
>> Greg Stark wrote:
>>> I'm beginning to think just excluding the hint bits would be simpler and
>>> safer. If we're double buffering then it might be possible to do that
>>> pretty cheaply. Copy the whole buffer with memcpy then loop through the
>>> line pointers unsetting the hint bits. Then do the crc. Though that would
>>> prevent us from doing "zero-copy" crc by doing it in the copy.
>
>> The downside to this idea is that we need to create a copy of the page
>> and call those routines when we read the page in, too.

oh, good point.

> Ugh. The cost on write was bad enough, but paying it on read is a lot
> worse ...

I think you could checksum the block including the hint bits then go back and
remove them from the checksum. I didn't realize you were handling more than
just the heap transaction hint bits though. It would be hard to do it in any
kind of abstract away like you were describing.

How happy are you with the wal logging entries? Have you done any tests to see
how much extra wal traffic it is? Are you sure you always generate enough logs
soon enough?

--
Gregory Stark
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