Re: 16-bit page checksums for 9.2

On Sun, Dec 25, 2011 at 5:08 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On Sat, Dec 24, 2011 at 8:06 PM, Greg Stark <stark(at)mit(dot)edu> wrote:
>> On Sat, Dec 24, 2011 at 4:06 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>>> Checksums merely detect a problem, whereas FPWs correct a problem if
>>> it happens, but only in crash situations.
>>>
>>> So this does nothing to remove the need for FPWs, though checksum
>>> detection could be used for double write buffers also.
>>
>> This is missing the point. If you have a torn page on a page that is
>> only dirty due to hint bits then the checksum will show a spurious
>> checksum failure. It will "detect" a problem that isn't there.
>
> It will detect a problem that *is* there, but one you are classifying
> it as a non-problem because it is a correctable or acceptable bit
> error.

I don't agree with this. We don't WAL-log hint bit changes precisely
because it's OK if they make it to disk and it's OK if they don't.
Given that, I don't see how we can say that writing out only half of a
page that has had hint bit changes is a problem. It's not.

(And if it is, then we ought to WAL-log all such changes regardless of
whether CRCs are in use.)

--
Robert Haas
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