On Sun, Dec 25, 2011 at 5:08 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
> On Sat, Dec 24, 2011 at 8:06 PM, Greg Stark <stark@mit.edu> wrote:
>> On Sat, Dec 24, 2011 at 4:06 PM, Simon Riggs <simon@2ndquadrant.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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
