Re: Enabling Checksums

On 16 April 2013 20:27, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Greg Stark <stark@mit.edu> writes:
>> On Fri, Apr 12, 2013 at 9:42 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>>> * WAL checksum is not used as the sole basis for end-of-WAL discovery.
>>> We reuse the WAL files, so the prev field in each WAL record shows
>>> what the previous end of WAL was. Hence if the WAL checksums give a
>>> false positive we still have a double check that the data really is
>>> wrong. It's unbelievable that you'd get a false positive and then have
>>> the prev field match as well, even though it was the genuine
>>> end-of-WAL.
>
>> This is kind of true and kind of not true. If a system loses power
>> while writing lots of data to WAL then the blocks at the end of the
>> WAL might not be written out in order. Everything since the last log
>> sync might be partly written out and partly not written out. That's
>> the case where the checksum is critical. The beginning of a record
>> could easily be written out including xl_prev and the end of the
>> record not written. 1/64,000 power losses would then end up with an
>> assertion failure or corrupt database.
>
> I have a hard time believing that it's a good idea to add checksums to
> data pages and at the same time weaken our ability to detect WAL
> corruption.  So this seems to me to be going in the wrong direction.
> What's it buying for us anyway?  A few CPU cycles saved during WAL
> generation?  That's probably swamped by the other costs of writing WAL,
> especially if you're using replication.

This part of the thread is dead now .... I said "lets drop this idea"
on 13 April.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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