Re: [WIP] Double-write with Fast Checksums

On Thu, Jan 12, 2012 at 12:09 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
>> On Wed, Jan 11, 2012 at 11:07 PM, Josh Berkus <josh(at)agliodbs(dot)com> wrote:
>>> On 1/11/12 1:25 PM, Dan Scales wrote:
>>>> And just wanted to reiterate one other benefit of double writes -- it greatly reduces the size of the WAL logs.
>
>>> Even if you're replicating?
>
>> Yes, but it will increase random I/O on the standby when we replay if
>> we don't have FPWs.
>
> The question is how you prevent torn pages when a slave server crashes
> during replay.  Right now, the presence of FPIs in the WAL stream,
> together with the requirement that replay restart from a checkpoint,
> is sufficient to guarantee that any torn pages will be fixed up.  If
> you remove FPIs from WAL and don't transmit some substitute information,
> ISTM you've lost protection against slave server crashes.

Sure, you need either FPW or DW to protect you. Whatever is used on
the primary must also be used on the standbys.

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