On 12/31/10, Simon Riggs <simon@2ndquadrant.com> wrote:
> On Fri, 2010-12-31 at 09:27 +0100, Stefan Kaltenbrunner wrote:
>
>> Maybe it has been discussed but I still don't see way it makes any
>> sense. If I declare a standby a sync standby I better want it sync - not
>> "maybe sync". consider the case of a 1 master and two identical sync
>> standbys - one sync standby is in the same datacenter the other is in a
>> backup location say 15km away.
>> Given there is a small constant latency to the second box (even if you
>> have fast networks) the end effect is that the second standby will NEVER
>> be sync (because the local one will always be faster) and you end up
>> with an async slave that cannot be used per your business rules?
>
> Your picture above is a common misconception. I will add something to
> the docs to explain this.
>
> 1. "sync" is a guarantee about how we respond to the client when we
> commit. If we wait for more than one response that slows things down,
> makes the cluster more fragile, complicates the code and doesn't
> appreciably improve the guarantee.
Whether it is more fragile depends on if you look at up-time fragility
or durability fragility. I think it can appreciably improve the
guarantee.
>
> 2. "sync" does not guarantee that the updates to the standbys are in any
> way coordinated. You can run a query on one standby and get one answer
> and at the exact same time run the same query on another standby and get
> a different answer (slightly ahead/behind). That also means that if the
> master crashes one of the servers will be ahead or behind. You can use
> pg_last_xlog_receive_location() to check which one that is.
If at least one of the standbys is in the same smoking crater as the
primary, then pg_last_xlog_receive_location on it is unlikely to
respond.
The guarantee goes away precisely when it is needed.
Cheers,
Jeff