Re: Hope for a new PostgreSQL era?

On Thu, 8 Dec 2011 16:34:49 -0800, Chris Travers <chris(dot)travers(at)gmail(dot)com>
wrote:
> On Thu, Dec 8, 2011 at 4:09 PM, Rodger Donaldson
> <rodgerd(at)diaspora(dot)gen(dot)nz> wrote:
>> On Thu, 08 Dec 2011 11:24:12 -0800, John R Pierce <pierce(at)hogranch(dot)com>
>> wrote:
>>> On 12/08/11 11:16 AM, Joshua D. Drake wrote:
>>>>>
>>>>> um, I believe this is referring to Oracle RAC clustering, not HA
>>>>> active/standby. I seriously doubt Oracle is dropping RAC.
>>>>
>>>> I meant worrying about it for Pg.
>>>
>>> the odds of Postgres developing something as complex and intricate as
>>> RAC are probably between zilch and none.  RAC was for many years
>>> completely unusable, and even now, its complicated, fragile, and
>> expensive.
>>
>> Happily, the complications and fragility are now utilised by Oracle to
>> help sell ExaData units, on the basis that if you give Oracle even more
>> money, they'll sell you a RAC that actually works!
>
> Looking at the general design of Postgres-XC compared to RAC, which
> workloads would the latter excel at as a matter of design that the
> former would not?

Not having touched -XC I can only theorycraft, but if I've understood the
shared-nothing model correctly, I'd expect it to hammer RAC in high-write
workloads, based on my experience of contention in RAC servers - a common
condition is that since the SGA is (effectively) shared across the RAC
interconnect, you're effectively limiting your peak write performance to
the throughput and latency of your ethernet link (which is, I assume, why
the ExaData uses Infiniband). We've had to do things like set up seperate
connection pools which write to one node at a time (with failover, of
course) for high-INSERT tables to avoid the RAC becoming painfully slow.

On the other hand I don't see anything that would suggest -XC will work as
seamlessly for failover as the RAC does. We went through a period where
our applications (and DBAs!) failed to notice kernel-panic induced reboots
of RAC members at least once every couple of weeks, with absolutely no
customer impact.

Anyway, my comment was more of a dig at the selling pitch for ExaData: I
have had it pitched with a straight face that I should want to buy one
because RAC is so hard to configure and maintain.

> Granted Postgres-XC is still pre-1.0 (latest
> release iirc is 0.9.6) and it doesn't yet support everything it needs
> to, but it looks very promising in this area, and it is open source.

It looks really interesting. Thanks for the pointer.