Re: PostgreSQL advocacy

On 3/22/16, 8:07 AM, "Bruce Momjian" <bruce(at)momjian(dot)us> wrote:

>On Mon, Mar 21, 2016 at 04:46:51PM +0000, Jernigan, Kevin wrote:
>> Disk is only a single point of failure in RAC if you configure
>> non-redundant storage. In general, Oracle recommends triple mirroring
>> to protect against disk failures, as they have had many experiences
>> over the years where customers with mirrored disks would see
>> consecutive disk failures within short periods of time.
>>
>> And RAC is widely used by Oracle’s larger customers, not only
>> for HA, but also in some cases for scale-out. Having said that,
>> it’s very true that any application running on Oracle RAC must be
>> configured to avoid hot block contention across RAC nodes, so it’s
>> not a completely transparent solution for scale out.
>
>I get asked about Oracle RAC often. My usual answer is that Oracle RAC
>gives you 50% of high reliability (storage is shared, mirroring helps)
>and 50% of scaling (CPU/memory is scaled, storage is not). The
>requirement to partition applications to specific nodes to avoid cache
>consistency overhead is another downside. (Slide 24 of my scaling
>presentation shows Oracle RAC,
>http://momjian.us/main/writings/pgsql/scaling.pdf .)
>
>I said the community is unlikely to go the Oracle RAC direction because
>it doesn't fully solve a single problem, and it is overly complex. The
>community prefers fully-solved problems and simpler solutions.
>
>For me, streaming replication fully solves the high reliability problem
>and sharding fully solves the scaling problem. Of course, if you need
>both, you have to deploy both, which gives you 100% of two solutions,
>rather than Oracle RAC which gives you 50% of each.
>
>However, I do think database upgrades are easier with Oracle RAC, and I
>think it is much easier to add/remove nodes than with sharding. For me,
>this chart summarizes it:
>
> HA Scaling Upgrade Add/Remove
> Oracle RAC 50% 50% easy easy
> Streaming Rep. 100% 25%* hard easy
> Sharding 0% 100% hard hard
>
> * Allows read scaling
>
>--
> Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
> EnterpriseDB http://enterprisedb.com
>
>+ As you are, so once was I. As I am, so you will be. +
>+ Roman grave inscription +

Implementing RAC-equivalent functionality is extremely hard, as evidenced by the lack of any directly comparable capability from any other relational db engine, until the release of IBM DB2 Shareplex a few years ago. And given the improvement of PostgreSQL and other open source solutions over the past 20 years, it’s not clear that it makes sense to go through the initial design and implementation work and then the ongoing maintenance overhead - most of what RAC provides can be achieved through other existing capabilities.

While I’m not sure that the percentage breakdowns in your chart are totally accurate, I agree with the general assessment, except for the highest-end applications which have zero-downtime requirements which can’t be met with streaming replication: the overhead of synchronous replication limits scalability, and the failover time for moving from primary to a failover target is significantly slower than RAC - which can be literally zero if configured correctly.

The higher-level point that I think is important is that while I may be able to win technical arguments that RAC is better for certain high-end extreme workloads - and maybe I can’t even win those arguments ;-) - the real issue is that there aren’t very many of those workloads, and the PostgreSQL community shouldn’t care: the vast majority of Oracle (and SQL Server etc) workloads don’t need all the fancy high-end RAC capabilities, or many of the other high-end commercial database capabilities. And those workloads can relatively easily be migrated to PostgreSQL, with minor disruption / change to schemas, data, triggers, constraints, procedural SQL…

-KJ