On 8/17/16 2:51 PM, Simon Riggs wrote:
> On 17 August 2016 at 12:19, Greg Stark <stark@mit.edu> wrote:
>> On Wed, Aug 17, 2016 at 1:36 AM, Jim Nasby <Jim.Nasby@bluetreble.com> wrote:
>>> Something I didn't see mentioned that I think is a critical point: last I
>>> looked, HOT standby (and presumably SR) replays full page writes. That means
>>> that *any* kind of corruption on the master is *guaranteed* to replicate to
>>> the slave the next time that block is touched. That's completely the
>>> opposite of trigger-based replication.
>>
>> Yes, this is exactly what it should be doing and exactly why it's
>> useful. Physical replication accurately replicates the data from the
>> master including "corruption" whereas a logical replication system
>> will not, causing divergence and possible issues during a failover.
>
> Yay! Completely agree.
>
> Physical replication, as used by DRBD and all other block-level HA
> solutions, and also used by other databases, such as Oracle.
>
> Corruption on the master would often cause errors that would prevent
> writes and therefore those changes wouldn't even be made, let alone be
> replicated.
My experience has been that you discover corruption after it's already
safely on disk, and more than once I've been able to recover by using
data on a londiste replica.
As I said originally, it's critical to understand the different
solutions and the pros and cons of each. There is no magic bullet.
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com
855-TREBLE2 (855-873-2532) mobile: 512-569-9461
