On 8/17/16 2:51 PM, Simon Riggs wrote:
On 17 August 2016 at 12:19, Greg Stark <st...@mit.edu> wrote:
On Wed, Aug 17, 2016 at 1:36 AM, Jim Nasby <jim.na...@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


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