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Re: Hot Standby query cancellation and Streaming Replication integration

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Greg Smith <greg(at)2ndquadrant(dot)com>
Cc: Dimitri Fontaine <dfontaine(at)hi-media(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Greg Stark <gsstark(at)mit(dot)edu>, Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Hot Standby query cancellation and Streaming Replication integration
Date: 2010-02-27 00:36:57
Message-ID: 201002270036.o1R0avG03960@momjian.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Greg Smith wrote:
> Bruce Momjian wrote:
> > Doesn't the system already adjust the delay based on the length of slave
> > transactions, e.g. max_standby_delay.  It seems there is no need for a
> > user switch --- just max_standby_delay really high.
> >   
> 
> The first issue is that you're basically saying "I don't care about high 
> availability anymore" when you increase max_standby_delay to a high 
> value.  Want to offload an 8 hour long batch report every day to the 
> standby?  You can do it with max_standby_delay=8 hours.  But the day 
> your master crashes 7 hours into that, you're in for a long wait before 
> your standby is available while it replays all the queued up segments.  
> Your 'hot standby' has actually turned into the old form of 'cold 
> standby' just when you need it to be responsive.

Well, I think the choice is either you delay vacuum on the master for 8
hours or pile up 8 hours of WAL files on the slave, and delay
application, and make recovery much slower.  It is not clear to me which
option a user would prefer because the bloat on the master might be
permanent.

-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com
  PG East:  http://www.enterprisedb.com/community/nav-pg-east-2010.do
  + If your life is a hard drive, Christ can be your backup. +

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Subject: Re: Hot Standby query cancellation and Streaming Replication integration
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