Re: Hot Standby query cancellation and Streaming Replication integration

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@momjian.us>        http://momjian.us EnterpriseDB
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