Re: "Hot standby"? - Mailing list pgsql-hackers

On 08/11/2009 11:19 PM, Robert Haas wrote: <blockquote
cite="mid:603c8f070908112019t7f62f170m6ed48bc651baae9d@mail.gmail.com"type="cite"><pre wrap="">On Tue, Aug 11, 2009 at
9:44PM, Greg Stark<a class="moz-txt-link-rfc2396E" href="mailto:gsstark@mit.edu"><gsstark@mit.edu></a> wrote:
</pre><blockquotetype="cite"><pre wrap="">No! This is *not* what "hot standby" means, at least not in the Oracle world.
 </pre></blockquote><pre wrap="">
 
I'm perplexed by this.  For example: <a class="moz-txt-link-freetext"
href="http://en.wikipedia.org/wiki/Hot_standby">http://en.wikipedia.org/wiki/Hot_standby</a>

Admittedly, wikipedia is not an authoritative source, but I've always
understood cold/warm/hot just as Peter described them upthread.  Cold
means it's on the shelf.  Warm means it's plugged in, but you have to
have to do something to get it going.  Hot means it just takes over
when needed.

But of course I guess Oracle can call their features what they want to... </pre></blockquote><br /> To further confuse
things,the "temperature" might apply to only a particular aspect of the solution. For example, "hot swappable disk
drives"are drives that probably do sit on a shelf until they are needed, and the "hot" aspect only implies that the
serverdoes not need to be shut down to install the drives, and initialize them for service. :-)<br /><br /> For
databases,people seem to be associating "hot" with the ability to issue read only queries. As somebody else said -
undera definition of hot that includes read-only clones, pg_dump/pg_restore could be considered a "hot standby"
strategy.<br/><br /> I don't agree with that definition. For the clone to be able to perform read-only queries does not
imply"hot" nor does it imply "standby". It implies "slave". The original poster correctly raised this concern.<br /><br
/>For myself, I associate "hot" to mean "ready to replace the master", which implies that the data is up-to-date or
nearlyup-to-date, and implies that the server is within a bit toggle of accepting mastership of the data and serving
ALLqueries that the master would serve. Key to this is "nearly up-to-date" (requires some sort of streaming) AND "ALL
queries"(not just read queries!).<br /><br /> If the server happens to be able to do read queries while it is waiting
instandby more - that's convenient and could be useful to somebody, but that's not the value of a "hot standby" -
that'sthe value of a "read-only slave". The feature being provided is not "hot standby".<br /><br /> In the case of
Oracle,I believe their "hot standby" provides the nearly up-to-date, and the capability to switch over, which satisfies
myrequirements. It might *also* allow read-only slave for the standby, but that's just convenience - it's not really
partof the definition or expectation of "hot standby".<br /><br /><br /><blockquote
cite="mid:603c8f070908112019t7f62f170m6ed48bc651baae9d@mail.gmail.com"type="cite"><blockquote type="cite"><pre
wrap="">"logbased replication", "read-only slaves", and "hot standby" are all
 
100% accurate descriptions of what the hot standby patch enables. I do
like "read only slaves" because it's the most precise and meaningful.   </pre></blockquote><pre wrap="">
Me too. </pre></blockquote><br /> Read only slave works for me.<br /><br /> Cheers,<br /> mark<br /><br /><pre
class="moz-signature"cols="72">-- 
 
Mark Mielke <a class="moz-txt-link-rfc2396E" href="mailto:mark@mielke.cc"><mark@mielke.cc></a>
</pre>