On 08/11/2009 11:19 PM, Robert Haas wrote:
On Tue, Aug 11, 2009 at 9:44 PM, Greg Stark<gsst...@mit.edu> wrote:
No! This is *not* what "hot standby" means, at least not in the Oracle world.
I'm perplexed by this. For example: http://en.wikipedia.org/wiki/Hot_standby
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...
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 server does not need
to be shut down to install the drives, and initialize them for service. :-)
For databases, people seem to be associating "hot" with the ability to
issue read only queries. As somebody else said - under a definition of
hot that includes read-only clones, pg_dump/pg_restore could be
considered a "hot standby" strategy.
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.
For myself, I associate "hot" to mean "ready to replace the master",
which implies that the data is up-to-date or nearly up-to-date, and
implies that the server is within a bit toggle of accepting mastership
of the data and serving ALL queries 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!).
If the server happens to be able to do read queries while it is waiting
in standby more - that's convenient and could be useful to somebody, but
that's not the value of a "hot standby" - that's the value of a
"read-only slave". The feature being provided is not "hot standby".
In the case of Oracle, I believe their "hot standby" provides the nearly
up-to-date, and the capability to switch over, which satisfies my
requirements. It might *also* allow read-only slave for the standby, but
that's just convenience - it's not really part of the definition or
expectation of "hot standby".
"log based 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.
Me too.
Read only slave works for me.
Cheers,
mark
--
Mark Mielke<m...@mielke.cc>
