Andrew Hammond wrote:
> On Jan 26, 2:38 pm, t...@sss.pgh.pa.us (Tom Lane) wrote:
>> Rick Gigger <r...@alpinenetworking.com> writes:
>>> I thought that the following todo item just barely missed 8.2:
>>> "Allow a warm standby system to also allow read-only statements [pitr]
>> No, it's a someday-wishlist item; the work involved is not small.
>
> Slony1 has supported log-shipping replication for about a year now. It
> provides similar functionality.

Yes but Slony is much more complicated, has significantly more
administrative overhead, and as far as I can tell is much more likely to
impact my production system than this method would.

Slony is a lot more flexible and powerful but I don't need that. I just
want a backup that is reasonably up to date that I can do queries on and
and failover to in case of hardware failure on my primary db.

I am going to be looking more closely at Slony now that it seems to be
the best option for this. I am not looking forward to how it will
complicate my life though. (Not saying it is bad, just complicated. At
least more complicated than simple postgres log shipping.


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Gregory Stark wrote:
> "Rick Gigger" <rick@alpinenetworking.com> writes:
>
>> I thought that the following todo item just barely missed 8.2:
>>
>> "Allow a warm standby system to also allow read-only statements [pitr]
>> This is useful for checking PITR recovery."
>
> No, nobody worked on it prior to 8.2. Afaik there's still nobody working on
> it. It's not trivial. Consider for example that your read-only query would
> still need to come up with a snapshot and there's nowhere currently to find
> out what transactions were in-progress at that point in the log replay.
>
> There's also the problem that currently WAL replay doesn't take have allow for
> any locking so there's no way for read-only queries to protect themselves
> against the WAL replay thrashing the buffer pages they're looking at.
>
> It does seem to be doable and I agree it would be a great feature, but as far
> as I know nobody's working on it for 8.3.

Thanks again for the update.


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On Feb 5, 2007, at 12:53 PM, Andrew Hammond wrote:
> On Jan 26, 2:38 pm, t...@sss.pgh.pa.us (Tom Lane) wrote:
>> Rick Gigger <r...@alpinenetworking.com> writes:
>>> I thought that the following todo item just barely missed 8.2:
>>> "Allow a warm standby system to also allow read-only statements
>>> [pitr]
>>
>> No, it's a someday-wishlist item; the work involved is not small.
>
> Slony1 has supported log-shipping replication for about a year now. It
> provides similar functionality.

Not really....

1) It's not possible for a PITR 'slave' to fall behind to a state
where it will never catch up, unless it's just on inadequate
hardware. Same isn't true with slony.
2) PITR handles DDL seamlessly
3) PITR is *much* simpler to configure and maintain
--
Jim Nasby jim@nasby.net
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)



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On 2/6/07, Jim Nasby <decibel@decibel.org> wrote:
> On Feb 5, 2007, at 12:53 PM, Andrew Hammond wrote:
> > On Jan 26, 2:38 pm, t...@sss.pgh.pa.us (Tom Lane) wrote:
> >> Rick Gigger <r...@alpinenetworking.com> writes:
> >>> I thought that the following todo item just barely missed 8.2:
> >>> "Allow a warm standby system to also allow read-only statements
> >>> [pitr]
> >>
> >> No, it's a someday-wishlist item; the work involved is not small.
> >
> > Slony1 has supported log-shipping replication for about a year now. It
> > provides similar functionality.
>
> Not really....
>
> 1) It's not possible for a PITR 'slave' to fall behind to a state
> where it will never catch up, unless it's just on inadequate
> hardware. Same isn't true with slony.

I imagine that there are ways to screw up WAL shipping too, but there
are plenty more ways to mess up slony.

> 2) PITR handles DDL seamlessly
> 3) PITR is *much* simpler to configure and maintain

4) You need 3 databases to do log shipping using slony1. An origin, a
subscriber which generates the logs and obviously the log-replica.

All of which is why I qualified my statement with "similar".

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Jim Nasby wrote:
> On Feb 5, 2007, at 12:53 PM, Andrew Hammond wrote:
>> On Jan 26, 2:38 pm, t...@sss.pgh.pa.us (Tom Lane) wrote:
>>> Rick Gigger <r...@alpinenetworking.com> writes:
>>>> I thought that the following todo item just barely missed 8.2:
>>>> "Allow a warm standby system to also allow read-only statements [pitr]
>>>
>>> No, it's a someday-wishlist item; the work involved is not small.
>>
>> Slony1 has supported log-shipping replication for about a year now. It
>> provides similar functionality.
>
> Not really....
>
> 1) It's not possible for a PITR 'slave' to fall behind to a state where
> it will never catch up, unless it's just on inadequate hardware. Same
> isn't true with slony.
> 2) PITR handles DDL seamlessly
> 3) PITR is *much* simpler to configure and maintain

Which is why I was hoping for a PITR based solution. Oh well, I will
have to figure out what is my best option now that I know it will not be
available any time in the near future.

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