Re: Summary and Plan for Hot Standby

On Sun, Nov 15, 2009 at 9:50 AM, Heikki Linnakangas
<heikki.linnakangas@enterprisedb.com> wrote:
> Simon Riggs wrote:
>> On Sun, 2009-11-15 at 16:07 +0200, Heikki Linnakangas wrote:
>>> - If WAL recovery runs out of lock space while acquiring an
>>> AccessExclusiveLock on behalf of a transaction that ran in the master,
>>> it will FATAL and abort recovery, bringing down the standby. Seems like
>>> it should wait/cancel queries instead.
>>
>> Hard resources will always be an issue. If the standby has less than it
>> needs, then there will be problems. All of those can be corrected by
>> increasing the resources on the standby and restarting. This effects
>> max_connections, max_prepared_transactions, max_locks_per_transaction,
>> as documented.
>
> There's no safe setting for those that would let you avoid the issue. No
> matter how high you set them, it will be possible for read-only backends
> to consume all the lock space, causing recovery to abort and bring down
> the standby.

OK, but... there will always be things that will bring down the
stand-by, just as there will always be things that can bring down the
primary.  A bucket of ice-water will probably do it, for example.  I
mean, it would be great to make it better, but is it so bad that we
can't postpone that improvement to a follow-on patch?  It's not clear
to me that it is.  I think we should really focus in on things that
are likely to make this (1) give wrong answers or (2) won't be able to
be fixed in a follow-on patch if they're not right in the original
one.  Only one or two of the items on your list of additional TODOs
seem like they might fall into category (2), and none of them appear
to fall into category (1).

I predict that if we commit a basic version of this with some annoying
limitations for 8.5, people who need the feature will start writing
patches to address some of the limitations.  No one else is going to
undertake any serious development work as long as this remains outside
the main tree, for fear of everything changing under them and all
their work being wasted.  I would like this feature to be as good as
possible, but I would like to have it at all more.

...Robert