Re: Snapshot synchronization, again...

On Feb 28, 2011, at 11:59 AM, Tom Lane wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Sun, Feb 27, 2011 at 8:33 PM, Joachim Wieland <joe@mcknight.de> wrote:
>>> Remember that it's not only about saving shared memory, it's also
>>> about making sure that the snapshot reflects a state of the database
>>> that has actually existed at some point in the past.
>
>> But you can do all of this with files too, can't you?  Just remove or
>> truncate the file when the snapshot is no longer valid.
>
> Yeah.  I think adopting a solution similar to 2PC state files is a very
> reasonable way to go here.  This isn't likely to be a high-usage or
> performance-critical feature, so it's not essential to keep the
> information in shared memory for performance reasons.

Dumb question: Is this something that could be solved by having the postmaster track this information in it's local
memoryand make it available via a variable-sized IPC mechanism, such as a port or socket? That would eliminate the need
toclean things up after a crash; I'm not sure if there would be other benefits. 
--
Jim C. Nasby, Database Architect                   jim@nasby.net
512.569.9461 (cell)                         http://jim.nasby.net