From: | Jim Nasby <jim(at)nasby(dot)net> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Joachim Wieland <joe(at)mcknight(dot)de>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Snapshot synchronization, again... |
Date: | 2011-03-02 02:28:06 |
Message-ID: | 8B6E94FB-57FA-4F13-AD91-294A53CF7ED3@nasby.net |
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Lists: | pgsql-hackers |
On Feb 28, 2011, at 11:59 AM, Tom Lane wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Sun, Feb 27, 2011 at 8:33 PM, Joachim Wieland <joe(at)mcknight(dot)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 truncate the file when the snapshot is no longer valid.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 memory and make it available via a variable-sized IPC mechanism, such as a port or socket? That would eliminate the need to clean things up after a crash; I'm not sure if there would be other benefits.
--
Jim C. Nasby, Database Architect jim(at)nasby(dot)net
512.569.9461 (cell) http://jim.nasby.net
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