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Re: Snapshot synchronization, again...

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Joachim Wieland <joe(at)mcknight(dot)de>
Cc: 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: 2011-02-28 17:38:21
Message-ID: (view raw or whole thread)
Lists: pgsql-hackers
On Sun, Feb 27, 2011 at 8:33 PM, Joachim Wieland <joe(at)mcknight(dot)de> wrote:
> On Sun, Feb 27, 2011 at 3:04 PM, Heikki Linnakangas
> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>>> Why exactly, Heikki do you think the hash is more troublesome?
>> It just feels wrong to rely on cryptography just to save some shared memory.
> 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. Furthermore, we
> can easily invalidate a snapshot that we have published earlier by
> deleting its checksum in shared memory as soon as the original
> transaction commits/aborts. And for these two a checksum seems to be a
> good fit. Saving memory then comes as a benefit and makes all those
> happy who don't want to argue about how many slots to reserve in
> shared memory or don't want to have another GUC for what will probably
> be a low-usage feature.

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truncate the file when the snapshot is no longer valid.

Robert Haas
The Enterprise PostgreSQL Company

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