Re: cheaper snapshots redux

On Thu, Aug 25, 2011 at 6:24 PM, Jim Nasby <jim(at)nasby(dot)net> wrote:
> On Aug 25, 2011, at 8:24 AM, Robert Haas wrote:
>> My hope (and it might turn out that I'm an optimist) is that even with
>> a reasonably small buffer it will be very rare for a backend to
>> experience a wraparound condition.  For example, consider a buffer
>> with ~6500 entries, approximately 64 * MaxBackends, the approximate
>> size of the current subxip arrays taken in aggregate.  I hypothesize
>> that a typical snapshot on a running system is going to be very small
>> - a handful of XIDs at most - because, on the average, transactions
>> are going to commit in *approximately* increasing XID order and, if
>> you take the regression tests as representative of a real workload,
>> only a small fraction of transactions will have more than one XID.  So
>
> BTW, there's a way to actually gather some data on this by using PgQ (part of Skytools and used by Londiste). PgQ works by creating "ticks" at regular intervals, where a tick is basically just a snapshot of committed XIDs. Presumably Slony does something similar.
>
> I can provide you with sample data from our production systems if you're interested.

Yeah, that would be great.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company