On Thu, Aug 25, 2011 at 6:24 PM, Jim Nasby <j...@nasby.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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
