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. -- Jim C. Nasby, Database Architect j...@nasby.net 512.569.9461 (cell) http://jim.nasby.net -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
