Re: cheaper snapshots redux
От | Jim Nasby |
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Тема | Re: cheaper snapshots redux |
Дата | |
Msg-id | D8367094-B5B3-4F5F-8713-59EF3BCBA367@nasby.net обсуждение исходный текст |
Ответ на | Re: cheaper snapshots redux (Robert Haas <robertmhaas@gmail.com>) |
Ответы |
Re: cheaper snapshots redux
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Список | pgsql-hackers |
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 bycreating "ticks" at regular intervals, where a tick is basically just a snapshot of committed XIDs. Presumably Slony doessomething similar. I can provide you with sample data from our production systems if you're interested. -- Jim C. Nasby, Database Architect jim@nasby.net 512.569.9461 (cell) http://jim.nasby.net
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