Thanks for the explanation! Best Regards, Marc-Olaf > Am 28.11.2016 um 22:47 schrieb Kevin Grittner <kgrittn@xxxxxxxxx>: > > On Fri, Nov 25, 2016 at 1:39 PM, Marc-Olaf Jaschke <moj@xxxxxxxxx> wrote: > >> 1. The following presentation uses serializable transaction >> isolation level to force a snapshot too old error. >> (http://momjian.us/main/writings/pgsql/features.pdf) >> Is it possible to get a snapshot too old error with active >> old_snapshot_threshold when the only used transaction isolation >> level is read committed? > > Yes. Transaction isolation level doesn't have much impact on this > feature. > >> 2. Does activating old_snapshot_threshold have a negative impact >> on peformance? > > The latest tests, near the end of development, were run by Tomas > Vondra and are reported here: > > https://www.postgresql.org/message-id/067acb3d-80eb-279d-fce0-90e0a36c6aa2@xxxxxxxxxxxxxxx > > Note that the "immediate" lines are using a setting of zero, which > is ridiculously aggressive, and only intended for testing purposes. > The lines ending in "-10-rw" are using a 10 minute expiration, > which is still far more aggressive than I would expect most > environments to use, but should give some idea what the impact > could be "worst case". Tomas says in the linked email, "I'm > personally convinced the performance impact is within noise." > > FWIW, it makes no sense to enable this feature with a read-only > workload, but that configuration was heavily tested, as it > represents a worst case. Also note that Tomas used a multi-socket > NUMA machine for these tests, because that is where we saw the > largest differences during early tests, before some optimizations > were applied. > > -- > Kevin Grittner > EDB: http://www.enterprisedb.com > The Enterprise PostgreSQL Company -- Sent via pgsql-admin mailing list (pgsql-admin@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-admin
