On 07/30/2015 02:55 AM, Jan Keirse wrote:
Hello, we have some very write heavy databases and I have our monitoring system watch the transaction age of my databases to be alerted before we get into problems in case autovacuum can't keep up to avoid transaction ID wraparound. The query I am executing is this: SELECT max(age(pg_database.datfrozenxid)) / 2147483648.0 * 100.0 AS "Percentage of transaction ID's used" FROM pg_database; My believe was that if this reaches 100 the database will stop accepting writes and one must vacuum. I have set alerts on 50 and 90, the result is around 9 so my believe was autovacuum is working fine for my workload. I often see autovacuum kicking in to prevent XID Wraparround, I thought that was just to be on the safe side and vacuum well before it's too late. However today I saw this post: http://blog.getsentry.com/2015/07/23/transaction-id-wraparound-in-postgres.html The following line has me worried: ... that database is going to reach a situation where the XID counter has reached its maximum value. The absolute peak is something around 2 billion, but it can be far lower than that in some situations... Could someone shed some light on this? Is my query insufficient? Can the transaction wrapparound freeze problem indeed occur earlier? And if so, could someone suggest a better query to monitor?
I would look at: http://www.postgresql.org/docs/9.4/interactive/routine-vacuuming.html#VACUUM-FOR-WRAPAROUND Which includes some query examples.
Kind Regards, Jan Keirse
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