Hello Jan,
I think your calculation is slightly off because per the docs when PostgreSQL comes within 1 million of the age at which an actual wraparound occurs it will go into the safety shutdown mode. Thus the calculation should be ((2^32)-1)/2-1000000 rather than just ((2^32)-1)/2 as I think you are using.
When I first started building out my group's PostgreSQL monitoring solution I too found the wording of transaction freeze to be a bit difficult to understand. For my team's internal documentation I have summarized it as follows, I hope it might be more clear:
...normal XIDs are compared using modulo-2^32 arithmetic, which means that ~(2^32-1)/2- transactions appear in the future and ~(2^32-1)/2 transactions appear in the past.
This [Transaction ID freeze] behavior of autovacuum is primarily dependent on the settings autovacuum_freeze_table_age and autovacuum_freeze_max_age, which are set as database defaults but can also be specified on a per table basis (as storage parameters in CREATE TABLE or ALTER TABLE)
- When a table's oldest transaction reaches autovacuum_freeze_table_age, the next autovacuum that is performed on that table will be a vacuum freeze
- PostgreSQL implicitly caps autovacuum_freeze_table_age at 0.95*autovacuum_freeze_max_age.
- When a table reaches autovacuum_freeze_max_age PostgreSQL will force an autovacuum freeze on that table, even if the table would not otherwise be autovacuumed or autovacuum is disabled.
- PostgreSQL implicitly caps autovacuum_freeze_max_age at 2 billion (2000000000)
The actual age that a wraparound occurs is ((2^32)/2)-1. When a PostgreSQL database comes within 1 million of this age (2^32/2-1-1000000) the database will go into the safety shutdown mode" and no longer accept commands, including the vacuum commands, and your only recovery option is to stop the server and use a single-user backend (where shutdown mode is not enforced) to execute VACUUM. This should, obviously, be avoided at all costs.
References:
Based on the above explanation we consider the following to be the most correct check for how close you are to an actual wraparound freeze:
CAST (age(relfrozenxid) AS real) / CAST(trunc(((2^32)/2)-1-1000000) AS real) AS perc_until_wraparound_server_freeze
(Note that we do this at the table level rather than the database level like you did, though, so that we have the information we need to tune the settings for individual tables.)
However it is better to set autovacuum max freeze age well below that value and monitor that instead. Autovacuum should always do a vacuum freeze for a table that has exceeded max freeze age, and if you are monitoring for that you should avoid a wrap around freeze:
CAST (age(relfrozenxid) AS real) / CAST ((least(autovacuum_freeze_max_age, 2000000000)) AS real) AS perc_until_freeze_max_age
And ensure that value does not exceed 100%. Though it is important to note that max freeze age can be set on a per table basis, so to get the true autovacuum_freeze_max_age of a table (or the real max of the database) you would need to check the reloptions field of pg_class for that table and only if there is no value specified for '%autovacuum_freeze_table_age%' use current_setting('autovacuum_freeze_max_age')
Let me know if I missed something or can clarify it further.
Will J. Dunn
On Thu, Jul 30, 2015 at 11:44 AM, Adrian Klaver <adrian.klaver@xxxxxxxxxxx> wrote:
Well if you click on the parameters in the above page you will go to their definitions:On 07/30/2015 08:41 AM, Jan Keirse wrote:
On Thu, Jul 30, 2015 at 2:56 PM, Adrian Klaver
<adrian.klaver@xxxxxxxxxxx> wrote:
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.
Yes, I have seen that documentation and it is because of it that I
believed that my queries were ok, but now I think I may be
misinterpreting or misunderstanding the documentation and have to look
at more information, like autovacuum_multixact_freeze_max_age?
So for autovacuum_multixact_freeze_max_age:
http://www.postgresql.org/docs/9.4/interactive/runtime-config-autovacuum.html#GUC-AUTOVACUUM-MULTIXACT-FREEZE-MAX-AGE
--
Adrian Klaver
adrian.klaver@xxxxxxxxxxx
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