Обсуждение: Finding out the aging tables for vacuuming to prevent data loss ontransaction id wraparound

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Finding out the aging tables for vacuuming to prevent data loss ontransaction id wraparound

От
Subramanian Krishnan
Дата:
Hi,

We are running PostgreSQL 9.4.14 and recently we received an alert for transaction id crossing the 1 billion mark. Since we can hit the transaction id wraparound issue for aging tables,
we investigated by running the following queries:

mpsdb=> select * from txid_current();
 txid_current
--------------
   1100826671
(1 row)

mpsdb=> SELECT datname, age(datfrozenxid) FROM pg_database ORDER BY 2 DESC LIMIT 20;
  datname  |    age
-----------+------------
 mpsdb     | 1087909867
 rdsadmin  | 1000562517
 postgres  |  950697143
 template0 |  900709601
 template1 |  900697643
(5 rows)

mpsdb=> SELECT c.relname as table_name, c.relkind as type, age(c.relfrozenxid) as age, c.relfrozenxid FROM pg_class AS c WHERE age(c.relfrozenxid) <> 2147483647 ORDER BY 3 DESC LIMIT 20;
    table_name     | type |    age     | relfrozenxid
-------------------+------+------------+--------------
 pg_toast_22985917 | t    | 1042877986 |     57941433
 pg_toast_22986301 | t    | 1042877877 |     57941542
 pg_toast_23823161 | t    | 1041791836 |     59027583
 pg_toast_23823242 | t    | 1041791776 |     59027643
 pg_toast_23904594 | t    | 1041658236 |     59161183
 pg_toast_23904629 | t    | 1041658221 |     59161198
 pg_toast_23904640 | t    | 1041658208 |     59161211
 pg_toast_23904648 | t    | 1041658200 |     59161219
 pg_toast_23904658 | t    | 1041658191 |     59161228
 pg_toast_23904666 | t    | 1041658183 |     59161236
 pg_toast_23904674 | t    | 1041658175 |     59161244
 pg_toast_23904682 | t    | 1041658169 |     59161250
 pg_toast_23904690 | t    | 1041658161 |     59161258
 pg_toast_23904698 | t    | 1041658156 |     59161263
 pg_toast_23904706 | t    | 1041658149 |     59161270
 pg_toast_23904714 | t    | 1041658129 |     59161290
 pg_toast_23904722 | t    | 1041658096 |     59161323
 pg_toast_23904730 | t    | 1041658090 |     59161329
 pg_toast_23904738 | t    | 1041658085 |     59161334
 pg_toast_23904746 | t    | 1041658077 |     59161342
(20 rows)


mpsdb=> SELECT datname, usename, pid, waiting, current_timestamp - xact_start AS xact_runtime, query FROM pg_stat_activity WHERE upper(query) like '%VACUUM%' ORDER BY xact_start;
datname | usename  |  pid  | waiting |  xact_runtime   |                                 query
---------+----------+-------+---------+-----------------+------------------------------------------------------------------------
 mpsdb   | rdsadmin | 13833 | f       | 00:00:14.161262 | autovacuum: VACUUM pg_toast.pg_toast_242649675 (to prevent wraparound)
 mpsdb   | rdsadmin | 14565 | f       | 00:00:11.735571 | autovacuum: VACUUM pg_toast.pg_toast_242649757 (to prevent wraparound)
 mpsdb   | rdsadmin | 15315 | f       | 00:00:09.303762 | autovacuum: VACUUM pg_toast.pg_toast_242649819 (to prevent wraparound)
 mpsdb   | rdsadmin | 14935 | f       | 00:00:06.893078 | autovacuum: VACUUM pg_toast.pg_toast_243226657 (to prevent wraparound)
 mpsdb   | rdsadmin | 15851 | f       | 00:00:04.322474 | autovacuum: VACUUM pg_toast.pg_toast_243227582 (to prevent wraparound)
 mpsdb   | rdsadmin | 15615 | f       | 00:00:01.768495 | autovacuum: VACUUM pg_toast.pg_toast_243226332 (to prevent wraparound)

It is clear that we need to vacuum the most aged pg_toast tables which are not yet picked up by the autovacuum process.

The question which is bothering us though is:
As per the queries we ran the most aged transaction and table is 1042877986 (pg_toast_22985917). And as per PostgreSQL pg_database documentation the most aged table/transaction in pg_class determines age of the database (via the datfrozenxid value). If that is the case we would have expected the age of mpsdb database to be 1042877986 (or in this range) but what we see as age is 1087909867.

Does this mean there are other aging tables we are not aware of? If yes, how do we get to those tables since we have already queried pg_class for the most aged table?

Request any help/guidance we can get regarding this.

Thanks and Regards,
Subu


Re: Finding out the aging tables for vacuuming to prevent data losson transaction id wraparound

От
Laurenz Albe
Дата:
Subramanian Krishnan wrote:
> We are running PostgreSQL 9.4.14 and recently we received an alert for transaction id
> crossing the 1 billion mark. Since we can hit the transaction id wraparound issue for aging tables,
> we investigated by running the following queries:
> 
> mpsdb=> select * from txid_current();
>  txid_current
> --------------
>    1100826671
> (1 row)
> 
> mpsdb=> SELECT datname, age(datfrozenxid) FROM pg_database ORDER BY 2 DESC LIMIT 20;
>   datname  |    age
> -----------+------------
>  mpsdb     | 1087909867
>  rdsadmin  | 1000562517
>  postgres  |  950697143
>  template0 |  900709601
>  template1 |  900697643
> (5 rows)
> 
> mpsdb=> SELECT c.relname as table_name, c.relkind as type, age(c.relfrozenxid) as age, c.relfrozenxid FROM pg_class
ASc WHERE age(c.relfrozenxid) <> 2147483647 ORDER BY 3 DESC LIMIT 20;
 
>     table_name     | type |    age     | relfrozenxid
> -------------------+------+------------+--------------
>  pg_toast_22985917 | t    | 1042877986 |     57941433
>  pg_toast_22986301 | t    | 1042877877 |     57941542
>  pg_toast_23823161 | t    | 1041791836 |     59027583
>  pg_toast_23823242 | t    | 1041791776 |     59027643
>  pg_toast_23904594 | t    | 1041658236 |     59161183
>  pg_toast_23904629 | t    | 1041658221 |     59161198
>  pg_toast_23904640 | t    | 1041658208 |     59161211
>  pg_toast_23904648 | t    | 1041658200 |     59161219
>  pg_toast_23904658 | t    | 1041658191 |     59161228
>  pg_toast_23904666 | t    | 1041658183 |     59161236
>  pg_toast_23904674 | t    | 1041658175 |     59161244
>  pg_toast_23904682 | t    | 1041658169 |     59161250
>  pg_toast_23904690 | t    | 1041658161 |     59161258
>  pg_toast_23904698 | t    | 1041658156 |     59161263
>  pg_toast_23904706 | t    | 1041658149 |     59161270
>  pg_toast_23904714 | t    | 1041658129 |     59161290
>  pg_toast_23904722 | t    | 1041658096 |     59161323
>  pg_toast_23904730 | t    | 1041658090 |     59161329
>  pg_toast_23904738 | t    | 1041658085 |     59161334
>  pg_toast_23904746 | t    | 1041658077 |     59161342
> (20 rows)
> 
> 
> mpsdb=> SELECT datname, usename, pid, waiting, current_timestamp - xact_start AS xact_runtime, query FROM
pg_stat_activityWHERE upper(query) like '%VACUUM%' ORDER BY xact_start;
 
> datname | usename  |  pid  | waiting |  xact_runtime   |                                 query
>
---------+----------+-------+---------+-----------------+------------------------------------------------------------------------
>  mpsdb   | rdsadmin | 13833 | f       | 00:00:14.161262 | autovacuum: VACUUM pg_toast.pg_toast_242649675 (to prevent
wraparound)
>  mpsdb   | rdsadmin | 14565 | f       | 00:00:11.735571 | autovacuum: VACUUM pg_toast.pg_toast_242649757 (to prevent
wraparound)
>  mpsdb   | rdsadmin | 15315 | f       | 00:00:09.303762 | autovacuum: VACUUM pg_toast.pg_toast_242649819 (to prevent
wraparound)
>  mpsdb   | rdsadmin | 14935 | f       | 00:00:06.893078 | autovacuum: VACUUM pg_toast.pg_toast_243226657 (to prevent
wraparound)
>  mpsdb   | rdsadmin | 15851 | f       | 00:00:04.322474 | autovacuum: VACUUM pg_toast.pg_toast_243227582 (to prevent
wraparound)
>  mpsdb   | rdsadmin | 15615 | f       | 00:00:01.768495 | autovacuum: VACUUM pg_toast.pg_toast_243226332 (to prevent
wraparound)
> 
> It is clear that we need to vacuum the most aged pg_toast tables which are not yet picked up by the autovacuum
process.
 
> 
> The question which is bothering us though is:
> As per the queries we ran the most aged transaction and table is 1042877986 (pg_toast_22985917).
> And as per PostgreSQL pg_database documentation the most aged table/transaction in pg_class determines
> age of the database (via the datfrozenxid value). If that is the case we would have expected the age
> of mpsdb database to be 1042877986 (or in this range) but what we see as age is 1087909867.
> 
> Does this mean there are other aging tables we are not aware of? If yes, how do we get to those
> tables since we have already queried pg_class for the most aged table?
> 
> Request any help/guidance we can get regarding this.

That is indeed strange.

Reading the code, I see that at the end of a VACUUM, "datfrozenxid" is set
to the minimum value of all "relfrozenxid" for all tables in the database
obtained with a sequential scan of "pg_class".

The only exception is that if a table is found where "relfrozenxid" is in the
future, nothing is done.  The comment suggests that that should normally not
happen, but "has been known to arise due to bugs in pg_upgrade".

- Can you find tables with negative "relfrozenxid" age in "pg_class"?

- The other option is that a VACUUM finished between the query of "pg_database"
  and the query of "pg_class".  Is that an option?

You should tune autovacuum to be more aggressive so that it gets done processing
the tables.  To get rid of the immediate problem, you could schedule a manual
VACUUM of the tables to which the TOAST table belong.

Yours,
Laurenz Albe


Re: Finding out the aging tables for vacuuming to prevent data losson transaction id wraparound

От
Subramanian Krishnan
Дата:
Hello Laurenz,

Thank you for taking the time out to read through and problem description and responding back.

After posting the question on the forum, I continued investigating possible root causes.  Through a flash of intuition I suspected if it could be a table which is strictly not a part of the database (not listed in pg_class) which could be contributing factor.

pg_shdepend fitted the bill. So I did a vacuum on pg_catalog.pg_shdepend and post that re-ran the DB age query. The trick worked and the age came down to match the max age entry in pg_class. To be on the safer side, I did vacuuming of pg_shdescription and pg_shseclabel as well.

Your suggestion of:
1) Making AV more aggressive and
2) Tiding over the present problem of aged TOAST tables though manual/scripted VACUUM is valid and we are working on that currently.

Thanks and Regards,
Subu

On Mon, Mar 5, 2018 at 2:54 PM, Laurenz Albe <laurenz.albe@cybertec.at> wrote:
Subramanian Krishnan wrote:
> We are running PostgreSQL 9.4.14 and recently we received an alert for transaction id
> crossing the 1 billion mark. Since we can hit the transaction id wraparound issue for aging tables,
> we investigated by running the following queries:
>
> mpsdb=> select * from txid_current();
>  txid_current
> --------------
>    1100826671
> (1 row)
>
> mpsdb=> SELECT datname, age(datfrozenxid) FROM pg_database ORDER BY 2 DESC LIMIT 20;
>   datname  |    age
> -----------+------------
>  mpsdb     | 1087909867
>  rdsadmin  | 1000562517
>  postgres  |  950697143
>  template0 |  900709601
>  template1 |  900697643
> (5 rows)
>
> mpsdb=> SELECT c.relname as table_name, c.relkind as type, age(c.relfrozenxid) as age, c.relfrozenxid FROM pg_class AS c WHERE age(c.relfrozenxid) <> 2147483647 ORDER BY 3 DESC LIMIT 20;
>     table_name     | type |    age     | relfrozenxid
> -------------------+------+------------+--------------
>  pg_toast_22985917 | t    | 1042877986 |     57941433
>  pg_toast_22986301 | t    | 1042877877 |     57941542
>  pg_toast_23823161 | t    | 1041791836 |     59027583
>  pg_toast_23823242 | t    | 1041791776 |     59027643
>  pg_toast_23904594 | t    | 1041658236 |     59161183
>  pg_toast_23904629 | t    | 1041658221 |     59161198
>  pg_toast_23904640 | t    | 1041658208 |     59161211
>  pg_toast_23904648 | t    | 1041658200 |     59161219
>  pg_toast_23904658 | t    | 1041658191 |     59161228
>  pg_toast_23904666 | t    | 1041658183 |     59161236
>  pg_toast_23904674 | t    | 1041658175 |     59161244
>  pg_toast_23904682 | t    | 1041658169 |     59161250
>  pg_toast_23904690 | t    | 1041658161 |     59161258
>  pg_toast_23904698 | t    | 1041658156 |     59161263
>  pg_toast_23904706 | t    | 1041658149 |     59161270
>  pg_toast_23904714 | t    | 1041658129 |     59161290
>  pg_toast_23904722 | t    | 1041658096 |     59161323
>  pg_toast_23904730 | t    | 1041658090 |     59161329
>  pg_toast_23904738 | t    | 1041658085 |     59161334
>  pg_toast_23904746 | t    | 1041658077 |     59161342
> (20 rows)
>
>
> mpsdb=> SELECT datname, usename, pid, waiting, current_timestamp - xact_start AS xact_runtime, query FROM pg_stat_activity WHERE upper(query) like '%VACUUM%' ORDER BY xact_start;
> datname | usename  |  pid  | waiting |  xact_runtime   |                                 query
> ---------+----------+-------+---------+-----------------+------------------------------------------------------------------------
>  mpsdb   | rdsadmin | 13833 | f       | 00:00:14.161262 | autovacuum: VACUUM pg_toast.pg_toast_242649675 (to prevent wraparound)
>  mpsdb   | rdsadmin | 14565 | f       | 00:00:11.735571 | autovacuum: VACUUM pg_toast.pg_toast_242649757 (to prevent wraparound)
>  mpsdb   | rdsadmin | 15315 | f       | 00:00:09.303762 | autovacuum: VACUUM pg_toast.pg_toast_242649819 (to prevent wraparound)
>  mpsdb   | rdsadmin | 14935 | f       | 00:00:06.893078 | autovacuum: VACUUM pg_toast.pg_toast_243226657 (to prevent wraparound)
>  mpsdb   | rdsadmin | 15851 | f       | 00:00:04.322474 | autovacuum: VACUUM pg_toast.pg_toast_243227582 (to prevent wraparound)
>  mpsdb   | rdsadmin | 15615 | f       | 00:00:01.768495 | autovacuum: VACUUM pg_toast.pg_toast_243226332 (to prevent wraparound)
>
> It is clear that we need to vacuum the most aged pg_toast tables which are not yet picked up by the autovacuum process.
>
> The question which is bothering us though is:
> As per the queries we ran the most aged transaction and table is 1042877986 (pg_toast_22985917).
> And as per PostgreSQL pg_database documentation the most aged table/transaction in pg_class determines
> age of the database (via the datfrozenxid value). If that is the case we would have expected the age
> of mpsdb database to be 1042877986 (or in this range) but what we see as age is 1087909867.
>
> Does this mean there are other aging tables we are not aware of? If yes, how do we get to those
> tables since we have already queried pg_class for the most aged table?
>
> Request any help/guidance we can get regarding this.

That is indeed strange.

Reading the code, I see that at the end of a VACUUM, "datfrozenxid" is set
to the minimum value of all "relfrozenxid" for all tables in the database
obtained with a sequential scan of "pg_class".

The only exception is that if a table is found where "relfrozenxid" is in the
future, nothing is done.  The comment suggests that that should normally not
happen, but "has been known to arise due to bugs in pg_upgrade".

- Can you find tables with negative "relfrozenxid" age in "pg_class"?

- The other option is that a VACUUM finished between the query of "pg_database"
  and the query of "pg_class".  Is that an option?

You should tune autovacuum to be more aggressive so that it gets done processing
the tables.  To get rid of the immediate problem, you could schedule a manual
VACUUM of the tables to which the TOAST table belong.

Yours,
Laurenz Albe

Re: Finding out the aging tables for vacuuming to prevent data losson transaction id wraparound

От
Fabio Pardi
Дата:
From my experience is good practice to:

- Monitor the age of your records (we use icinga/nagios for that) to know on time when things are going wrong

- Schedule a periodic VACUUM of your db, if possible. Eg: if your db is heavily used during the day but resting at
night,then during the night is a good idea to run a VACUUM. This will also offload your db during the day, because less
autovacuumjobs will be triggered.
 

As additional note, if I remember correctly, a vacuum will be triggered to prevent wraparound also if you set
'autovacuum=off'in your configuration. That specific vacuum will have priority over normal vacuums.
 

For statistical purposes, a good formula to understand your actual 'burn rate' is to periodically run: 

SELECT * FROM txid_current();

and compare results over time to have a good estimate on how many xid you are using daily.


Regards,

Fabio





On 03/06/2018 12:54 PM, Subramanian Krishnan wrote:
> Hello Laurenz,
> 
> Thank you for taking the time out to read through and problem description and responding back.
> 
> After posting the question on the forum, I continued investigating possible root causes.  Through a flash of
intuitionI suspected if it could be a table which is strictly not a part of the database (not listed in pg_class) which
couldbe contributing factor.
 
> 
> pg_shdepend fitted the bill. So I did a vacuum on pg_catalog.pg_shdepend and post that re-ran the DB age query. The
trickworked and the age came down to match the max age entry in pg_class. To be on the safer side, I did vacuuming of
pg_shdescriptionand pg_shseclabel as well.
 
> 
> Your suggestion of:
> 1) Making AV more aggressive and
> 2) Tiding over the present problem of aged TOAST tables though manual/scripted VACUUM is valid and we are working on
thatcurrently.
 
> 
> Thanks and Regards,
> Subu
> 
> On Mon, Mar 5, 2018 at 2:54 PM, Laurenz Albe <laurenz.albe@cybertec.at <mailto:laurenz.albe@cybertec.at>> wrote:
> 
>     Subramanian Krishnan wrote:
>     > We are running PostgreSQL 9.4.14 and recently we received an alert for transaction id
>     > crossing the 1 billion mark. Since we can hit the transaction id wraparound issue for aging tables,
>     > we investigated by running the following queries:
>     >
>     > mpsdb=> select * from txid_current();
>     >  txid_current
>     > --------------
>     >    1100826671
>     > (1 row)
>     >
>     > mpsdb=> SELECT datname, age(datfrozenxid) FROM pg_database ORDER BY 2 DESC LIMIT 20;
>     >   datname  |    age
>     > -----------+------------
>     >  mpsdb     | 1087909867
>     >  rdsadmin  | 1000562517
>     >  postgres  |  950697143
>     >  template0 |  900709601
>     >  template1 |  900697643
>     > (5 rows)
>     >
>     > mpsdb=> SELECT c.relname as table_name, c.relkind as type, age(c.relfrozenxid) as age, c.relfrozenxid FROM
pg_classAS c WHERE age(c.relfrozenxid) <> 2147483647 ORDER BY 3 DESC LIMIT 20;
 
>     >     table_name     | type |    age     | relfrozenxid
>     > -------------------+------+------------+--------------
>     >  pg_toast_22985917 | t    | 1042877986 |     57941433
>     >  pg_toast_22986301 | t    | 1042877877 |     57941542
>     >  pg_toast_23823161 | t    | 1041791836 |     59027583
>     >  pg_toast_23823242 | t    | 1041791776 |     59027643
>     >  pg_toast_23904594 | t    | 1041658236 |     59161183
>     >  pg_toast_23904629 | t    | 1041658221 |     59161198
>     >  pg_toast_23904640 | t    | 1041658208 |     59161211
>     >  pg_toast_23904648 | t    | 1041658200 |     59161219
>     >  pg_toast_23904658 | t    | 1041658191 |     59161228
>     >  pg_toast_23904666 | t    | 1041658183 |     59161236
>     >  pg_toast_23904674 | t    | 1041658175 |     59161244
>     >  pg_toast_23904682 | t    | 1041658169 |     59161250
>     >  pg_toast_23904690 | t    | 1041658161 |     59161258
>     >  pg_toast_23904698 | t    | 1041658156 |     59161263
>     >  pg_toast_23904706 | t    | 1041658149 |     59161270
>     >  pg_toast_23904714 | t    | 1041658129 |     59161290
>     >  pg_toast_23904722 | t    | 1041658096 |     59161323
>     >  pg_toast_23904730 | t    | 1041658090 |     59161329
>     >  pg_toast_23904738 | t    | 1041658085 |     59161334
>     >  pg_toast_23904746 | t    | 1041658077 |     59161342
>     > (20 rows)
>     >
>     >
>     > mpsdb=> SELECT datname, usename, pid, waiting, current_timestamp - xact_start AS xact_runtime, query FROM
pg_stat_activityWHERE upper(query) like '%VACUUM%' ORDER BY xact_start;
 
>     > datname | usename  |  pid  | waiting |  xact_runtime   |                                 query
>     >
---------+----------+-------+---------+-----------------+------------------------------------------------------------------------
>     >  mpsdb   | rdsadmin | 13833 | f       | 00:00:14.161262 | autovacuum: VACUUM pg_toast.pg_toast_242649675 (to
preventwraparound)
 
>     >  mpsdb   | rdsadmin | 14565 | f       | 00:00:11.735571 | autovacuum: VACUUM pg_toast.pg_toast_242649757 (to
preventwraparound)
 
>     >  mpsdb   | rdsadmin | 15315 | f       | 00:00:09.303762 | autovacuum: VACUUM pg_toast.pg_toast_242649819 (to
preventwraparound)
 
>     >  mpsdb   | rdsadmin | 14935 | f       | 00:00:06.893078 | autovacuum: VACUUM pg_toast.pg_toast_243226657 (to
preventwraparound)
 
>     >  mpsdb   | rdsadmin | 15851 | f       | 00:00:04.322474 | autovacuum: VACUUM pg_toast.pg_toast_243227582 (to
preventwraparound)
 
>     >  mpsdb   | rdsadmin | 15615 | f       | 00:00:01.768495 | autovacuum: VACUUM pg_toast.pg_toast_243226332 (to
preventwraparound)
 
>     >
>     > It is clear that we need to vacuum the most aged pg_toast tables which are not yet picked up by the autovacuum
process.
>     >
>     > The question which is bothering us though is:
>     > As per the queries we ran the most aged transaction and table is 1042877986 (pg_toast_22985917).
>     > And as per PostgreSQL pg_database documentation the most aged table/transaction in pg_class determines
>     > age of the database (via the datfrozenxid value). If that is the case we would have expected the age
>     > of mpsdb database to be 1042877986 (or in this range) but what we see as age is 1087909867.
>     >
>     > Does this mean there are other aging tables we are not aware of? If yes, how do we get to those
>     > tables since we have already queried pg_class for the most aged table?
>     >
>     > Request any help/guidance we can get regarding this.
> 
>     That is indeed strange.
> 
>     Reading the code, I see that at the end of a VACUUM, "datfrozenxid" is set
>     to the minimum value of all "relfrozenxid" for all tables in the database
>     obtained with a sequential scan of "pg_class".
> 
>     The only exception is that if a table is found where "relfrozenxid" is in the
>     future, nothing is done.  The comment suggests that that should normally not
>     happen, but "has been known to arise due to bugs in pg_upgrade".
> 
>     - Can you find tables with negative "relfrozenxid" age in "pg_class"?
> 
>     - The other option is that a VACUUM finished between the query of "pg_database"
>       and the query of "pg_class".  Is that an option?
> 
>     You should tune autovacuum to be more aggressive so that it gets done processing
>     the tables.  To get rid of the immediate problem, you could schedule a manual
>     VACUUM of the tables to which the TOAST table belong.
> 
>     Yours,
>     Laurenz Albe
> 
>