Re: BDR concern/issue

On 3 March 2016 at 07:54, cchee-ob <carter(dot)chee(at)objectbrains(dot)com> wrote:

> I queried pg_replication_slots after I removed an BDR node and I noticed a
> slot_name that isn't in bdr.bdr_node_slots. And active is 'f' and it has
> been retaining bytes. Should I be concerned and is there a way to remove
> it. I do still have one UDR node which is running
> (bdr_16385_6228994276814368133_1_16384). Any suggestions?
>
> svp2=# SELECT
> slot_name, database, active,
> pg_xlog_location_diff(pg_current_xlog_insert_location(), restart_lsn)
> AS retained_bytes
> FROM pg_replication_slots
> WHERE plugin = 'bdr';
> slot_name | database | active |
> retained_bytes
>
> -----------------------------------------+----------+--------+----------------
> bdr_16385_6206441431541275808_1_16385__ | svp2 | f |
> 410036551440
> bdr_16385_6228994276814368133_1_16384__ | svp2 | t |
> 285760
> (2 rows)
>
> svp2=# SELECT * FROM pg_stat_replication;
> -[ RECORD 1 ]----+-------------------------------------------
> pid | 1122
> usesysid | 10
> usename | postgres
> application_name | bdr (6228994276814368133,1,16384,):receive
> client_addr | 10.253.0.8
> client_hostname |
> client_port | 43724
> backend_start | 2016-02-25 17:53:21.10519-08
> backend_xmin |
> state | streaming
> sent_location | 184/AE210BC8
> write_location | 184/AE210BC8
> flush_location | 184/AE20E748
> replay_location | 184/AE210BC8
> sync_priority | 0
> sync_state | async
>
> svp2=# select * from bdr.bdr_node_slots;
> node_name | slot_name
> -----------+-----------
> (0 rows)
>
> svp2=# SELECT * FROM bdr.bdr_nodes;
> -[ RECORD 1 ]------+------------------------------------------
> node_sysid | 6206439726032130602
> node_timeline | 1
> node_dboid | 16385
> node_status | r
> node_name | BDR1
> node_local_dsn | host=10.253.228.105 port=5432 dbname=svp2
> node_init_from_dsn |
> -[ RECORD 2 ]------+------------------------------------------
> node_sysid | 6206440469625465777
> node_timeline | 1
> node_dboid | 16385
> node_status | k
> node_name | BDR2
> node_local_dsn | host=10.253.16.25 port=5432 dbname=svp2
> node_init_from_dsn | host=10.253.228.105 port=5432 dbname=svp2
>
>
If you have a slot you know is unused, drop it. You can check it's the slot
for the parted node by comparing the slot name against the bdr local node
identity for the parted node (see the bdr docs for relevant functions to
get node identity).

BDR makes a best-effort attempt at dropping slots when parting a node but
there are known race conditions. We really need a two-phase part, where we
first agree to part and *then* actually remove the node, but that's not yet
implemented.

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services