Обсуждение: backend_xmin in pg_stat_replication
Hi,
application_name | txid_snapshot_xmin | backend_xmin | ?column?
------------------+--------------------+--------------+------------
xxxxxxxxxx | 6957042833 | 2662075435 | 4294967398
if I understand it correctly, backend_xmin in pg_stat_replication is the xmin that's reported back by hot_standby_feedback. Given there are no long-running transactions on the replica, I presume that value should be pretty close to the xmin field of any recent snapshots on the master. This is true for all my databases but one:
select application_name,
txid_snapshot_xmin(txid_current_snapshot()),
backend_xmin::TEXT::BIGINT,
txid_snapshot_xmin(txid_current_snapshot())-backend_xmin::TEXT::BIGINT
from pg_stat_replication;
application_name | txid_snapshot_xmin | backend_xmin | ?column?
------------------+--------------------+--------------+------------
xxxxxxxxxx | 6957042833 | 2662075435 | 4294967398
Over time, that backend_xmin is slowly advancing.
If I call txid_current_snapshot() in a new session on the replica, I get reasonable numbers but the backend_xmin in pg_stat_activity is equally off.
select backend_xmin, txid_current_snapshot()
from pg_stat_activity
where backend_xmin is not null;
backend_xmin | txid_current_snapshot
--------------+------------------------
2662207433 | 6957174729:6957174729:
--------------+------------------------
2662207433 | 6957174729:6957174729:
Is that expected behavior? Or is there anything wrong?
All other backends are idle and all but one pretty fresh. One has been running for about 2 months with short-lasting transactions every now and again.
Thanks,
Torsten
Hi, On 2018-10-01 12:20:26 +0200, Torsten Förtsch wrote: > if I understand it correctly, backend_xmin in pg_stat_replication is the > xmin that's reported back by hot_standby_feedback. Given there are no > long-running transactions on the replica, I presume that value should be > pretty close to the xmin field of any recent snapshots on the master. This > is true for all my databases but one: > > select application_name, > txid_snapshot_xmin(txid_current_snapshot()), > backend_xmin::TEXT::BIGINT, > > txid_snapshot_xmin(txid_current_snapshot())-backend_xmin::TEXT::BIGINT > from pg_stat_replication; > > application_name | txid_snapshot_xmin | backend_xmin | ?column? > ------------------+--------------------+--------------+------------ > xxxxxxxxxx | 6 957 042 833 | 2 662 075 435 | 4 294 967 398 I don't think the calculation you're doing here is correct. backend_xmin is an xid (max 2^32-1), whereas txid_snapshot_xmin returns an xid *with* epoch (max 2^64-1). What you're measuring here is simply the fact that the xid counter has wrapped around. Greetings, Andres Freund
Thanks a lot! So, the correct calculation is like this:
application_name | txid_snapshot_xmin | backend_xmin | ?column?
------------------+--------------------+--------------+----------
xxxxxxxxxx | 6960964080 | 2665996642 | 142
select application_name,
txid_snapshot_xmin(txid_current_snapshot()),
backend_xmin::TEXT::BIGINT,
(txid_snapshot_xmin(txid_current_snapshot())-backend_xmin::TEXT::BIGINT)%(2^32)::BIGINT
from pg_stat_replication;
application_name | txid_snapshot_xmin | backend_xmin | ?column?
------------------+--------------------+--------------+----------
xxxxxxxxxx | 6960964080 | 2665996642 | 142
That makes more sense.
On Mon, Oct 1, 2018 at 5:32 PM Andres Freund <andres@anarazel.de> wrote:
Hi,
On 2018-10-01 12:20:26 +0200, Torsten Förtsch wrote:
> if I understand it correctly, backend_xmin in pg_stat_replication is the
> xmin that's reported back by hot_standby_feedback. Given there are no
> long-running transactions on the replica, I presume that value should be
> pretty close to the xmin field of any recent snapshots on the master. This
> is true for all my databases but one:
>
> select application_name,
> txid_snapshot_xmin(txid_current_snapshot()),
> backend_xmin::TEXT::BIGINT,
>
> txid_snapshot_xmin(txid_current_snapshot())-backend_xmin::TEXT::BIGINT
> from pg_stat_replication;
>
> application_name | txid_snapshot_xmin | backend_xmin | ?column?
> ------------------+--------------------+--------------+------------
> xxxxxxxxxx | 6 957 042 833 | 2 662 075 435 | 4 294 967 398
I don't think the calculation you're doing here is correct.
backend_xmin is an xid (max 2^32-1), whereas txid_snapshot_xmin returns
an xid *with* epoch (max 2^64-1). What you're measuring here is simply
the fact that the xid counter has wrapped around.
Greetings,
Andres Freund