Обсуждение: live and dead tuples are zero on slave running on tmpfs

Hello,

I have a master slave (read replica) setup running pg 9.4.4. I'm
trying to find the dead tuples out both the master and slave

Here's what it looks like on master:

# select relname ,n_live_tup ,n_dead_tup from pg_stat_user_tables;
     relname      | n_live_tup | n_dead_tup
------------------+------------+------------
 test_52     |    4998366 |          0
 test_v2     |   25182728 |    4086591
 test_1mrows |    1000127 |          0

That seems legit because I did an update to ~4million rows just before this.

Here's what it looks on slave though

 #select relname ,n_live_tup ,n_dead_tup from pg_stat_user_tables;
     relname      | n_live_tup | n_dead_tup
------------------+------------+------------
 test_52     |          0 |          0
 test_v2     |          0 |          0
 test_1mrows |          0 |          0

the postgres data directory on the slave is configured to a tmpfs
mounted store. Would this cause it to have zero live & dead tuples ?

postgres version on master and slave:
PostgreSQL 9.4.4 on x86_64-unknown-linux-gnu, compiled by gcc (Ubuntu
4.8.2-19ubuntu1) 4.8.2, 64-bit

-- Karthik

On Thu, Aug 20, 2015 at 12:55 AM, Karthik Viswanathan
<krthkv26@gmail.com> wrote:
> Hello,
>
> I have a master slave (read replica) setup running pg 9.4.4. I'm
> trying to find the dead tuples out both the master and slave
>
> Here's what it looks like on master:
>
> # select relname ,n_live_tup ,n_dead_tup from pg_stat_user_tables;
>      relname      | n_live_tup | n_dead_tup
> ------------------+------------+------------
>  test_52     |    4998366 |          0
>  test_v2     |   25182728 |    4086591
>  test_1mrows |    1000127 |          0
>
> That seems legit because I did an update to ~4million rows just before this.
>
> Here's what it looks on slave though
>
>  #select relname ,n_live_tup ,n_dead_tup from pg_stat_user_tables;
>      relname      | n_live_tup | n_dead_tup
> ------------------+------------+------------
>  test_52     |          0 |          0
>  test_v2     |          0 |          0
>  test_1mrows |          0 |          0
>
> the postgres data directory on the slave is configured to a tmpfs
> mounted store. Would this cause it to have zero live & dead tuples ?

Autovacuum process doesn't run on slave servers. This is the process
which updates the live and dead tuples. And also pg_stat_tmp files are
not copied from master to slave during backup.

Because of the above reasons, there are no stats available for the tables
in the slave server because of this reason it shows the 0 live and 0 dead
rows. This is expected.

Regards,
Hari Babu
Fujitsu Australia