Re: Weird spikes in delay for async streaming replication on 9.1

On Thu, 26 Feb 2015 10:51:55 -0500
John Scalia <jayknowsunix(at)gmail(dot)com> wrote:

> OK, if it's asynchronous, is your script checking that primary isn't
> holding up closing out and transmitting the latest WAL segment during
> these times?

No. The checking script is dead simple. It's a perl script that does
more-or-less the equivalent of this. $master_dbi is a DBI handle
pointing to the master database server and $standby_dbi is one pointing
to the standby. Pseudocode is shown below (the real code actually measures
the duration in much finer increments than 1s, but you get the idea...)

my $time = time();

# There's one row in the table with key column 'Timestamp'
$master_dbi->do("UPDATE table SET value = ? WHERE key = 'Timestamp'", undef, $time);

my $start = time();
while(..) {
$row = $standby_dbi->fetchrow_arrayref("SELECT * FROM table WHERE key = 'Timestamp'");
# Check that we got back the same $time we put in and if so,
# break out of the while loop; otherwise pause for a bit.
}
# See how long that took
my $duration = time() - $start;

> And if the Standby really needs to be up to date, why
> not try synchronous replication?

It doesn't need to be totally up-to-date. It can lag by up to 30s without
impacting our application. And we don't use synchronous replication because
then a network problem would stall all the write transactions on the primary
and that would be fatal.

Regards,

David.