Обсуждение: [GENERAL] Transaction apply speed on the standby
Ver 9.6.1 In a streaming replication can it be assumed that if both primary and standby are of the same hardware, then the rate atwhich transactions are applied on the standby will be same as that on primary. Or standbys are always slower than primaryin applying transactions because of the way replication works. The reason why I am asking is that I am trying to find what are the circumstances when there can be a large gap between primaryand standby in async replication mode. Thanks
On Thu, Jan 26, 2017 at 8:34 AM, Rakesh Kumar <rakeshkumar464@outlook.com> wrote:
Ver 9.6.1
In a streaming replication can it be assumed that if both primary and standby are of the same hardware, then the rate at which transactions are applied on the standby will be same as that on primary. Or standbys are always slower than primary in applying transactions because of the way replication works.
It could go either way. The standby only has to apply the changes, not compute them, so if the primary does something like:
UPDATE foobar set col1 = (<slow select query returning one row and one column>) where col2=? ;
then the standby will replay it much faster than the primary needed to execute it.
On the other hand, replay is done single-threaded. If the primary has a lot of active concurrent connections, replaying them serially could be much slower than it took to produce them in the first place. This might be true of both CPU and of IO. If your IO is a big RAID system, the primary could keep multiple spindles active simultaneously by having multiple connections waiting on different pieces of data independently, while replay will wait on them serially. There is currently not a prefetch mechanism for replay.
Cheers,
Jeff
On Fri, Jan 27, 2017 at 3:34 AM, Rakesh Kumar <rakeshkumar464@outlook.com> wrote:
Ver 9.6.1
In a streaming replication can it be assumed that if both primary and standby are of the same hardware, then the rate at which transactions are applied on the standby will be same as that on primary. Or standbys are always slower than primary in applying transactions because of the way replication works.
The reason why I am asking is that I am trying to find what are the circumstances when there can be a large gap between primary and standby in async replication mode.
As already said, the replication sync between master and standby depends on lot of factors CPUs, Disks, Network bandwidth etc.
If it is the application demand that data has to be in complete sync on Standby at all times, then, the best way to go is with synchronous replication. In this mode PostgreSQL ensures all the changes on master are replicated to standby and master waits until changes are committed on standby. This is even more better in 9.6 with "remote_apply" to synchronous_commit parameter, which ensures data from master is actually replayed at synchronous standby. However, this type of replication mode can be a risk when there is a failure (Example : Network break down) and standby is not connected to master and during this time transactions on master wait until standby responds back.
Regards,
Venkata B N
Database Consultant