Measuring replication lag time

[Stuart Bishop]

Hi.

I need to measure how far in the past a hot standby is, async
streaming replication.

On the Hot Standby, "select
age(current_timestamp,pg_last_xact_replay_timestamp())" gets me this
(or close enough for my purposes - I understand that if there are no
updates, there are no logs to replay and the lag time will increase).

Is there some way to get this same information on the master?
pg_stat_replication contains the log information, but I can't see how
to map this to a timestamp.

Is there a better way of measuring this?

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Stuart Bishop <[hidden email]>
http://www.stuartbishop.net/

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[Venkat Balaji]

On Wed, Feb 22, 2012 at 5:40 PM, Stuart Bishop <[hidden email]> wrote:

Hi.

I need to measure how far in the past a hot standby is, async
streaming replication.

On the Hot Standby, "select
age(current_timestamp,pg_last_xact_replay_timestamp())" gets me this
(or close enough for my purposes - I understand that if there are no
updates, there are no logs to replay and the lag time will increase).

Is there some way to get this same information on the master?
pg_stat_replication contains the log information, but I can't see how
to map this to a timestamp.

Is there a better way of measuring this?

Comparing "pg_controldata" output on prod and standby might help you with this. 

Thanks,

VB

[Greg Williamson-3]

Stuart Bishop shaped the aether to ask:

> Hi.
>
> I need to measure how far in the past a hot standby is, async
> streaming replication.
>


Not sure if this will help, but we are using repmgr <https://github.com/greg2ndQuadrant/repmgr>; it sets up a monitoring schema which we poll )see the "Monitoring and Testing" section ... study their source code some and see how they come up with lag times.

HTH,

Greg WiIliamson
DBA
Powerreviews dot com


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[Stuart Bishop]

On Thu, Feb 23, 2012 at 2:58 AM, Greg Williamson
<[hidden email]> wrote:
> Stuart Bishop shaped the aether to ask:
>
>> Hi.
>>
>> I need to measure how far in the past a hot standby is, async
>> streaming replication.
>
> Not sure if this will help, but we are using repmgr <https://github.com/greg2ndQuadrant/repmgr>; it sets up a monitoring schema which we poll )see the "Monitoring and Testing" section ... study their source code some and see how they come up with lag times.

Might help indeed. My existing solution already has a small daemon (I
can't always query the Slony-I sl_status view fast enough for load
balancing web requests, so I maintain a cache). But repmgr seems to
cover other work I need to do to keep ops happy so something for me to
look closer at.


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[Samba]

Comparing "pg_controldata" output on prod and standby might help you with this.

We do use this approach and it is pretty reliable and gives time lag up to the granularity of checkpoint_timeout.

On Thu, Feb 23, 2012 at 11:51 AM, Stuart Bishop <[hidden email]> wrote:

On Thu, Feb 23, 2012 at 2:58 AM, Greg Williamson
<[hidden email]> wrote:
> Stuart Bishop shaped the aether to ask:
>
>> Hi.
>>
>> I need to measure how far in the past a hot standby is, async
>> streaming replication.
>
> Not sure if this will help, but we are using repmgr <https://github.com/greg2ndQuadrant/repmgr>; it sets up a monitoring schema which we poll )see the "Monitoring and Testing" section ... study their source code some and see how they come up with lag times.

Might help indeed. My existing solution already has a small daemon (I
can't always query the Slony-I sl_status view fast enough for load
balancing web requests, so I maintain a cache). But repmgr seems to
cover other work I need to do to keep ops happy so something for me to
look closer at.

--
Stuart Bishop <[hidden email]>
http://www.stuartbishop.net/

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