Re: Calculating Replication Lag - units

On 06/26/2012 05:11 AM, Stuart Bishop wrote:
> On Tue, Jun 26, 2012 at 6:21 AM, David Kerr<dmk@mr-paradox.net>  wrote:
>> On Mon, Jun 25, 2012 at 02:17:22PM -0700, Steve Crawford wrote:
>> - On 06/25/2012 01:17 PM, David Kerr wrote:
>> ->Howdy,
>> ->
>> ->When calculating Replication lag, I know that we have to compare the
>> ->pg_current_xlog_location
>> ->to pg_last_xlog_receive_location, etc. but what I'm trying to figure out
>> ->is what are
>> ->the units that I'm left with after the calculation.
>> ->
>> ->(i.e., does the xlog_location imply some time value?)
>> ->
>> ->Here's the output of the (slightly modified script)
>> ->Master: 5003964876715
>> ->Receive: 5003964876715
>> ->Replay: 5003964765203
>> ->
>> ->receive.value 0
>> ->apply.value 111512
>> ->
>> ->111512 isn't inherently useful to me on its own.
>> ->
>> ->Any tips?
>> ->
>> - How about now()-pg_last_xact_replay_timestamp() (however this can be a
>> - large number if there have not been any recent transactions on the
>> - master). I suppose you could do something like:
>> -
>> - case when pg_last_xlog_receive_location() =
>> - pg_last_xlog_replay_location() then '0 seconds'::interval
>> - else now()-pg_last_xact_replay_timestamp() end as log_delay;
>>
>> i don't know for sure that 111512 is a time value.. that's kind of
>> what i'm wondering. If i knew that it was like miliseconds or something
>> that would be helpful.
>
> On the hot standby:
>
>      SELECT now()-pg_last_xact_replay_timestamp() AS lag;
>
> This gives you the lag time as a PostgreSQL interval.
>
> (It also might give you a value if you run it on a database that is
> not a hot standby if it started in recovery mode).
>
> It seems difficult or impossible to calculate this on the master.
>
>

d'oh.

looking closer at the script I was using, it says in there that's it's
kb xlog, so not time but size. That explains a lot.

I like the the replay_timestamp method better though fits in better to
my management's question of "how much data would we lose?".

Thanks all.