Re: [PROPOSAL] VACUUM Progress Checker.

On 2015/12/10 20:46, Michael Paquier wrote:
> On Thu, Dec 10, 2015 at 7:23 PM, Amit Langote
> <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>> AIUI, the counts published via stats collector are updated asynchronously
>> w.r.t. operations they count and mostly as aggregate figures. For example,
>> PgStat_StatTabEntry.blocks_fetched. IOW, we never see
>> pg_statio_all_tables.heap_blks_read updating as a scan reads blocks. Maybe
>> that helps keep traffic to pgstat collector to sane levels. But that is
>> not to mean that I think controlling stats collector levels was the only
>> design consideration behind how such counters are published.
>>
>> In case of reporting counters as progress info, it seems we might have to
>> send too many PgStat_Msg's, for example, for every block we finish
>> processing during vacuum. That kind of message traffic may swamp the
>> collector. Then we need to see the updated counters from other counters in
>> near real-time though that may be possible with suitable (build?)
>> configuration.
>
> As far as I understand it, the basic reason why this patch exists is
> to allow a DBA to have a hint of the progress of a VACUUM that may be
> taking minutes, or say hours, which is something we don't have now. So
> it seems perfectly fine to me to report this information
> asynchronously with a bit of lag. Why would we need so much precision
> in the report?

Sorry, I didn't mean to overstate this requirement. I agree precise
real-time reporting of progress info is not such a stringent requirement
from the patch. The point regarding whether we should storm the collector
with progress info messages still holds, IMHO.

Thanks,
Amit