Re: Performance query about large tables, lots of concurrent access

Bill Moran wrote:
> In response to Karl Wright <kwright@metacarta.com>:
>
>> Alvaro Herrera wrote:
>>> Karl Wright wrote:
>>>> Alvaro Herrera wrote:
>>>>> Karl Wright wrote:
>>>>>
>>>>>> This particular run lasted four days before a VACUUM became essential.
>>>>>> The symptom that indicates that VACUUM is needed seems to be that the
>>>>>> CPU usage of any given postgresql query skyrockets.  Is this essentially
>>>>>> correct?
>>>>> Are you saying you weren't used to run VACUUM all the time?  If so,
>>>>> that's where the problem lies.
>>>> Postgresql 7.4 VACUUM runs for so long that starting it with a cron job
>>>> even every 24 hours caused multiple instances of VACUUM to eventually be
>>>> running in my case.  So I tried to find a VACUUM schedule that permitted
>>>> each individual vacuum to finish before the next one started.  A vacuum
>>>> seemed to require 4-5 days with this particular database - or at least
>>>> it did for 7.4.  So I had the VACUUM schedule set to run every six days.
>>> How large is the database?  I must admit I have never seen a database
>>> that took 4 days to vacuum.  This could mean that your database is
>>> humongous, or that the vacuum strategy is wrong for some reason.
>> The database is humongus, and the machine is under intense load.  On the
>> instance where this long vacuum occurred, there were several large
>> tables - one with 7,000,000 rows, one with 14,000,000, one with
>> 140,000,000, and one with 250,000,000.
>
> Don't rule out the possibility that the only way to fix this _might_ be to
> throw more hardware at it.  Proper configuration can buy you a lot, but if
> your usage is exceeding the available bandwidth of the IO subsystem, the
> only way you're going to get better performance is to put in a faster IO
> subsystem.
>
>>> You know that you can run vacuum on particular tables, right?  It would
>>> be probably a good idea to run vacuum on the most updated tables, and
>>> leave alone those that are not or little updated (hopefully the biggest;
>>> this would mean that an almost-complete vacuum run would take much less
>>> than a whole day).
>> Yeah, sorry, that doesn't apply here.
>
> Why not?  I see no reason why an appropriate autovaccum schedule would not
> apply to your scenario.  I'm not saying it does, only that your response
> does not indicate that it doesn't, and thus I'm concerned that you're
> writing autovacuum off without proper research.
>

I'm not writing off autovacuum - just the concept that the large tables
aren't the ones that are changing.  Unfortunately, they *are* the most
dynamically updated.

Karl