Re: Multiple buffer cache?

Bret,

Thank you for your response!

>> Greetings,
>>
>> Is there a way of configuring PostgreSQL so that one specific table would
>> use, say, 4GB of buffer cache while other tables would use the rest?
>>
>> I would like to keep the table and its indexes always in "hot"
>> state, so that
>> other queries won't pollute this part of the buffer cache. It would ensure
>> reliable performance and much less disk IOPS working with the table.
>>
>
> Fiddling with the buffer cache like that would require some sort of
> OS support, if I'm not mistaken in what you're asking for.
>
I am talking about PostgreSQL buffer cache not OS level. I believe it
has nothing to do with
OS support.

It would be great to have support of multiple cache buffers assigned to
different set of tables.
Having this implemented, I would assign frequently accessed
configuration tables (selects
and updates) to one buffer and historical tables (lots of insert
operations) to another buffer, so
the sets would use independent buffers and won't affect each other.

> And then, even if the support is there, you'd need to outline exactly
> how you're planning on pushing this button.
>
> Specifically, what's your usage pattern that would make this a
> win for you?
>
Let me explain. I have a very busy application generating thousands of
SQLs per second.
There is an application level cache built into the application already.

The important part is that once per hour the application writes
collected data to huge historical
tables (100M up-to billions of records, partitioned). Since it happens
every hour database buffer
cache is already overwritten by data and indexes of other tables, so the
write operation is very
slow and requires huge amount of disk seeks causing 50-100x drop of
performance.

So, my idea is to assign a separate buffer cache for the historical
tables. It would guarantee that
index data is always cached, so the write operation will be very fast.

Is it possible? Is there any other techniques available?

> If the table and its indexes can already fit into the buffer cache,
> and it's as commonly accessed as you think it is, the OS should
> probably have it cached anyway.
>
I see what you are saying but the problem is that it is normally
accessed once per hour only.

Any thoughts?

Alexei