Re: pg_rewarm status

On Tue, Dec 17, 2013 at 8:02 AM, Jim Nasby <jim(at)nasby(dot)net> wrote:

> On 12/17/13, 8:34 AM, Robert Haas wrote:
>
>> On Tue, Dec 17, 2013 at 12:09 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
>> wrote:
>>
>>> I have used pg_prewarm during some of work related to Buffer Management
>>> and
>>> other performance related work. It is quite useful utility.
>>> +1 for reviving this patch for 9.4
>>>
>>
>> Any other votes?
>>
>
> We've had to manually code something that runs EXPLAIN ANALYZE SELECT *
> from a bunch of tables to warm our caches after a restart, but there's
> numerous flaws to that approach obviously.
>
> Unfortunately, what we really need to warm isn't the PG buffers, it's the
> FS cache, which I suspect this won't help. But I still see where just
> pg_buffers would be useful for a lot of folks, so +1.

Since it doesn't use directIO, you can't warm the PG buffers without also
warming FS cache as a side effect. That is why I like 'buffer' as the
default--if the data fits in shared_buffers, it warm those, otherwise it at
least warms the FS. If you want to only warm the FS cache, you can use
either the 'prefetch' or 'read' modes instead.

Cheers,

Jeff