On Fri, Mar 9, 2012 at 8:25 AM, Dimitri Fontaine <dimitri@2ndquadrant.fr> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> It's been bugging me for a while now that we don't have a prewarming
>> utility, for a couple of reasons, including:
>>
>> 1. Our customers look at me funny when I suggest that they use
>> pg_relation_filepath() and /bin/dd for this purpose.
>
> Try telling them about pgfincore maybe.
>
> https://github.com/klando/pgfincore
Oh, huh. I had no idea that pgfincore could do that. I thought that
was just for introspection; I didn't realize it could actually move
data around for you.
>> 2. Sometimes when I'm benchmarking stuff, I want to get all the data
>> cached in shared_buffers. This is surprisingly hard to do if the size
>> of any relation involved is >=1/4 of shared buffers, because the
>> BAS_BULKREAD stuff kicks in. You can do it by repeatedly seq-scanning
>> the relation - eventually all the blocks trickle in - but it takes a
>> long time, and that's annoying.
>
> That reminds me of something…
>
> cedric=# select * from pgfadvise_willneed('pgbench_accounts');
> relpath | os_page_size | rel_os_pages | os_pages_free
> --------------------+--------------+--------------+---------------
> base/11874/16447 | 4096 | 262144 | 169138
> base/11874/16447.1 | 4096 | 65726 | 103352
> (2 rows)
>
> Time: 4462,936 ms
That's not the same thing. That's pulling them into the OS cache, not
shared_buffers.
> Is it possible with your tool to snapshot the OS and PostgreSQL cache in
> order to warm an Hot Standby server?
Nope. It doesn't have any capabilities to probe for information,
because I knew those things already existed in pg_buffercache and
pgfincore, and also because they weren't what I needed to solve my
immediate problem, which was a way to get the entirety of a relation
into shared_buffers.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
