Thread: Estimating hot data size

Dne 16.2.2011 21:51, Chris Hoover napsal(a):
> All,
>
> I'm trying to estimate the size of my hot data set, and wanted to get
> some validation that I'm doing this correctly.
>
> Basically, I'm using the sum(heap_blks_read + idx_blks_read) from
> pg_statio_all_tables, and diffing the numbers over a period of time (1
> hour at least).  Is this a fair estimate?  The reason for doing this is
> we are looking at new server hardware, and I want to try and get enough
> ram on the machine to keep the hot data in memory plus provide room for
> growth.
>
> Thanks,
>
> Chris
>
> Example:
>
>
>
> *Time*
>
>
>
> *Total Blocks*
>
> 2011-02-16 11:25:34.621874-05
>
>
>
> 123,260,464,427.00
>
> 2011-02-16 12:25:46.486719-05
>
>
>
> 123,325,880,943.00
>
>
>
> To get the hot data for this hour (in KB), I'm taking:
>
>
>  (123,325,880,943.00 - 123,260,464,427.00)* 8 = 523,332,128KB
>
>
> Correct?

I doubt that, although I'm not sure what exactly you mean by hot data
set. I guess it's the data set you're working with frequently, right?

The first gotcha is that heap_blks_read counts only blocks not found in
shared buffers, so those 500MB is actually the amount of data read from
the disk (or filesystem cache). It does not say anything about how
frequently the data are used.

The second gotcha is that the same block may be counted repeatedly,
especially if it is not frequently used. It's counted for query A, then
it's removed from the cache (to be replaced by another block), and then
for another query B. So the number heap_blks_read does not mean there
were that many different blocks read from the disk.

What I'd recommend is to measure the cache hit ratio, i.e. this

   heap_blks_hit / (heap_blks_read + heap_blks_hit)

which means how efficient the cache is. Increase shared buffers until it
stops to increase - that's the hot data set size.

regards
Tomas

PS: The value heap_blks_hit does not actually mean the blocks were read
    from the disk - it might be read from filesystem cache (and there's
    not easy way to find out this AFAIK).

Chris Hoover wrote:
> Basically, I'm using the sum(heap_blks_read + idx_blks_read) from
> pg_statio_all_tables, and diffing the numbers over a period of time (1
> hour at least).  Is this a fair estimate?  The reason for doing this
> is we are looking at new server hardware, and I want to try and get
> enough ram on the machine to keep the hot data in memory plus provide
> room for growth.

Those two are measuring reads to the operating system, which isn't
really a good measure of the working data set.  If you switch to the
internal counters that measure what's already cached, that won't be
quite right either.  Those will be repeatedly measuring the same block,
on the truly hot ones, which inflates how big you'll think the working
set is relative to its true size.

If you visit http://projects.2ndquadrant.com/talks you'll find a talk
called "Inside the PostgreSQL Buffer Cache" that goes over how the cache
is actually managed within the database.  There's also some sample
queries that run after you install the pg_buffercache module into a
database.  Check out "Buffer contents summary, with percentages".
That's the only way to really measure what you're trying to see.  I will
sometimes set shared_buffers to a larger value than would normally be
optimal for a bit, just to get a better reading on what the hot data is.

If you also want to get an idea what's in the operating system cache,
the pgfincore module from http://pgfoundry.org/projects/pgfincore/ will
allow that on a Linux system.

--
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support  www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books