On 17-Jan-07, at 3:41 PM, Steve wrote:
Hey there;
I've been lurking on this list awhile, and I've been working with
postgres for a number of years so I'm not exactly new to this. But
I'm still having trouble getting a good balance of settings and I'd
like to see what other people think. We may also be willing to
hire a contractor to help tackle this problem if anyone is interested.
I've got an application here that runs large (in terms of length --
the queries have a lot of conditions in them) queries that can
potentially return millions of rows but on average probably return
tens of thousands of rows. It's read only for most of the day, and
pretty much all the queries except one are really fast.
However, each night we load data from a legacy cobol system into
the SQL system and then we summarize that data to make the reports
faster. This load process is intensely insert/update driven but
also has a hefty amount of selects as well. This load process is
taking ever longer to complete.
SO ... our goal here is to make this load process take less time.
It seems the big part is building the big summary table; this big
summary table is currently 9 million rows big. Every night, we
drop the table, re-create it, build the 9 million rows of data (we
use COPY to put hte data in when it's prepared, not INSERT), and
then build the indexes on it -- of which there are many.
Unfortunately this table gets queried in a lot of different ways
and needs these indexes; also unfortunately, we have operator class
indexes to support both ASC and DESC sorting on columns so these
are for all intents and purposes duplicate but required under
Postgres 8.1 (we've recently upgraded to Postgres 8.2, is this
still a requirement?)
Building these indexes takes forever! It's a long grind through
inserts and then building the indexes takes a hefty amount of time
too. (about 9 hours). Now, the application is likely part at
fault, and we're working to make it more efficient, but it has
nothing to do with the index building time. I'm wondering what we
can do to make this better if anything; would it be better to leave
the indexes on? It doesn't seem to be. Would it be better to use
INSERTs instead of copies? Doesn't seem to be.
Anyway -- ANYTHING we can do to make this go faster is
appreciated :) Here's some vital statistics:
- Machine is a 16 GB, 4 actual CPU dual-core opteron system using
SCSI discs. The disc configuration seems to be a good one, it's
the best of all the ones we've tested so far.
The basic problem here is simply writing all the data to disk. you
are building 9M rows of data plus numerous index's. How much data are
you actually writing to the disk. Try looking at iostat while this is
going on.
My guess is you are maxing out the disk write speed.
- The load process itself takes about 6 gigs of memory, the rest is
free for postgres because this is basically all the machine does.
- If this was your machine and situation, how would you lay out the
emmory settings? What would you set the FSM to? Would you leave
teh bgwriter on or off? We've already got FSYNC off because "data
integrity" doesn't matter -- this stuff is religeously backed up
and we've got no problem reinstalling it. Besides, in order for
this machine to go down, data integrity of the DB is the least of
the worries :)
Do wal_buffers/full_page_writes matter of FSYNC is off? If so,
what settings? What about checkpoints?
Not reallly, I'd have WAL buffers write to a ram disk
Any finally, any ideas on planner constants? Here's what I'm using:
seq_page_cost = 0.5 # measured on an arbitrary
scale
random_page_cost = 1.0 # same scale as above
cpu_tuple_cost = 0.001 # same scale as above
cpu_index_tuple_cost = 0.0001 # same scale as above
cpu_operator_cost = 0.00025 # same scale as above
effective_cache_size = 679006
as a general rule make shared buffers about 25% of free mem,
effective cache 75% but with a write intensive load like you have I
think the first thing to look at is write speed.
I really don't remember how I came up with that
effective_cache_size number....
Anyway... any advice would be appreciated :)
Steve
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