On Mon, 20 Apr 2009, Stephen Frost wrote:
David,
* da...@lang.hm (da...@lang.hm) wrote:
I am working with the rsyslog developers to improve it's performance in
inserting log messages to databases.
Great!
currently they have a postgres interface that works like all the other
ones, where rsyslog formats an insert statement, passes that the the
interface module, that sends it to postgres (yes, each log as a seperate
transaction)
Ouch.
yep
the big win is going to be in changing the core of rsyslog so that it can
process multiple messages at a time (bundling them into a single
transaction)
Yup.
1. begin; insert; insert;...;end
Doing the insert in a transaction should definitely improve your
performance. Doing them as prepared statements would be good too, and
using binary mode would very likely help.
2. insert into table values (),(),(),()
Using this structure would be more database agnostic, but won't perform
as well as the COPY options I don't believe. It might be interesting to
do a large "insert into table values (),(),()" as a prepared statement,
but then you'd have to have different sizes for each different number of
items you want inserted.
on the other hand, when you have a full queue (lots of stuff to insert) is
when you need the performance the most. if it's enough of a win on the
database side, it could be worth more effort on the applicaiton side.
3. copy from stdin
(how do you tell it how many records to read from stdin, or that you
have given it everything without disconnecting)
Assuming you're using libpq, you just call PQputCopyEnd(). Then you
call PQgetResult() to check that everything worked ok.
one of the big questions is what value we will get by making things
database specififc (more below)
4. copy from stdin in binary mode
Binary mode, in general, should be faster. You should consider what
format the data is inside your application though (it's less useful to
use binary copy if you're having to convert from text to binary in your
application).
any idea what sort of difference binary mode would result in?
and each of the options above can be done with prepared statements,
stored procedures, or functions.
I know that using procedures or functions can let you do fancy things
like inserting the row(s) into the appropriate section of a partitioned
table
We would normally recommend having the database handle the partitioning
by using a trigger on the base table to call a stored procedure. The
application really doesn't need to know about this.
well, the trigger or stored procedure/funcion can be part of the database
config, or loaded by the app when it starts.
one very big question is how much of a gain there is in moving from a
database agnostic approach to a database specific approach.
currently rsyslog makes use of it's extensive formatting capabilities to
format a string along the lines of
$DBformat="insert into table X values ('$timestamp','$msg');"
then it hands the resulting string to the database interface module. This
is a bit of a pain to setup, and not especially efficiant, but it has the
ability to insert data into whatever schema you want to use (unlike a lot
of apps that try to force you to use their schema)
I proposed a 5 variable replacement for this to allow for N log entries to
be combined into one string to be sent to the database:
DBinit (one-time things like initialinzing prepared statements, etc)
DBstart (string for the start of a transaction)
DBjoin (tring to use to join multiple DBitems togeather)
DBend (string for the end of a transaction)
DBitem (formatting of a single action )
so you could do something like
DBstart = "insert into table X values"
DBjoin = ","
DBend = ";"
DBitem = "('$timestampe','$msg')"
and it would create a string like #2
this is extremely flexible. I think it can do everything except binary
mode operations, including copy. It is also pretty database agnostic.
but people are asking about how to do binary mode, and some were thinking
that you couldn't do prepared statements in Oracle with a string-based
interface.
so I decided to post here to try and get an idea of (1) how much
performance would be lost by sticking with strings, and (2) of all the
various ways of inserting the data, what sort of performance differences
are we talking about
David Lang
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