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Re: INSERT - UPDATE throughput oscillating and SSD activity after stopping the client

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On Fri, 2016-12-02 at 13:45 -0800, Adrian Klaver wrote:
> 
> On 12/02/2016 09:40 AM, Tom DalPozzo wrote:
> > 
> > 
> > Hi,
> > I've two tables, t1 and t2, both with one bigint id indexed field
> > and
> > one 256 char data field; t1 has always got 10000 row, while t2 is
> > increasing as explained in the following.
> > 
> > My pqlib client countinously updates  one row in t1 (every time
> > targeting a different row) and inserts a new row in t2. All this in
> > blocks of 1000 update-insert per commit, in order to get better
> > performance.
> > Wal_method is fsync, fsync is on, attached my conf file.
> > I've a 3.8ghz laptop with evo SSD.
> > 
> > Performance is  measured every two executed blocks and related to
> > these
> > blocks.
> > 
> > Over the first few minutes performance is around 10Krow/s then it
> > slowly
> > drops, over next few minutes to 4Krow/s, then it slowly returns
> > high and
> > so on, like a wave.
> > I don't understand this behaviour. Is it normal? What does it
> > depend on?
> Have you looked at the Postgres log entries that cover these
> episodes?
> 
> Is there anything of interest there?
> 
In particular look at checkpoints. In the config file you've changed
checkpoint_timeout, but you haven't changed max_wal_size, so my guess
is the checkpoints happen every few minutes, and run for about 1/2 the
time (thanks for completion_target=0.5). That would be consistent with
pattern of good/bad performance.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services



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