Hi Fabien,

Hello Tomas.

On 2016-01-11 14:45:16 +0100, Andres Freund wrote:
I measured it in a different number of cases, both on SSDs and spinning
rust. I just reproduced it with:

postgres-ckpt14 \
        -D /srv/temp/pgdev-dev-800/ \
        -c maintenance_work_mem=2GB \
        -c fsync=on \
        -c synchronous_commit=off \
        -c shared_buffers=2GB \
        -c wal_level=hot_standby \
        -c max_wal_senders=10 \
        -c max_wal_size=100GB \
        -c checkpoint_timeout=30s

What kernel, filesystem and filesystem option did you measure with?

Andres did these measures, not me, so I do not know.

I was/am using ext4, and it turns out that, when abling flushing, the
results are hugely dependant on barriers=on/off, with the latter making
flushing rather advantageous. Additionally data=ordered/writeback makes
measureable difference too.

These are very interesting tests, I'm looking forward to have a look at the results.

The fact that these options change performance is expected. Personnaly the test I submitted on the thread used ext4 with default mount options plus "relatime".

If I had a choice, I would tend to take the safest options, because the point of a database is to keep data safe. That's why I'm not found of the "synchronous_commit=off" chosen above.

Reading kernel sources trying to understand some more of the performance



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