> Then the
> database makes the fsync call, and suddenly the OS wants to flush 2-6GB of data
> straight to disk. Without that background trickle, you now have a flood that
> only the highest-end disk controller or a backing-store full of SSDs or PCIe
> NVRAM could ever hope to absorb.
Isn't checkpoint_completion_target supposed to deal exactly with that problem?
Plus: if 2-6GB is too much, why not decrease checkpoint_segments? Or
checkpoint_timeout?
> The kernel
> developers agree, or we wouldn't have dirty_bytes, or
> dirty_background_bytes, and they wouldn't have changed the defaults to 5%
> and 10% instead of 10% and 40%.
I'm not saying that those kernel parameters are "useless"; I'm saying they are used
in the same way as the checkpoint_segments, checkpoint_timeout and
checkpoint_completion_target are used by postgresql; and on a postgresql-only system
I would rather have postgresql look after the fsync calls, not the OS.
