Re: dirty_ratio & dirty_background_ratio settings with huge memory

pinker wrote:
> Laurenz Albe wrote
> > Yes, you should set vm.dirty_background_bytes and vm.dirty_bytes
> > and not use the *_ratio settings.
> >
> > 2 GB for vm.dirty_background_bytes and 1 GB for vm.dirty_bytes sounds
> > fine.

To repeat (for the archive): it should be the other way round.

> Thank you Laurenz. I was always wondering why not to set bigger window
> between those 2. Because setting dirty_background_bytes seems quite natural
> for me, i.e. start to write asynchronously faster, but why to provoke stalls
> faster? is it not better to stretch the window much wider, like 1GB for
> dirty_background_bytes and for instance 20 GB dirty_bytes? it's the Approach
> 3: Both Ways from
> https://lonesysadmin.net/2013/12/22/better-linux-disk-caching-performance-vm-dirty_ratio/

I'd say that setting vm.dirty_background_bytes low and vm.dirty_bytes
very high is not a good idea, at least for databases:

- If flushing data to disk in the background can keep up with the write
activity, you're fine. But then you'd also be fine with a lower setting
for vm.dirty_bytes, right?

- If flushing to disk cannot keep up with the write activity, you'll get
an I/O spike when everything is flushed to disk at checkpoint time and
the database may become unresponsive for a while.

The idea behind a low vm.dirty_bytes setting is to avoid one big I/O spike
and have several smaller ones instead.

Yours,
Laurenz Albe
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