[ovirt-devel] VDSM changes Linux memory dirty ratios - why?

[Martin Polednik]

On 29/11/16 22:01 +0200, Yaniv Kaul wrote:
>It appears that VDSM changes the following params:
>vm.dirty_ratio = 5
>vm.dirty_background_ratio = 2
>
>Any idea why? Because we use cache=none it's irrelevant anyway?

It's not really irrelevant, the host still uses disk cache. Anyway,
there is BZ[1] with a presentation[2] that (imho reasonably) states:

"Reduce dirty page limits in KVM host to allow
direct I/O writer VMs to compete successfully
with buffered writer processes for storage
access"

I wonder why virtual-host tuned profile doesn't contain these values:

$ grep vm.dirty /usr/lib/tuned/virtual-host/tuned.conf
vm.dirty_background_ratio = 5

[1]https://bugzilla.redhat.com/show_bug.cgi?id=740887
[2]http://perf1.lab.bos.redhat.com/bengland/laptop/rhev/rhev-vm-rsptime.pdf

>TIA,
>Y.

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