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Re: qcow2 overlay performance
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From: |
Alberto Garcia |
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Subject: |
Re: qcow2 overlay performance |
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Date: |
Tue, 27 Oct 2020 18:57:26 +0100 |
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User-agent: |
Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu) |
On Thu 22 Oct 2020 10:56:46 PM CEST, Yoonho Park wrote:
> I am still seeing the performance degradation, but I did find something
> interesting (and promising) with qemu 5.1.50. Enabling the subcluster
> allocation support in qemu 5.1.50 (extended_l2=on) eliminates the
> performance degradation of adding an overlay. Without subcluster allocation
> enabled, I still see the performance degradation in qemu 5.1.50 when adding
> an overlay. For these experiments, I used 64K blocks and 2M qcow2 cluster
> size.
Well, 2MB clusters have 64KB subclusters, so your request size is equal
to the subcluster size. If the requests are aligned there should be no
copy-on-write and therefore no performance degradation if you have
backing images.
Berto