blockcopy preserve original cache mode value

[Jiatong Shen]

Thank you for the clarification.

On Wed, Nov 16, 2022 at 5:11 PM Peter Krempa <pkrempa at redhat.com> wrote:

> On Wed, Nov 16, 2022 at 09:17:43 +0800, Jiatong Shen wrote:
> > Hello Community,
> >
> >    We are observing that after a blockcopy, the disk backend will inherit
> > from original disk's cache mode configuration even though a new cache
> mode
> > is given. Specifically, the function
> >
> https://github.com/libvirt/libvirt/blob/cd94d891fb4b5cdda229f58b1dee261d5514082b/src/qemu/qemu_domain.c#L10887
> > which eventually calls  qemuDomainPrepareDiskSourceData and override
> input
> > config.
> > My question is is this on purpose? Because seems like qemu allows
> different
> > configs.
> > Thnak you very much for the help.
>
> This is an historical artifact from the time when the block copy API
> used the 'drive-mirror' QMP API of qemu which didn't allow configuring
> any image specific parameters.
>
> Nowadays we use 'blockdev-mirror' which takes an image instantiated via
> 'blockdev-add' which can configure everything the same way as the disk,
> so it would be possible to honour the cache mode as specified in the
> block copy XML.
>
> Note that if you intend to implement it, the code must still preserve
> the original cache mode if the user doesn't specify it in the XML.
>
>

-- 

Best Regards,

Jiatong Shen
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