Re: [PATCH v3 2/2] block: Add backend_defaults property

[Stefan Hajnoczi]

On Thu, Mar 11, 2021 at 12:39:16AM +0900, Akihiko Odaki wrote:
> backend_defaults property allow users to control if default block
> properties should be decided with backend information.
> 
> If it is off, any backend information will be discarded, which is
> suitable if you plan to perform live migration to a different disk backend.
> 
> If it is on, a block device may utilize backend information more
> aggressively.
> 
> By default, it is auto, which uses backend information from physical
> devices and ignores one from file. It is consistent with the older
> versions, and should be aligned with the user expectation that a file
> backend is more independent of host than a physical device backend.

Can BlockLimits pdiscard_alignment and opt_transfer be used instead of
adding discard_granularity and opt_io fields to BlockSizes? They seem to
do the same thing except the QEMU block layer currently uses BlockLimits
values internally to ensure that requests are suitable for the host
device.

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