Re: What does qemu-img convert -W documentation mean?

[Jakob Bohm]

On 2021-05-26 15:40, Richard W.M. Jones wrote:
> Firstly there's a real interesting thread about nbdcopy vs qemu-img
> convert performance:
> https://listman.redhat.com/archives/libguestfs/2021-May/thread.html#00119
>
> I have a question about qemu-img convert -W flag.  The documentation
> says:
>
>   -W  Allow out-of-order writes to the destination.  This  option  im‐
>       proves performance, but is only recommended for preallocated de‐
>       vices like host devices or other raw block devices.
>
> I don't understand why out of order writes are bad for things like raw
> files and qcow2 files, or for non-preallocated block devices.
> Wouldn't they always be a win?
>
> Rich.
Out of order writes to things that allocate one block at a time from the
lower layer would cause the mapping from virtual clusters to clusters at
the lower layer to befragmented, literally out of order, which hurts
performanceof future access.


Enjoy

Jakob
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Jakob Bohm, CIO, Partner, WiseMo A/S.  https://www.wisemo.com
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