Re: [Qemu-discuss] Difference between commit and rebase

[Max Reitz]

On 23.08.19 03:52, lampahome wrote:
>     Generally, rebase is going to be slower because it reads some clusters
>     and compares the old with the new backing file to see whether they are
>     the same.  commit will not do that.  (OTOH, if there are many clusters
>     in the old backing chain that happen to contain the same data as the new
>     one, this will save space, because it won’t copy those clusters from the
>     old backing chain.)
> 
> But commit also can use -b to choose base image, won't it compare the
> old with the new base?

It won’t do so by reading any clusters.  It just checks metadata to see
which clusters are allocated above the base image, and then writes those
to the base.

Max

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