Re: [Qemu-devel] [RFC PATCH 07/17] block: make high level discard operation always zero

[Avi Kivity]

On 03/08/2012 07:15 PM, Paolo Bonzini wrote:
> While the formats/protocols are free to implement a discard operation
> that does *not* zero the data, providing a common view to the guests is
> the only way to avoid configuration and migration nightmares.  (We don't
> want the admin to manually set up discard_zeroes_data/discard_granularity!)
>
> This has a couple of drawbacks:
>
> 1) QEMU effectively will not try to use discard unless discard_zeroes_data
> is true.  To do this we could add a flag such as BDRV_ALWAYS_DISCARD,
> which the device models could use if their discard_zeroes_data property is
> set to false.  However, I haven't done this, estimating that no strictly
> positive integer could represent the number of people using it.
>
> 2) it may turn a thin-provisioning operation into a full preallocation,
> which is not nice.
>
> 3) it may cost memory for a bounce buffer that only needs to be filled
> with zeroes.
>
> While (3) can be worked around, the only way around the other two,
> unfortunately, is support in the formats and protocols.  We will still
> provide device options to opt out of this, but with raw and qed covered
> (+ qcow2 without backing file, and qcow3 in the future) it should not
> be too bad.

Can't qcow2 with a backing file also be supported?  Zero out the first
cluster, and remember it.  The following discards can reuse this zero
cluster, as long as it hasn't been overwritten.

-- 
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