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

[Kevin Wolf]

Am 08.03.2012 18:55, schrieb 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.

qcow2 can't handle clusters that are referenced twice from the same L1
table. This would require a reverse lookup to adjust the QCOW_O_COPIED
flags in the L2 tables containing the other references.

Kevin