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Re: discard and v2 qcow2 images

From: Alberto Garcia
Subject: Re: discard and v2 qcow2 images
Date: Fri, 20 Mar 2020 20:41:27 +0100
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

On Fri 20 Mar 2020 08:35:44 PM CET, Eric Blake <address@hidden> wrote:
>> This flag is however only supported when qcow_version >= 3. In older
>> images the cluster is simply deallocated, exposing any possible
>> previous data from the backing file.
> Discard is advisory, and has no requirements that discarded data read
> back as zero.  However, if write zeroes uses discard under the hood,
> then THAT usage must guarantee reading back as zero.

write_zeroes doesn't seem to use discard in any case, so no problem

>> @@ -3763,6 +3763,10 @@ static coroutine_fn int 
>> qcow2_co_pdiscard(BlockDriverState *bs,
>>       int ret;
>>       BDRVQcow2State *s = bs->opaque;
>> +    if (s->qcow_version < 3) {
>> +        return -ENOTSUP;
>> +    }
>> +
> This changes it so you no longer see stale data, but doesn't change
> the fact that you don't read zeroes (just that your stale data is now
> from the current layer instead of the backing layer, since we did
> nothing at all).

discard can already fail if the request is not aligned, in this case you
get -ENOTSUP and stay with the same data as before.

What's different in this case is that you can actually get stale data,
that doesn't seem like a desirable outcome.


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