Re: [PATCH 7/8] block: Pass truncate exact=true where reasonable

[Max Reitz]

On 18.09.19 22:52, Maxim Levitsky wrote:
> On Wed, 2019-09-18 at 11:51 +0200, Max Reitz wrote:
>> This is a change in behavior, so all instances need a good
>> justification.  The comments added here should explain my reasoning.
>>
>> qed already had a comment that suggests it always expected
>> bdrv_truncate()/blk_truncate() to behave as if exact=true were passed
>> (c743849bee7 came eight months before 55b949c8476), so it was simply
>> broken until now.
>>
>> Signed-off-by: Max Reitz <address@hidden>
>> ---
>>  block/parallels.c | 11 +++++++++--
>>  block/qcow2.c     |  6 +++++-
>>  block/qed.c       |  2 +-
>>  qemu-img.c        |  7 ++++++-
>>  qemu-io-cmds.c    |  7 ++++++-
>>  5 files changed, 27 insertions(+), 6 deletions(-)

[...]

>> diff --git a/block/qed.c b/block/qed.c
>> index 7c2a65af40..8005cfc305 100644
>> --- a/block/qed.c
>> +++ b/block/qed.c
>> @@ -674,7 +674,7 @@ static int coroutine_fn 
>> bdrv_qed_co_create(BlockdevCreateOptions *opts,
>>      l1_size = header.cluster_size * header.table_size;
>>  
>>      /* File must start empty and grow, check truncate is supported */
> I would update the above comment, with something like
> 
> "QED format requires the underlying file to have the exact expected length,
> which is 0 on creation"
> Or something similar.

I’ll change it to:

The QED format associates file length with allocation status,
so a new file (which is empty) must have a length of 0.

Hope that’s OK. :-)

Max

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