Re: JFYI: ext4 bug triggerable by kvm

[Anthony Liguori <anthony@codemonkey.ws>]

On 08/17/2010 04:07 AM, Christoph Hellwig wrote:
> On Mon, Aug 16, 2010 at 03:34:12PM -0500, Anthony Liguori wrote:
>    
>> On 08/16/2010 01:42 PM, Christoph Hellwig wrote:
>>      
>>> On Mon, Aug 16, 2010 at 09:43:09AM -0500, Anthony Liguori wrote:
>>>        
>>>>> Also, ext4 is _very_ slow on O_SYNC writes (which is
>>>>> used in kvm with default cache).
>>>>>            
>>>> Yeah, we probably need to switch to sync_file_range() to avoid the
>>>> journal commit on every write.
>>>>
>>>>          
>>> No, we don't.  sync_file_range does not actually provide any data
>>> integrity.
>>>        
>> What do you mean by data integrity?
>>      
> sync_file_range only does pagecache-level writeout of the file data.
> It nevers calls into the actual filesystem, that means any block
> allocations (for filling holes / converting preallocated space in normal
> filesystems, or every write in COW-based filesstems like qcow2) never
> get flushes to disk,

But assuming that you had a preallocated disk image, it would 
effectively flush the page cache so it sounds like the only real issue 
is sparse and growable files.

>   and even more importantly the disk write cache is
> never flushed.
>    

The point is that we don't want to flush the disk write cache.  The 
intention of writethrough is not to make the disk cache writethrough but 
to treat the host's cache as writethrough.

Regards,

Anthony Liguori

> In short it's completely worthless for any real filesystem.
>
>