[Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive

[Anthony Liguori <anthony@codemonkey.ws>]

On 05/17/2010 09:04 AM, Alexander Graf wrote:
> Anthony Liguori wrote:
>    
>> On 05/17/2010 08:17 AM, Alexander Graf wrote:
>>      
>>> On 17.05.2010, at 15:09, Anthony Liguori wrote:
>>>
>>>
>>>        
>>>> On 05/17/2010 08:02 AM, Alexander Graf wrote:
>>>>
>>>>          
>>>>>> My concern is that ext3 exaggerates the cost of fsync() which will
>>>>>> result in diminishing value over time for this feature as people
>>>>>> move to ext4/btrfs.
>>>>>>
>>>>>>
>>>>>>              
>>>>> There will be ext3 file systems for years out. Just because people
>>>>> can use better and faster file systems doesn't mean they do. And
>>>>> I'm sure they can't always choose. If anything, I can try and see
>>>>> what the numbers look like for xfs.
>>>>>
>>>>>
>>>>>            
>>>> But ext3 with barrier=1 is pretty uncommon in practice.  Another
>>>> data point would be an ext3 host file system with barrier=0.
>>>>
>>>>          
>>> Who defines what is common and what not? To me, the SLES11 default is
>>> common. In fact, the numbers in the referred mail were done on an
>>> 11.1 system.
>>>
>>>        
>> But it wasn't the SLES10 default so there's a smaller window of
>> systems that are going to be configured this way.  But this is
>> orthogonal to the main point.  Let's quantify how important this
>> detail is before we discuss the affected user base.
>>      
> Alright. I took my Netbook (2GB of RAM) and a USB hard disk, so I can
> easily remount the data fs the vmdk image is on. Here are the results:
>
> # mkfs.ext3 /dev/sdc1
> # mount /dev/sdc1 /mnt -obarrier=1
>
> cache=writeback
>
> real    0m52.801s
> user    0m16.065s
> sys     0m6.688s
>
> cache=volatile
>
> real    0m47.876s
> user    0m15.921s
> sys     0m6.548s
>
> # mount /dev/sdc1 /mnt -obarrier=0
>
> cache=writeback
>
> real    0m53.588s
> user    0m15.901s
> sys     0m6.576s
>
> cache=volatile
>
> real    0m48.715s
> user    0m16.581s
> sys     0m5.856s
>
> I don't see a difference between the results. Apparently the barrier
> option doesn't change a thing.
>    

Ok.  I don't like it, but I can see how it's compelling.  I'd like to 
see the documentation improved though.  I also think a warning printed 
on stdio about the safety of the option would be appropriate.

Regards,

Anthony Liguori

>
> Alex
>
>