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

[Anthony Liguori]

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
>
>