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Re: [Qemu-devel] [PULL for-2.1 13/24] qemu-img create: add 'nocow' optio


From: Chun Yan Liu
Subject: Re: [Qemu-devel] [PULL for-2.1 13/24] qemu-img create: add 'nocow' option
Date: Tue, 01 Jul 2014 22:34:41 -0600


>>> On 7/2/2014 at 11:10 AM, in message <address@hidden>, Eric Blake
<address@hidden> wrote: 
> On 07/01/2014 02:48 AM, Stefan Hajnoczi wrote: 
> > From: Chunyan Liu <address@hidden> 
> >  
> > Add 'nocow' option so that users could have a chance to set NOCOW flag to 
> > newly created files. It's useful on btrfs file system to enhance  
> performance. 
> >  
> > Btrfs has low performance when hosting VM images, even more when the guest 
> > in those VM are also using btrfs as file system. One way to mitigate this  
> bad 
> > performance is to turn off COW attributes on VM files. Generally, there are 
> > two ways to turn off NOCOW on btrfs: a) by mounting fs with nodatacow, then 
> > all newly created files will be NOCOW. b) per file. Add the NOCOW file 
> > attribute. It could only be done to empty or new files. 
> >  
> > This patch tries the second way, according to the option, it could add  
> NOCOW 
> > per file. 
> >  
> > For most block drivers, since the create file step is in raw-posix.c, so we 
> > can do setting NOCOW flag ioctl in raw-posix.c only. 
> >  
> > But there are some exceptions, like block/vpc.c and block/vdi.c, they are 
> > creating file by calling qemu_open directly. For them, do the same setting 
> > NOCOW flag ioctl work in them separately. 
> >  
> > [Fixed up 082.out due to the new 'nocow' creation option 
> > --Stefan] 
> >  
> > Signed-off-by: Chunyan Liu <address@hidden> 
> > Signed-off-by: Stefan Hajnoczi <address@hidden> 
>  
> I see this is already in qemu.git, so it will need a followup to fix the 
> user-visible typos (s/trun/turn/, s/Capitabl/Capital/) 
>  

http://thread.gmane.org/gmane.comp.emulators.qemu/284536

-Chunyan

> --  
> Eric Blake   eblake redhat com    +1-919-301-3266 
> Libvirt virtualization library http://libvirt.org 
>  
>  





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