Re: [Qemu-devel] [PATCH] qemu-img: set nocow flag to new file

[Kevin Wolf <kwolf@redhat.com>]

Am 14.11.2013 um 10:44 hat Alex Bennée geschrieben:
> 
> cyliu@suse.com writes:
> 
> > Set NOCOW flag to newly created images to solve performance issues on btrfs.
> >
> > Btrfs has terrible 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 (since having copy on
> > write for this kind of data is not useful).
> <snip>
> 
> It's been a while since I had to mess with this performance mystery but
> I recall you also need to ensure the partition needs to be mounted with
> the nodatacow mount option. Unless this has changed we should probably
> warn the user about that.

>From the information I read today while looking at this patch, the
nodatacow mount option seems just to influence the default for newly
created files. That is, if you set the flag like this patch is doing,
it has the same effect as running qemu-img create while having mounted
the file system with nodatacow.

Never tried it on my own, though, so take it with a grain of salt.

Kevin