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

[Stefan Hajnoczi]

On Mon, Nov 18, 2013 at 12:54:59PM +0800, Chunyan Liu wrote:
> 2013/11/15 Stefan Hajnoczi <address@hidden>
> 
> > On Thu, Nov 14, 2013 at 04:15:28PM +0800, Chunyan Liu wrote:
> > > 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).
> > >
> > > Signed-off-by: Chunyan Liu <address@hidden>
> > > ---
> > >  block/raw-posix.c     |    6 ++++++
> > >  block/vdi.c           |    7 +++++++
> > >  block/vmdk.c          |    7 +++++++
> > >  include/qemu-common.h |    9 +++++++++
> > >  4 files changed, 29 insertions(+), 0 deletions(-)
> > >
> > > diff --git a/block/raw-posix.c b/block/raw-posix.c
> > > index f6d48bb..4a3e9d0 100644
> > > --- a/block/raw-posix.c
> > > +++ b/block/raw-posix.c
> > > @@ -1072,6 +1072,12 @@ static int raw_create(const char *filename,
> > QEMUOptionParameter *options,
> > >          result = -errno;
> > >          error_setg_errno(errp, -result, "Could not create file");
> > >      } else {
> > > +#ifdef __linux__
> > > +        /* set NOCOW flag to solve performance issue on fs like btrfs */
> > > +        int attr;
> > > +        attr = FS_NOCOW_FL;
> > > +        ioctl(fd, FS_IOC_SETFLAGS, &attr);
> > > +#endif
> > This should be optional and I'm not sure it should be the default.
> >
> > Rationale: If you're on btrfs you probably expect the copy-on-write and
> > snapshot features of the file system.  We shouldn't silently disable
> > that unless the user asks for it.
> >
> >
> The problem is: if users want to use copy-on-write (e.g, for snapshotting)
> and
> don't care about performance degrade, they still be able to issue "chattr"
> to
> change it to be COW. However, if a file is created as COW, but later users
> care
> about performance, there is no way to switch to NOCOW per file. NOCOW
> should be
> set to new or empty file only on btrfs.

When the NOCOW attribute is set on a file, reflink copying (aka
file-level snapshots) do not work:

$ cp --reflink test.img test-snapshot.img

This produces EINVAL.

It is a regression if qemu-img create suddenly starts breaking this
standard btrfs feature for existing users.

Please make it a .bdrv_create() option which is off by default to avoid
breaking existing users' workflows/scripts.  The result should be
something like:

$ qemu-img create test.img 8G # file has NOCOW cleared
$ qemu-img create -o nocow=on test.img 8G # file has NOCOW set

Stefan