On Wed, Feb 01, 2023 at 04:12:32PM -0500, Stefan Hajnoczi wrote: > Modernize the description and document the size=/offset= runtime > options. > > Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> > --- > docs/system/qemu-block-drivers.rst.inc | 32 ++++++++++++++++++++++---- > 1 file changed, 27 insertions(+), 5 deletions(-) > > diff --git a/docs/system/qemu-block-drivers.rst.inc > b/docs/system/qemu-block-drivers.rst.inc > index be6eec1eb6..ec9ebb2066 100644 > --- a/docs/system/qemu-block-drivers.rst.inc > +++ b/docs/system/qemu-block-drivers.rst.inc > @@ -16,11 +16,11 @@ options that are supported for it. > .. option:: raw > > Raw disk image format. This format has the advantage of > - being simple and easily exportable to all other emulators. If your > - file system supports *holes* (for example in ext2 or ext3 on > - Linux or NTFS on Windows), then only the written sectors will reserve > - space. Use ``qemu-img info`` to know the real size used by the > - image or ``ls -ls`` on Unix/Linux. > + being simple and easily exportable to all other emulators. Modern > + file systems support *holes* (for example in btrfs/XFS/ext4 on > + Linux or NTFS on Windows) where space is allocated on demand as sectors are > + written. Use ``qemu-img info`` to know the real size used by the image or > + ``ls -ls`` on Unix/Linux. > > Supported create options: > > @@ -33,6 +33,28 @@ options that are supported for it. > for image by writing data to underlying storage. This data may or > may not be zero, depending on the storage location. > > + Supported runtime options: > + > + .. program:: raw > + .. option:: offset > + > + The byte position in the underlying file where the virtual disk starts. > + This is handy when you want to present just a single partition from a > + physical disk as the virtual disk. This option is usually used in > + conjunction with the ``size`` option. > + > + .. option:: size > + > + Limit the virtual disk size to the given number of bytes, regardless of > how > + large the underlying file is. This option is usually used in conjunction > + with the ``offset`` option. > + > + Note the raw format can be omitted when no runtime options are being used. > In > + that case the raw format does nothing besides forwarding I/O requests to > the > + protocol blockdev. You can improve performance slightly by eliminating > + ``--blockdev raw,file=file0,node-name=drive0`` and renaming the "file0" > + blockdev to "drive0".
This paragraphs reads a bit softly. I would word this such that we explicitly and strongly recommend against using the 'raw' format driver. Almost no one will have a need for this. All the protocol drivers expose a raw format, which can be consumed directly. So the 'raw' format driver should NEVER be used unless the user needs to apply a limited window over the underlying disk capacity, which is pretty rare. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
