Emergency UNIX and Linux Support

Hi guys.

I'm studding XEN virtualization with 'The Book Of XEN'. in page 46 it has a statement about how to use xm block-attach command that I don't understand.
here is the command:

Code:

xm block-attach 0 tap:aio:/opt/xen/anthony.img /dev/xvda1 w 0

and here is sentence:

Code:

we are attaching anthony.img read/write using tap:aio driver to /dev/xvda1 
in domain 0 using domain 0 to mediate access (because we tend to avoid using non-dom0 driver domains)

what exactly means? is the 'w' means read/write? why should we need it? i mean is there a read only image?

Short Answer:

Yup! That 'w' means read/write. You might have read only drives, such as a CD. You can mount read-only drives on multiple DomUs and you can't do that (without major craziness or a network/cluster filesystem) with a read/write drive.

Long Answer:

Typing "xm block-attach" into google, got me this man page

xm(1): Xen management user interface - Linux man page

with this section on block-attach

block-attach domain-id be-dev fe-dev mode [bedomain-id]
Create a new virtual block device. This will trigger a hotplug event for the guest.

OPTIONS
domain-id
The domain id of the guest domain that the device will be attached to.
be-dev
The device in the backend domain (usually domain 0) to be exported. This can be specified as a physical partition (phy:sda7) or as a file mounted as loopback (file://path/to/loop.iso).
fe-dev
How the device should be presented to the guest domain. It can be specified as either a symbolic name, such as /dev/hdc, for common devices, or by device id, such as 0x1400 (/dev/hdc device id in hex).
mode
The access mode for the device from the guest domain. Supported modes are w (read/write) or r (read-only).
bedomain-id
The back end domain hosting the device. This defaults to domain 0.
EXAMPLES
Mount an ISO as a Disk
xm block-attach guestdomain file://path/to/dsl-2.0RC2.iso /dev/hdc ro

This will mount the dsl iso as /dev/hdc in the guestdomain as a read only device. This will probably not be detected as a cdrom by the guest, but mounting /dev/hdc manually will work.