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Tips and tricks: How do I use the “xm block-attach” command to attach additional storage (like USB drive) to my running para-virtualized Xen guests?

by The editorial team

Contributed by Chris Lalancette and Eduardo Damato

Release Found: Red Hat Enterprise Linux 5

The xm block-attach command can be used to add additional storage to a running para-virtualized Xen guest. To ensure this succeeds, use the xvd device as the frontend device, and NOT sd or hd. For instance, the following command:

# xm block-attach  /var/lib/xen/images/disk2.img /dev/xvdb w

will export the file /var/lib/xen/images/disk2.img as /dev/xvdb inside the guest. The last parameter is the mode for exporting the disk. There are 3 modes currently available: r for read only, w for read/write, and w! for read/write with sharing.

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This entry was posted by The editorial team on Monday, August 20th, 2007 at 8:27 am and is filed under Red Hat Enterprise Linux, tips and tricks. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

5 responses to “Tips and tricks: How do I use the “xm block-attach” command to attach additional storage (like USB drive) to my running para-virtualized Xen guests?”

  1. polasure says:
    August 28th, 2007 at 9:05 pm

    How could I find the file disk2.img? In fact, there is no file in this directory /var/lib/xen/images/

  2. Laurentiu Coica says:
    August 30th, 2007 at 4:10 am

    I think that the correct syntax for xm block-attach is
    xm block-attach file://path/to/images.dsk

    For creating disk image, use something like:
    dd if=/dev/zero of=/path/to/image.dsk bs=1024k count=8192
    (for 8 Gb image size)

  3. polasure says:
    September 7th, 2007 at 5:32 am

    In fact, I found that “xm block-attach 1 phy:/dev/sde xvdb w” can work for para-virtualized Xen guests, but not for full-virtualized Xen guests.

    I also tried the commands as you said, but still failed.

    Hope for your help! Thank you!

  4. polasure says:
    September 7th, 2007 at 5:34 am

    In this conmmand: xm block-attach 1 phy:/dev/sde xvdb w
    1 is the ID of the guest.

  5. Chris says:
    January 16th, 2009 at 6:46 am

    Keep in mind that if you want to mount a tap:aio volume on dom0 that you likely need to run “modprobe xenblk” to load the blkfront driver first.

    Then you can run something like “xm block-attach 0 tap:aio:/var/lib/xen/images/server.img /dev/xvda w”, which will create the /dev/xvda device(s) which can then be mounted (if LVM is used then “vgchange -a y” and “vgmknodes” need to be run to make the volumes appear in /dev).

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