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I can successfully attach disks to a running KVM instance using virsh attach-disk...

virsh attach-disk node-1 /dev/vg_lunsr/lun1  vdb
Disk attached successfully

...but these new devices aren't seen by the guest without a reboot, which almost defeats the purpose of dynamic attachment. If these were SCSI devices I would use e.g. /sys/class/scsi_host/host0/scan to request the SCSI drivers to scan for new devices. Is there an equivalent capability for the virtio block driver?

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up vote 0 down vote accepted

Yes, it mostly depends on what distro is running inside the guest. On Debian/Ubuntu, you certainly want to load this kernel module:

modprobe acpiphp

and then try to hot plug your devices. It will appear without guest reboot. You can find more information about hot plugging in libvirt here

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That was it. Thanks! – larsks Dec 21 '12 at 21:37

This most likely depends on your guests kernel/KVM/udev version. Just tried it with centos 6.3 and as soon as i added the disk. Device /dev/vdb was available on the running guest.

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