6.7

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Before you change the state of the arbitrator, make sure that USB devices residing on the host are
not attached to a virtual machine. If USB devices become unavailable to a virtual machine, a host
administrator might have disabled the arbitrator. When an administrator stops or disconnects the
arbitrator for troubleshooting or other purposes, USB devices attached to that host become
unavailable to the virtual machine. If a data transfer is taking place at this time, you might lose the
data. To reestablish the arbitrator, you must restart the host.
Connecting USB Devices to an ESXi Host
You can connect and chain multiple USB hubs and devices to an ESXi host. Careful planning and
knowledge of hub behavior and limitations can help ensure that your devices work optimally.
USB physical bus topology defines how USB devices connect to the host. Support for USB device
passthrough to a virtual machine is available if the physical bus topology of the device on the host does
not exceed tier seven. The first tier is the USB host controller and root hub. The last tier is the target USB
device. You can cascade up to five tiers of external or internal hubs between the root hub and the target
USB device. An internal USB hub attached to the root hub or built into a compound device counts as one
tier.
The quality of the physical cables, hubs, devices, and power conditions can affect USB device
performance. To ensure the best results, keep the host USB bus topology as simple as possible for the
target USB device, and use caution when you deploy new hubs and cables into the topology. The
following conditions can affect USB behavior:
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Communication delay between the host and virtual machine increases as the number of cascading
hubs increases.
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Connecting or chaining multiple external USB hubs increases device enumeration and response time,
which can make the power support to the connected USB devices uncertain.
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Chaining hubs together also increases the chance of port and hub error, which can cause the device
to lose connection to a virtual machine.
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Certain hubs can cause USB device connections to be unreliable, so use care when you add a new
hub to an existing setup. Connecting certain USB devices directly to the host rather than to a hub or
extension cable might resolve their connection or performance issues.
Note To prevent additional problems, be aware of the physical constraints of long-term deployment in a
machine room environment. Small devices are easily damaged by being stepped on or knocked loose.
In some cases, you must hard reset the device and hub to restore the device to a working state.
For a list of supported USB devices for passthrough from an ESXi host to a virtual machine, see the
VMware knowledge base article at http://kb.vmware.com/kb/1021345.
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