vSphere Storage - ESXi 5.1 - Documentation - VMware
vSphere Storage - ESXi 5.1 - Documentation - VMware
vSphere Storage - ESXi 5.1 - Documentation - VMware
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<strong>vSphere</strong> <strong>Storage</strong><br />
Identifying Device Connectivity Problems<br />
When your <strong>ESXi</strong> host experiences a problem while connecting to a storage device, the host treats the problem<br />
as permanent or temporary depending on certain factors.<br />
<strong>Storage</strong> connectivity problems are caused by a variety of reasons. Although <strong>ESXi</strong> cannot always determine the<br />
reason for a storage device or its paths being unavailable, the host differentiates between a permanent device<br />
loss (PDL) state of the device and a transient all paths down (APD) state of storage.<br />
Permanent Device Loss<br />
(PDL)<br />
A condition that occurs when a storage device permanently fails or is<br />
administratively removed or excluded. It is not expected to become available.<br />
When the device becomes permanently unavailable, <strong>ESXi</strong> receives appropriate<br />
sense codes or a login rejection from storage arrays, and is able to recognize<br />
that the device is permanently lost.<br />
All Paths Down (APD) A condition that occurs when a storage device becomes inaccessible to the host<br />
and no paths to the device are available. <strong>ESXi</strong> treats this as a transient condition<br />
because typically the problems with the device are temporary and the device<br />
is expected to become available again.<br />
Detecting PDL Conditions<br />
A storage device is considered to be in the permanent device loss (PDL) state when it becomes permanently<br />
unavailable to your <strong>ESXi</strong> host.<br />
Typically, the PDL condition occurs when a device is unintentionally removed, or its unique ID changes, or<br />
when the device experiences an unrecoverable hardware error.<br />
When the storage array determines that the device is permanently unavailable, it sends SCSI sense codes to<br />
the <strong>ESXi</strong> host. The sense codes allow your host to recognize that the device has failed and register the state of<br />
the device as PDL.<br />
NOTE The sense codes must be received on all paths to the device for the device to be considered permanently<br />
lost.<br />
The following VMkernel log example of a SCSI sense code indicates that the device is in the PDL state.<br />
H:0x0 D:0x2 P:0x0 Valid sense data: 0x5 0x25 0x0 or Logical Unit Not Supported<br />
For information about SCSI sense codes, see Troubleshooting <strong>Storage</strong> in <strong>vSphere</strong> Troubleshooting.<br />
In the case of iSCSI arrays with a single LUN per target, PDL is detected through iSCSI login failure. An iSCSI<br />
storage array rejects your host's attempts to start an iSCSI session with a reason Target Unavailable. As with<br />
the sense codes, this response must be received on all paths for the device to be considered permanently lost.<br />
After registering the PDL state of the device, the host stops attempts to reestablish connectivity or to issue<br />
commands to the device to avoid becoming blocked or unresponsive. The I/O from virtual machines is<br />
terminated.<br />
NOTE <strong>vSphere</strong> HA can detect PDL and restart failed virtual machines.<br />
The <strong>vSphere</strong> Web Client displays the following information for the device:<br />
n The operational state of the device changes to Lost Communication.<br />
n All paths are shown as Dead.<br />
n Datastores on the device are grayed out.<br />
138 <strong>VMware</strong>, Inc.
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