Saturday, 27 February 2016

APD and PDL in VMware VSphere 6.0 HA APD and PDL in VMware VSphere 6.0 HA

APD and PDL in VMware VSphere 6.0 HA

by on in

There are two distinct states a device can be in when storage connectivity is lost; All Paths Down (APD) or Permanent Device Loss (PDL). For each of these states:
All Paths Down (APD) is a condition where all paths to the storage device are lost or the storage device is removed. The state is caused because the change happen in an uncontrolled manner, and the VMkernal core storage stack does not know how long the loss of access to the device will last. The APD is a condition that is treated as temporary (transient), since the storage device might come back online; or it could be permanent, which is referred to as a Permanent Device Loss (PDL) .Till 5.x there was no support for handling APD state.
PDL (Permanent Device Loss) Esxi considers device loss permanent. It can be caused by making a LUN inaccessible to a host, either by unmapping or deleting it. In this case, the storage array informs the host of a PDL state through a SCSI command response. The removal is considered permanent when all paths have the PDL error. PDL was introduced in 5.1
 In vSphere 6.0, VMware has enhanced APD and PL and introduced vSphere VMCP (VM component protection) feature. If an APD or a PDL condition occurs and VM is running on a host which has got connectivity issue with a datastore, HA will kick in and restart that VM to other host which has not connectivity issue with the same storage.

There are two variants of PDL, planned and unplanned:


 NFS doesn't have a PDL as it’s not a block level storage from where the LUN access can be removed. VMCP for APD works totally fine with NFS.
VMCP protects VMs against storage connectivity failures and misconfiguration. It covers all datastores used by a VM.

VMCP has an options for APD a) aggressive  b) conservative" failover.
The difference between the two is in Conservative failover mode, HA would first look out for a place where to restart the VM and then terminates it. In aggressive failover mode, HA would terminate the VM first and then would look out where to place the V

For more details related to APD/PDC VMCP topic please refer the below link


https://pubs.vmware.com/vsphere-51/index.jsp?topic=%2Fcom.vmware.vcli.examples.doc%2Fcli_manage_files.5.7.html

http://blogs.vmware.com/vsphere/2015/06/vm-component-protection-vmcp.html

http://pubs.vmware.com/vsphere-60/index.jsp?topic=%2Fcom.vmware.vsphere.avail.doc%2FGUID-F01F7EB8-FF9D-45E2-A093-5F56A788D027.html


By at
Labels:

3 comments:

Subscribe to: Post Comments (Atom)