Upgrade from 7.6 to xenserver 8.0

while using the RPU (http method) for xenserver upgrade from 7.6 to 8.0 I m seeing this error:

Failure communicating with plugin. when we checked the logs, we see that :

hardware: Dell poweredge R810

Jun 17 10:53:18 XXXXXX prepare_host_upgrade.py ERROR: Root disk /dev/disk/by-id/scsi-SDell_Internal_Dual_S_0123456789AB not found
Jun 17 10:53:18 XXXXXXX prepare_host_upgrade.py ERROR: There was an error in preparing the host for upgrade.
Jun 17 10:53:52 xxxx prepare_host_upgrade.py INFO: Verifying repo...

xencenter logs:

019-06-17 06:01:57,994 INFO XenAPI.Session [53] - Invoking JSON-RPC method 'PBD.get_currently_attached'
2019-06-17 06:01:58,103 INFO XenAPI.Session [53] - Invoking JSON-RPC method 'Async.host.evacuate'
2019-06-17 06:06:18,327 INFO XenAPI.Session [53] - Invoking JSON-RPC method 'task.destroy'
2019-06-17 06:06:18,421 INFO XenAdmin.Wizards.PatchingWizard.PlanActions.PlanAction [53] - Host 'xx' upgrading from version '7.6.0.release/lima/master/8'
2019-06-17 06:06:18,421 INFO XenAPI.Session [53] - Invoking JSON-RPC method 'host.call_plugin'
2019-06-17 06:06:21,556 ERROR XenAdmin.Wizards.PatchingWizard.AutomatedUpdatesBasePage [53] - Failed to carry out plan. There was a failure communicating with the plugin.
2019-06-17 06:18:49,354 INFO XenAPI.Session [Connection to xx] - Invoking XML-RPC method session.login_with_password
2019-06-17 06:18:49,946 INFO XenAPI.Session [Connection to xx] - Invoking XML-RPC method pool.get_all_records
2019-06-17 06:18:50,149 INFO XenAPI.Session [Connection to xx] - Invoking XML-RPC method host.get_record
2019-06-17 06:18:57,372 INFO XenAdmin.Network.XenConnection [Connection to 172.20.1.201] - Connected to IADXenBF01 (xxx:443)
2019-06-17 06:18:57,372 INFO XenAdmin.Network.XenConnection [Connection to 172.20.1.201] - Connecting to IADXenBF01 (xx) in progress.
2019-06-17 06:18:58,012 INFO XenAdmin.MainWindow [128] - Connected to iadxenbf201 (version 7.5, build 7.5.0.release/kolkata/master/29) with XenCenter 8.0 (build 8.0.0.7086)
2019-06-17 06:18:58,292 INFO Audit [92] - Operation started: TransferHealthCheckSettingsAction: IADXenBF01: Pool 0c61a7b0-f304-8dc3-37f7-18e8494b6d8c (IADXenBF01): Transfer Health Check settings
2019-06-17 06:18:58,308 INFO Audit [92] - Operation success: TransferHealthCheckSettingsAction: IADXenBF01: Pool 0c61a7b0-f304-8dc3-37f7-18e8494b6d8c (IADXenBF01): Transfer Health Check settings
2019-06-17 06:18:58,776 INFO Audit [128] - Operation started: TransferXenCenterMetadataAction: Connection unknown: Transfer Health Check settings
2019-06-17 06:18:58,792 INFO Audit [44] - Operation success: TransferXenCenterMetadataAction: Connection unknown: Transfer Health Check settings
2019-06-17 06:18:58,854 INFO XenAdmin.Network.XenConnection [128] - Connection to 172.20.1.201 successful.
2019-06-17 06:20:33,500 WARN XenAdmin.Commands.DisconnectCommand [Main program thread] - Connection to 172.20.1.201 closed.
2019-06-17 06:20:33,890 INFO Audit [Main program thread] - Operation started: TransferXenCenterMetadataAction: Connection unknown: Transfer Health Check settings
2019-06-17 06:20:33,906 INFO Audit [7] - Operation success: TransferXenCenterMetadataAction: Connection unknown: Transfer Health Check settings
2019-06-17 06:36:49,547 INFO Audit [Main program thread] - Operation started: EnableHostAction: IADXenBFTest: Pool 2264c6b0-b68d-7d86-a54f-7e3f8cb4de23 (IADXenBFTest): Host d925e684-87b3-4f1d-8bfd-35e83fff4b89 (IADXenTest001): Waiting
2019-06-17 06:36:49,609 INFO XenAPI.Session [85] - Invoking JSON-RPC method 'host.remove_from_other_config'
2019-06-17 06:36:49,750 INFO XenAPI.Session [85] - Invoking JSON-RPC method 'Async.host.enable'
2019-06-17 06:36:49,859 INFO XenAPI.Session [85] - Invoking JSON-RPC method 'task.remove_from_other_config'
2019-06-17 06:36:49,953 INFO XenAPI.Session [85] - Invoking JSON-RPC method 'task.add_to_other_config'
2019-06-17 06:36:50,046 INFO XenAPI.Session [85] - Invoking JSON-RPC method 'task.remove_from_other_config'
2019-06-17 06:36:50,124 INFO XenAPI.Session [85] - Invoking JSON-RPC method 'task.add_to_other_config'
2019-06-17 06:36:51,325 INFO XenAPI.Session [85] - Invoking JSON-RPC method 'task.destroy'