Symptoms

You have Agent for Linux with cPanel/Plesk integration. This Agent is an old build and you want to update it to the latest build . You notice that its status briefly changes to Upgrading, then returns to the previous state. 

In installation logs, you can find this error:

Mount points of kernel module 'snumbd26' are detected. Unmount them manually, and then repeat the installation.

Cause

snumbd26 mount points cannot be unmounted correctly, since p.2 jbd2 kernel daemon (thread) created and also writes to snumbd block device.

In situation when write to mount ending with error, due to out of free space, then error is returned to Snumbd block device and p.2 jbd2 kernel daemon.

Solution

Reboot the machine and re-attempt update.  Explanation - need to forcefully end Snumbd and jbd2 kernel daemon. (We can't gracefully end kernel daemon without rebooting OS)

In case of out of free space for temp folder, then after OS reboot, and before browsing operations, change temp folder location using following steps:

1. Create config file /usr/lib/Acronis/system_libs/8.2.1/config with the following content
#!/bin/bash
export ACRONIS_MOUNT_TMP_DIR="<DIR_NAME>"
 2. Restart the Acronis MMS service by releasing command service acronis_mms restart