Max Backups and error 'no space left on device'

Hi,
i have configured a backup set and the storage MaxBackup to 10.

Now in the folder there are 10 backups for every vm.
Launcing a manual backup the server answer that it has reached the maximun number of backups and refuse to work.
I have found 'no space left on device' error looking in the line of the console.
I have just see that root partition is near full.
Reading forums i have red that this is normal because the backups failed and it stored there the files.
Now, after setting MaxBackup to 15, the first backup in the set, failed with ' qmp command 'query-backup' failed - got timeout'

So, i need to free root and to solve these backps' errors and i'm not familiar with linux....




Thank you
Fabrizio