We've had reports of users being unable to perform backups when using EFS; when using 6.6.x to backup a 6.6.x cluster, this is a supported configuration.
This is due to the fact that a single process may only hold 256 locks in EFS and with small (ish) buckets, it's quite possible for SQLite to try acquire many more than this.
We can disable SQLite file locking by switching out the VFS for either unix-none, or win32-none which have no-op locking operations.
To some degree, yes. Changing (increasing) the number of threads being used to backup (with '--threads' can help, however, this isn't a guaranteed solution.