nbackup with switch -D ON does not work on ZFS

As you know, nbackup has an option -D to use direct IO to decrease negative effect on file system cache, and usually, it is a faster way to do incremental backups. However, it appeared, that nbackup with switch -D ON does not work on ZFS

Code

./nbackup -b 0 localhost:/opt/firebird/examples/empbuild/employee.fdb /opt/test2.nbk -user SYSDBA -pass masterkey -D ON

Example failure:

Code

Failure: Error (22) opening database file: /opt/firebird/examples/empbuild/employee.fdb

The error means the following:

Code

EINVAL The filesystem does not support the O_DIRECT flag.

So, better use ext3 and ext4 as volumes for database and backups on Linux.