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
[root@server1 bin]# ./nbackup -b 0 localhost:/opt/firebird/examples/empbuild/employee.fdb /opt/test2.nbk -user SYSDBA -pass masterkey -D ON
Failure: Error (22) opening database file: /opt/firebird/examples/empbuild/employee.fdb
The error means the following:
EINVAL The filesystem does not support the O_DIRECT flag.
So, better use ext3 and ext4 as volumes for database and backups on Linux.