SG_SET_COMMAND_Q 0x2271 [_GET_ 0x2270] .  The default it the original sg driver was not to allow commands to be queued on the same file descriptor (actually it was more restrictive, commands could not be queued on a SCSI device). The version 2 sg driver kept this action as its default (for backward compatibility) and offered these ioctl()s to change and monitor the command queuing state.