questions regarding O_DIRECT with SCSI, and timeout handling

questions regarding O_DIRECT with SCSI, and timeout handling

[guy keren]

Hi,

i have two questions, for which i didn't manage to find a clear answer 
so far. i hope it is ok to ask them here.

1. when opening a SCSI disk block device file with O_DIRECT - am i 
guranteed that requests being send using read() or write() system calls 
are transfered all the way to the device before returning, or might they 
get queued in the low-level device driver? am i guaranteed that the 
read() or write() system call will return only when all the underlying 
SCSI Commands have completed fully? if not - is there a general way to 
achieve this, or do i have to resort to device-specific solutions?

does it matter if this is a real disk device, or a SCSI-over-something 
(E.g. iscsi) connection?


2. when sending a SCSI command, a time-out is being delivered with it 
from the SCSI mid-layer down to the lower-layer driver (HBA driver, 
iscsi driver, etc.). when does the system start to count the time - when 
the command was queued to the low-level driver? or when the low-level 
driver sent the command to the underlying device (e.g. an internal disk, 
external RAID, etc)? or does the system start to count the time before this?

thanks,
--guy