Addressing when lun spans disks

Addressing when lun spans disks

[MShetty]

Hi,

What exactly is the value of 'target id' when my lun spans multiple
disks ?? I am not able to understand how the physical disk address would
be obtained when a lun spans multiple disks

Thanks and Regards,
M Shetty

Re: Addressing when lun spans disks

[Doug Ledford]

On Mon, Oct 21, 2002 at 09:28:00PM +0530, MShetty wrote:
> Hi,
> 
> What exactly is the value of 'target id' when my lun spans multiple
> disks ?? I am not able to understand how the physical disk address would
> be obtained when a lun spans multiple disks

If you are using an external drive chassis that turns multiple disks into 
a single lun, then the target id is whatever the target id for the lun is.  
The drives that are actually behind the logical lun are *never* exposed to 
the scsi layer.  If you are talking about using the md raid code to map 
several disks into a single md device, then it's the job of the md code to 
remap all incoming requests to point to the proper hard disk, and in the 
process of doing so it rewrites the target id, the lun, and the block 
address to the correct values for the underlying physical disk.

-- 
  Doug Ledford <dledford@redhat.com>     919-754-3700 x44233
         Red Hat, Inc. 
         1801 Varsity Dr.
         Raleigh, NC 27606
  

Re: Addressing when lun spans disks

[Bryan Henderson]

>What exactly is the value of 'target id' when my lun spans multiple
>disks ?? I am not able to understand how the physical disk address would
>be obtained when a lun spans multiple disks

Do you understand that a target is bigger than a logical unit?  A physical
unit has a target ID and is subdvided into logical units which have logical
unit numbers.  The logical unit number identifies a logical unit within a
physical unit.

There is no such thing as a logical unit that spans physical units.

In some advanced storage subsystems, the physical subsystem contains a
whole bunch of disks and space on those disks is allocated arbitrarily to
logical units.  In that case, the whole subsystem is the physical unit and
has one target ID.  And there is no SCSI identifier that identifies the
individual disks -- they are invisible to the SCSI protocol.