1TB issue

[Kenneth D. Merry]

On Wed, Jun 02, 2004 at 10:49:39 +0100, David Malone wrote:
> On Tue, Jun 01, 2004 at 05:40:10PM -0600, Kenneth D. Merry wrote:
> > See another recent message on this list from Brooks Davis.  It sounds like
> > sysinstall has issues with large arrays in -current, but fdisk works okay.
> 
> I've heard from some people that there was a 2TB limit on SCSI
> devices, but it wasn't clear to me if this was a limit of the SCSI
> implementation they were using, or if it arose from the SCSI spec.
> What's actually the case here?

It's a limitation in the device they're using and/or the SCSI layer they're
using.  With 10 or 12 byte read/write CDBs, you're limited to 32 bits to
specify the LBA.  For a device with 512 byte sectors, that limits you to
2TB.

With the SBC-2 spec, they (T10) introduced 16 byte read, write and read
capacity commands.  That allows you to get the size of a device larger
than 2TB, and address blocks past the 2TB limit.  (The size of the LBA
field was increased to 8 bytes for the 16 byte commands, so you can
address 2^64 blocks.)

CAM in FreeBSD-current has had support for devices larger than 2TB for a
little over a year.  -stable doesn't have that support, because it has other
limitations (pointed out by Bruce) that basically mean CAM support for
large devices wouldn't really matter.

Ken
-- 
Kenneth Merry
ken at kdm.org