backup & low downtime for home network

[Lennart Sorensen]

On Thu, Dec 06, 2007 at 12:41:40PM -0500, Chris Aitken wrote:
> Okay, I set Master and Slave on each IDE cable. I'm starting up the 
> computer now (I'm installing ubuntu 7.10). The first thing I see is,
> 
> The folowing configuration options were automatically updated:
> 
> Disk: 137438MB   WDC WD1600AAJB-00PVA0
> CD-ROM: CDD-1480
> 
> The problem is that the salesman told me these are 160 GB drives. I just 
> spoke with a guy at the local Krazy Krazy - he says that the "DOS on the 
> motherboard" uses the missing 22 GB. Is that true? I didn't know CMOS 
> settings were DOS. And I'm surprised it needs 22 GB of hard disk space. 
> That's bigger thatn the biggest hard drive I've ever owned (which was 20 
> GB until now).
> 
> Also, I'm surprised that the second hard drive (Master on the other IDE 
> cable) wasn't detected - or maybe that is going to be on the next POST 
> output message...

There is two types of IDE protocols.  LBA28 and LBA48.  LBA28 can only
address 2^28 sectors of 512bytes each, which is 137GB.  Anything bigger
gets clipped.  A modern IDE controller can use 48bit addressing instead,
which gives you 137PB maximum (or something near that).

512 bytes/sector * 1024*1024*256 (that's 2^28) = 137438953472 bytes

I know on my motherboard a BIOS update actually allowed me to start
using LBA48 rather than LBA28 on the onboard controller.  I had
previously bought the add in card because I needed an LBA48 compatible
IDE controller.  Any promise IDE card you buy will do 48bit LBA and
hence support the full size, while your motherboard if it is P3 era will
not unless the maker happens to be nice enough to have made an updated
the BIOS and assuming the chipset is physically capable of allowing it.

Of course even older systems don't even have LBA support, or they had
BIOS bugs that limited you to 33GB (1/4 of the 137GB), or 8GB, or 2GB,
or 512MB if you had no LBA and no support for more than 1024 cylinders.
The 33GB bios bugs linux could work around and allow you to use the
whole drive anyhow even if you told the drive to lie to the bios about
it's size (soft clipping was available for a number of drives to do that
trick).

--
Len Sorensen
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