Re: [Xen-users] Basic questions - numbered

[Tim Deegan <Tim.Deegan@xxxxxxxxxxxx>]

Hi Ted,

On Fri, Apr 01, 2005 at 12:26:13PM -0700, Ted Hilts wrote:
> So, theoretically speaking,  if I were to use /dev/hde (the fith (5th) 
> drive -- in the sequence a, b, c, d, e, f,  if there were no gaps) I 
> would have  /hde1 and if on hde I allocated an extended partion with 64 
> logical partitions (numbered 1 to 64) I could install a bootable (using 
> Grub) linux distribution in each of these logical paritions.

Almost, but you can't use quite that many partitions (see below).

> But I don't think this variability is 
> the case with Grub and the designations are reserved, as in (hd4, n) 
> which always refers to the 5th hard drive and a partition.

Yep.  Linux names drives based on how they are connected, e.g., /dev/sda
for the first SCSI drive, /dev/hdd for the second IDE drive on the
second channel, and so on. 

Linux uses partition numbers 1 to 4 for primary partitions, and then
counts all the logical partitions from 5 upwards, to a maximum of 63 for
IDE disks (so you can only have 59 logical partitions) and of 15 for
SCSI disks (so you can only have 11 logical partitions).

http://lxr.linux.no/source/Documentation/devices.txt#L192
http://lxr.linux.no/source/Documentation/devices.txt#L272

GrUB allocates drive numbers in the order the BIOS presents them,
regardless of whether they're SCSI or IDE, so they may not necessarily
be in the same order that linux sees them in.  I believe if you run the
grub command in a running linux it will try to figure out the mapping
for you.

GrUB uses partition numbers 0 to 3 for primary partitions, and then
counts all the logical partitions from 4 upwards.

http://www.gnu.org/software/grub/manual/html_node/Naming-convention.html#Naming%20convention

> Continuing theoretically: If that drive contained just logical 
> partitions I would have on hde (hde1, hde2, hde3...hden) where n=1 to 64 
> -- I believe 0 refers to the extended partition itself but I am not sure 
> as it is not used just assumed.
> And if this were translated into GRUB terminology I would have ((hd4,0), 
> (hd4,1), (hd4,2)...(hdn-1,n-1) where n =1 to 64

If you have 59 logical partitions on the 5th IDE drive (and that drive
is the master device on the third controller, and you don't have any
non-IDE drives...), GrUB should call the logical partitions (hd4,4) to
(hd4,62).  Linux should call them /dev/hde5 to /dev/hde63.

Have you thought about using LVM? :)

HTH,

Tim.

-- 
Tim Deegan                           (My opinions, not the University's)
Systems Research Group
University of Cambridge Computer Laboratory

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