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The Friday 2007-08-24 at 16:31 -0400, Felix Miata wrote:
> >> Because it's a kernel design issue, something SUSE/Novell has little or no
> >> control over.
>
> > Well, they can assign developers to scsi and develop patches that add
> > support for more than 16 partitions to scsi modules. If technically
> > possible, of course (I don't know what is the origin of that limit). Or
> > invent something different than scsi for both scsi and ata.
>
> This is nothing ordinary patching can solve. It's a fundamental kernel design
> problem. SCSI has always had the 15 partition limit because its device node
> major is 8. To change it to something less, which would be required to allow
> more than 15 SCSI partitions, would require major backward compatibility
> breakage in kernel design. Novell developers wouldn't likely try anything
> significant without major participation from the kernel development community
> and the Linux community generally.
Huh? Mmm? pata uses larger major numbers. What...? :-?
[...] [reading]
Ah, from what you said I looked at
"/usr/src/linux/Documentation/devices.txt":
3 block First MFM, RLL and IDE hard disk/CD-ROM interface
0 = /dev/hda Master: whole disk (or CD-ROM)
64 = /dev/hdb Slave: whole disk (or CD-ROM)
For partitions, add to the whole disk device number:
0 = /dev/hd? Whole disk
1 = /dev/hd?1 First partition
2 = /dev/hd?2 Second partition
...
63 = /dev/hd?63 63rd partition
22 block Second IDE hard disk/CD-ROM interface
0 = /dev/hdc Master: whole disk (or CD-ROM)
64 = /dev/hdd Slave: whole disk (or CD-ROM)
8 block SCSI disk devices (0-15)
0 = /dev/sda First SCSI disk whole disk
16 = /dev/sdb Second SCSI disk whole disk
32 = /dev/sdc Third SCSI disk whole disk
...
240 = /dev/sdp Sixteenth SCSI disk whole disk
Partitions are handled in the same way as for IDE
disks (see major number 3) except that the limit on
partitions is 15.
I think I understand. The minor device number is used to distinguish both
the device (16 of them) and the partitions (16 of them). If I understand
correctly, it is one byte. But pata distributes that byte differently,
allowing for more partitions and less disks.
Very curious! Indeed, a kernel design decision (or unix design?) having a
long reach. Reminds me of the "1 megabyte is enough memory" decision :-p
Still, I suppose they could use a different device name, like nsd (new s
d), with a different number scheme (like the /dev/sg? devices). But that
would be a wild speculation on my part.
> Kernel people have said a solution should come from userspace, and I think
> this is where Novell developers are mostly spending some effort quietly. I
> found some also from Mandriva, while none from Fedora.
Let's hope! :-)
> The problem as I see it is little or no use cases for systems the Distro
> makers are paid to support to require more than 15 partitions. Outside
> experimentation and development environments, I can't think of a use case for
> more than 15. This makes it hard for a developer to get paid to work on a
> solution, meaning fewer resources available to task this problem.
I understand.
I don't really know about current linux enterprise usage, but I guess
somebody should do a good, serious, poll. But I used a big unix machine,
very serious (not a pc, but with scsi disks) that used several partitions.
If there was one for the system, there would be another one replicating
the system (kept umounted). Several programs had dedicated partitions
(raw), including a database. Things that could grow had their own
partition, like the logs, for instance. However, I don't remember how many
partitions it had. Several, for sure.
- --
Cheers,
Carlos E. R.
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