Re: GEOM architecture and the (lack of) need for foot-shooting

On Thu, Apr 07, 2005, Poul-Henning Kamp wrote:
> First of all, there are tools which do not do the right thing.
> Amongst these are bsdlabel, fdisk, boot0cfg and sysinstall.  Most
> of them do something right but none of them gets everything right.
[...]
> The correct way to do that is to use the g_ctl() api because what
> is needed is an out-of-band mechanism to tell that we want to loose
> one of the partitions.
> 
> g_ctl() has not been fully implemented in all classes yet, and
> therefore what we currently do is open one of the partitions and
> issue an ioctl which hits the GEOM_MBR instance.
> 
> This worked fine until recently where it was discovered that one
> could issue ioctls which did "write like" stuff on a filedescriptor
> open only for read.  This is pretty counter to what people exepect
> and we fixed it.
> 
> The problem with that is that there may not be any partition we can
> open for write, they may all be opened by something else (mounted)
> and therefore our attempt to open will fail.

Thanks for the nice description of what needs to be done, and for
taking a constructive attitude towards addressing these problems.

So I don't have time to implement an API for disk partition layout
changes.  But before I read your message, I was looking into
getting sysinstall to issue a DIOCSMBR ioctl to the disk instead
of trying to write it directly.  Would that suffice to allow users
to safely modify their partition tables, while preserving the
anti-foot-shooting mechanism?  Of course, when someone has time
to implement a g_ctl() to do this as you suggest, it would be easy
to adapt sysinstall at that time.

> Now, why havn't you finished GEOM ?  I hear.
> 
> Well, many reasons.
> 
> For one thing I wanted to see how it panned out in all sorts of
> ways before I went any further, it is important to stop up every
> so often and see if the direction is still sound.
> 
> Second, there was a lot of talk about sysinstallNG at the time and
> I thought that would be a great time to revise the entire
> userland-edit-disk-layout thing.
> 
> Third, I needed a break from it.

FWIW, my complaint was not about what you did or didn't manage to
accomplish single-handedly.  My complaint was that you turned on the
anti-footshooting knob by default before userland was ready for it.

> 1. Find out which partition format we migrate to instead of BSDlabel
>    which runs out of steam around 2TB.  GPT has been proposed but
>    seems to be a rather dead end with Itanic sinking fast.

Regardless of what happens to IA-64 and EFI, GPTs probably aren't
going away.  Within a few years, Intel and Microsoft will be
forced to address the 2TB limit for run-of-the-mill desktops, and
I really can't see them inventing yet another format.  But those
are just my two cents; you're right that this should be discussed
further.