Accessing disks via their serial numbers.

[Poul-Henning Kamp]

In message <20060626031636.GK82074 at funkthat.com>, John-Mark Gurney writes:

>Can't we expand the disk api?  add a const char *d_serial to the struct
>disk, and have the disk api automaticly propegate the serial number up
>to the geom layer?

This is more or less what Pawel asked me for, and I am not at all
convinced that is how we want to do it.

There are several attributes which could be relevant in a context
like what Pawel proposes, the serial number (which one ? drive or
media ?) the physical address cam-{bus,id,lun}, FC WWN's and so on.

If we disregard mechanism for a moment, and assume that g_label gets
hold of the info and creates label devices matching these various
informations, then we have to address the problem of obsesity in
the geom mesh:

In addition to ad0, ad0s1 and ad0s1a we will then have label/SOMESERIAL,
label/SOMESERIALs1, label/SOMESERIALs1a etc etc.

The better way of implementing it is by using the devfs 'device-on-demand'
facility to create symlinks to the geom_dev nodes based on specific
lookups, that would not pollute /dev with nodes people don't really
use and it would avoid the combinatorial explosion of the geom mesh.

And this brings me to the next point:  As nice as this feature
sounds to begin with, is it really useful in practice ?

I suspect that the proponents of this scheme will object to my proposal
because they can not do a "ls /dev/mumble/*" to get a list of disk
serial numbers, and if so, that dooms the proposal:  If the only objective
is to get a list of serial numbers, then this is not how it should be
done, instead we should add a real VPD registry to the kernel.

Summary:

I'm against until somebody have explained what the use is, and if
use is proven, it should be done with devfs device-on-demand(/cloning)
instead of g_label.

Poul-Henning

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