Re: Linux 2.4.2ac20

Re: Linux 2.4.2ac20

[David Balazic]

Nathan Walp (faceprint@faceprint.com) wrote :

> Also, sometime between ac7 and ac18 (spring break kept me from testing 
> stuff inbetween), i assume during the new aic7xxx driver merge, the 
> order of detection got changed, and now the ide-scsi virtual host is 
> host0, and my 29160N is host1. Is this on purpose? It messed up a 
> bunch of my stuff as far as /dev and such are concerned. 

SCSI adapters are enumerated randomly(*) , relying on certain numbering
will get you into trouble, sooner or later.
There is no commonly accepted solution, AFAIK.
The same thing can happent to disk enumeration ( sdb becomes sdc )
or partition enumeration ( hda6 becomes hda5 ).

* - theoreticaly no, but practicaly yes ( most of the time )

-- 
David Balazic
--------------
"Be excellent to each other." - Bill & Ted
- - - - - - - - - - - - - - - - - - - - - -

Re: Linux 2.4.2ac20

[Nathan Walp]

David Balazic wrote:
> 
> Nathan Walp (faceprint@faceprint.com) wrote :
> 
> > Also, sometime between ac7 and ac18 (spring break kept me from testing
> > stuff inbetween), i assume during the new aic7xxx driver merge, the
> > order of detection got changed, and now the ide-scsi virtual host is
> > host0, and my 29160N is host1. Is this on purpose? It messed up a
> > bunch of my stuff as far as /dev and such are concerned.
> 
> SCSI adapters are enumerated randomly(*) , relying on certain numbering
> will get you into trouble, sooner or later.
> There is no commonly accepted solution, AFAIK.
> The same thing can happent to disk enumeration ( sdb becomes sdc )
> or partition enumeration ( hda6 becomes hda5 ).
> 
> * - theoreticaly no, but practicaly yes ( most of the time )
> 
> --
> David Balazic
> --------------
> "Be excellent to each other." - Bill & Ted
> - - - - - - - - - - - - - - - - - - - - - -

SCSI adapters are given host numbers in a random order?  Even with no
hardware changes?  Does this make less than sense to anyone else?  Every
kernel EVER up till now has had the real scsi cards (in some particular
order) then ide-scsi.  Have I just been lucky???

Nathan

Re: Linux 2.4.2ac20

[Tim Wright]

On Tue, Mar 13, 2001 at 02:24:54PM -0500, Nathan Walp wrote:
> David Balazic wrote:
> > 
> > SCSI adapters are enumerated randomly(*) , relying on certain numbering
> > will get you into trouble, sooner or later.
> > There is no commonly accepted solution, AFAIK.
> > The same thing can happent to disk enumeration ( sdb becomes sdc )
> > or partition enumeration ( hda6 becomes hda5 ).
> > 
> > * - theoreticaly no, but practicaly yes ( most of the time )
> 
> SCSI adapters are given host numbers in a random order?  Even with no
> hardware changes?  Does this make less than sense to anyone else?  Every
> kernel EVER up till now has had the real scsi cards (in some particular
> order) then ide-scsi.  Have I just been lucky???
> 

No, it's not truly random. That would be insane. And, no, if you don't change
the kernel or the hardware, then they won't jump around.
But, yes, if you change the hardware, or someone changes the probe order
in the kernel, you can suffer from device name slippage. This is a minor
problem on a small home system, and a massive PITA on a large server.
You can at least mandate the probe order on a 2.4 system (see the scsihosts
parameter).

Tim

-- 
Tim Wright - timw@splhi.com or timw@aracnet.com or twright@us.ibm.com
IBM Linux Technology Center, Beaverton, Oregon
Interested in Linux scalability ? Look at http://lse.sourceforge.net/
"Nobody ever said I was charming, they said "Rimmer, you're a git!"" RD VI

Re: Linux 2.4.2ac20

[David Balazic]

Nathan Walp wrote:
> 
> David Balazic wrote:
> >
> > Nathan Walp (faceprint@faceprint.com) wrote :
> >
> > > Also, sometime between ac7 and ac18 (spring break kept me from testing
> > > stuff inbetween), i assume during the new aic7xxx driver merge, the
> > > order of detection got changed, and now the ide-scsi virtual host is
> > > host0, and my 29160N is host1. Is this on purpose? It messed up a
> > > bunch of my stuff as far as /dev and such are concerned.
> >
> > SCSI adapters are enumerated randomly(*) , relying on certain numbering
> > will get you into trouble, sooner or later.
> > There is no commonly accepted solution, AFAIK.
> > The same thing can happent to disk enumeration ( sdb becomes sdc )
> > or partition enumeration ( hda6 becomes hda5 ).
> >
> > * - theoreticaly no, but practicaly yes ( most of the time )
> >
> > --
> > David Balazic
> > --------------
> > "Be excellent to each other." - Bill & Ted
> > - - - - - - - - - - - - - - - - - - - - - -
> 
> SCSI adapters are given host numbers in a random order?  Even with no
> hardware changes?  Does this make less than sense to anyone else?  Every
> kernel EVER up till now has had the real scsi cards (in some particular
> order) then ide-scsi.  Have I just been lucky???
> 
> Nathan

What I mean that too many factors are affecting the enumeration,
so that you can not rely on it :

-  kernel changes
-  driver changes ( partly overlaps with the previous poit )
-  hardware changes
-  something else ?

There is no policy for this enumeration ( AFAIK ) , so there is
nothing to rely on , except luck :-)

-- 
David Balazic
--------------
"Be excellent to each other." - Bill & Ted
- - - - - - - - - - - - - - - - - - - - - -

Re: Linux 2.4.2ac20

[Nathan Walp]

David Balazic wrote:
> 
> Nathan Walp wrote:
> >
> > David Balazic wrote:
> > >
> > > Nathan Walp (faceprint@faceprint.com) wrote :
> > >
> > > > Also, sometime between ac7 and ac18 (spring break kept me from testing
> > > > stuff inbetween), i assume during the new aic7xxx driver merge, the
> > > > order of detection got changed, and now the ide-scsi virtual host is
> > > > host0, and my 29160N is host1. Is this on purpose? It messed up a
> > > > bunch of my stuff as far as /dev and such are concerned.
> > >
> > > SCSI adapters are enumerated randomly(*) , relying on certain numbering
> > > will get you into trouble, sooner or later.
> > > There is no commonly accepted solution, AFAIK.
> > > The same thing can happent to disk enumeration ( sdb becomes sdc )
> > > or partition enumeration ( hda6 becomes hda5 ).
> > >
> > > * - theoreticaly no, but practicaly yes ( most of the time )
> > >
> > > --
> > > David Balazic
> > > --------------
> > > "Be excellent to each other." - Bill & Ted
> > > - - - - - - - - - - - - - - - - - - - - - -
> >
> > SCSI adapters are given host numbers in a random order?  Even with no
> > hardware changes?  Does this make less than sense to anyone else?  Every
> > kernel EVER up till now has had the real scsi cards (in some particular
> > order) then ide-scsi.  Have I just been lucky???
> >
> > Nathan
> 
> What I mean that too many factors are affecting the enumeration,
> so that you can not rely on it :
> 
> -  kernel changes
> -  driver changes ( partly overlaps with the previous poit )
> -  hardware changes
> -  something else ?
> 
> There is no policy for this enumeration ( AFAIK ) , so there is
> nothing to rely on , except luck :-)

See, that all makes sense.  You can't depend on hardware to detect in
the same order, whether it's SCSI cards, network cards, or anything
really.  But the software psuedo-device that is ide-scsi, shouldn't that
pick a spot before or after the hardware and stay there?  That's my
point, i guess.

Nathan

magic device renumbering was -- Re: Linux 2.4.2ac20

[John Jasen]

The problem:

drivers change their detection schemes; and changes in the kernel can
change the order in which devices are assigned names.

For example, the DAC960(?) drivers changed their order of
detecting controllers, and I did _not_ have fun, given that the machine in
question had about 40 disks to deal with, spread across two controllers.

This can create a lot of problems for people upgrading large, production
quality systems -- as, in the worst case, the system won't complete the
boot cycle; or in middle cases, the user/sysadmin is stuck rewriting X
amount of files and trying again; or in small cases, you find out that
your SMC and Intel ethernet cards are reversed, and have to go fix things
...

Possible solutions(?):

Solaris uses an /etc/path_to_inst file, to keep track of device ordering,
et al.

Maybe we should consider something similar, where a physical device to
logical device map is kept and used to keep things consistent on
kernel/driver changes; device addition/removal, and so forth ...

I am, of course, open to better solutions.

--
-- John E. Jasen (jjasen1@umbc.edu)
-- In theory, theory and practise are the same. In practise, they aren't.

Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[Tim Wright]

On Wed, Mar 14, 2001 at 10:36:40AM -0500, John Jasen wrote:
> 
> The problem:
> 
[ Device name slippage ]
> 
> Possible solutions(?):
> 
> Solaris uses an /etc/path_to_inst file, to keep track of device ordering,
> et al.
> 
> Maybe we should consider something similar, where a physical device to
> logical device map is kept and used to keep things consistent on
> kernel/driver changes; device addition/removal, and so forth ...
> 
> I am, of course, open to better solutions.
> 

This would currently be massive overkill for Linux, but DYNIX/ptx avoids this
problem entirely by keeping a device naming database. This became necessary
when we added support for multi-path fibre-channel connected disks. Most
device-naming conventions rely on "physical" addresses i.e. this disk at the end
of this bus connected to this controller in this PCI slot is /dev/sdd. The
Solaris scheme mentioned above is no different in that respect. Unfortunately,
it doesn't work with multi-path FC-connected devices.

Very briefly, devices that are "id-able" i.e. already have a unique id are
simply entered into the database (SCSI drives have a unique id that you can
read at autoconf time). For elements that are not "id-able", we establish
a derived id by recording their relation to "id-able" elements. At boot time,
we scan (in parallel) the system and compare what we find to the database.
That way, you get consistent naming for devices, and, at least in the case of
the SCSI (or FC) drives, the name doesn't change, even if you pull a drive
from one bus and plug it into a different bus entirely.

As I say, this would be massive overkill for Linux, but it's a rather thorough
solution :-)

Tim

-- 
Tim Wright - timw@splhi.com or timw@aracnet.com or twright@us.ibm.com
IBM Linux Technology Center, Beaverton, Oregon
Interested in Linux scalability ? Look at http://lse.sourceforge.net/
"Nobody ever said I was charming, they said "Rimmer, you're a git!"" RD VI

Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[Greg KH]

On Wed, Mar 14, 2001 at 08:27:10AM -0800, Tim Wright wrote:
> This would currently be massive overkill for Linux, but DYNIX/ptx avoids this
> problem entirely by keeping a device naming database. This became necessary
> when we added support for multi-path fibre-channel connected disks. Most
> device-naming conventions rely on "physical" addresses i.e. this disk at the end
> of this bus connected to this controller in this PCI slot is /dev/sdd. The
> Solaris scheme mentioned above is no different in that respect. Unfortunately,
> it doesn't work with multi-path FC-connected devices.
> 
> Very briefly, devices that are "id-able" i.e. already have a unique id are
> simply entered into the database (SCSI drives have a unique id that you can
> read at autoconf time). For elements that are not "id-able", we establish
> a derived id by recording their relation to "id-able" elements. At boot time,
> we scan (in parallel) the system and compare what we find to the database.
> That way, you get consistent naming for devices, and, at least in the case of
> the SCSI (or FC) drives, the name doesn't change, even if you pull a drive
> from one bus and plug it into a different bus entirely.

This comes up a lot with regards to USB devices too.  One of the
usb-serial drivers (the edgeport driver) did something like this by
looking at the topology of the USB bus and where a specific device was
(it was also helped by unique serial numbers) and allowed the devices to
be assigned device nodes based on the topology and a small user space
program.  I'm going to try to do this for all usb-serial devices in 2.5

I can see a scheme like this being very useful for all USB, FireWire,
scsi, etc type of devices.

And no, I don't think that having some type of device naming "database"
is overkill and will eventually make it into parts of the kernel (the
"database" living outside of the kernel of course...)

greg k-h

-- 
greg@(kroah|wirex).com

Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[Tim Wright]

On Wed, Mar 14, 2001 at 10:15:26AM -0800, Greg KH wrote:
[My ramblings on device naming database deleted]
> This comes up a lot with regards to USB devices too.  One of the
> usb-serial drivers (the edgeport driver) did something like this by
> looking at the topology of the USB bus and where a specific device was
> (it was also helped by unique serial numbers) and allowed the devices to
> be assigned device nodes based on the topology and a small user space
> program.  I'm going to try to do this for all usb-serial devices in 2.5
> 
> I can see a scheme like this being very useful for all USB, FireWire,
> scsi, etc type of devices.
> 
> And no, I don't think that having some type of device naming "database"
> is overkill and will eventually make it into parts of the kernel (the
> "database" living outside of the kernel of course...)
> 

Well, if it sounds useful, I can look into putting up the design documentation
(yes, shock, horror, there is some :-). It's pretty thorough and covers most
of the issues involved, and hence might be a good talking point, even if we
chose to implement quite differently.

Tim

-- 
Tim Wright - timw@splhi.com or timw@aracnet.com or twright@us.ibm.com
IBM Linux Technology Center, Beaverton, Oregon
Interested in Linux scalability ? Look at http://lse.sourceforge.net/
"Nobody ever said I was charming, they said "Rimmer, you're a git!"" RD VI

Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[Greg KH]

On Wed, Mar 14, 2001 at 05:53:16PM -0800, Tim Wright wrote:
> Well, if it sounds useful, I can look into putting up the design documentation
> (yes, shock, horror, there is some :-). It's pretty thorough and covers most
> of the issues involved, and hence might be a good talking point, even if we
> chose to implement quite differently.

I'd be interested in seeing it, and I'm sure other developers would too.
If you need a place to host it, I can offer a spot on the linux-hotplug
sourceforge site for it.

thanks,

greg k-h

-- 
greg@(kroah|wirex).com

Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[Christoph Hellwig]

In article <Pine.SGI.4.31L.02.0103141026460.532128-100000@irix2.gl.umbc.edu> you wrote:

> The problem:

> drivers change their detection schemes; and changes in the kernel can
> change the order in which devices are assigned names.
>
> For example, the DAC960(?) drivers changed their order of
> detecting controllers, and I did _not_ have fun, given that the machine in
> question had about 40 disks to deal with, spread across two controllers.

Put LABEL=<label set with e2label> in you fstab in place of the device name.

	Christoph

P.S. UUID= work, too - but I prefer a human-readable label...
-- 
Of course it doesn't work. We've performed a software upgrade.

Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[Peter Svensson]

On Wed, 14 Mar 2001, Christoph Hellwig wrote:

> Put LABEL=<label set with e2label> in you fstab in place of the device name.
>
> 	Christoph
>
> P.S. UUID= work, too - but I prefer a human-readable label...

There are a lot of different devices besides disks, e.g. tape drives etc.
I seem to remember from the last round this came up that modern FC fabrics
have some dynamic properties that may require more intelligence in the
kernel.

Peter
--
Peter Svensson      ! Pgp key available by finger, fingerprint:
<petersv@psv.nu>    ! 8A E9 20 98 C1 FF 43 E3  07 FD B9 0A 80 72 70 AF
<petersv@df.lth.se> !
------------------------------------------------------------------------
Remember, Luke, your source will be with you... always...

Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[Lars Kellogg-Stedman]

> Put LABEL=<label set with e2label> in you fstab in place of the device name.

Which is great, for filesystems that support labels.  Unfortunately,
this isn't universally available -- for instance, you cannot mount
a swap partition by label or uuid, so it is not possible to completely
isolate yourself from the problems of disk device renumbering.

-- Lars

-- 
Lars Kellogg-Stedman <lars@larsshack.org>

Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[Andreas Dilger]

Lars writes:
> > Put LABEL=<label set with e2label> in you fstab in place of the device name.
> 
> Which is great, for filesystems that support labels.  Unfortunately,
> this isn't universally available -- for instance, you cannot mount
> a swap partition by label or uuid, so it is not possible to completely
> isolate yourself from the problems of disk device renumbering.

There is room for a LABEL and/or UUID in the swap superblock, if you
would want to implement support for this.  I took a look once, and it
should be possible to add in a compatible way.  Of course, you can
always put swap into LVM, which also makes it (along with filesystems
other than ext2) immune from the nasty device name changes.

Cheers, Andreas
-- 
Andreas Dilger  \ "If a man ate a pound of pasta and a pound of antipasto,
                 \  would they cancel out, leaving him still hungry?"
http://www-mddsp.enel.ucalgary.ca/People/adilger/               -- Dogbert

Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[Richard B. Johnson]

On Wed, 14 Mar 2001, Lars Kellogg-Stedman wrote:

> > Put LABEL=<label set with e2label> in you fstab in place of the device name.
> 
> Which is great, for filesystems that support labels.  Unfortunately,
> this isn't universally available -- for instance, you cannot mount
> a swap partition by label or uuid, so it is not possible to completely
> isolate yourself from the problems of disk device renumbering.
> 
> -- Lars
> 
> -- 
> Lars Kellogg-Stedman <lars@larsshack.org>
> 
When my BIOS finds IDE disks, it starts at the lowest address of
the port. It then looks for the first master, then the slave(s), etc.
Then it tries the second, etc.

When my SCSI BIOS finds disks, it starts at the first controller,
the first LUN, the first drive, etc.

This used to even be the way disks were located by the kernel
drivers. Now, these are found in some "random" order.

If whatever is causing the "random" order was fixed, put back like
it used to be, etc., we wouldn't have these problems.


Cheers,
Dick Johnson

Penguin : Linux version 2.4.1 on an i686 machine (799.53 BogoMips).

"Memory is like gasoline. You use it up when you are running. Of
course you get it all back when you reboot..."; Actual explanation
obtained from the Micro$oft help desk.

Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[John Jasen]

On Wed, 14 Mar 2001, Richard B. Johnson wrote:

> This used to even be the way disks were located by the kernel
> drivers. Now, these are found in some "random" order.
>
> If whatever is causing the "random" order was fixed, put back like
> it used to be, etc., we wouldn't have these problems.

Another alternative to path2inst or a database, I suppose, would be to use
bus/pci slot information (like in /proc/pci?) to order multiple devices, so
at least there's some consistency.

You might have a serious headache, however, when adding a device, under
that scheme.

--
-- John E. Jasen (jjasen1@umbc.edu)
-- In theory, theory and practise are the same. In practise, they aren't.

Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[Christoph Hellwig]

On Wed, Mar 14, 2001 at 02:11:57PM -0500, Lars Kellogg-Stedman wrote:
> > Put LABEL=<label set with e2label> in you fstab in place of the device name.
> 
> Which is great, for filesystems that support labels.  Unfortunately,
> this isn't universally available -- for instance, you cannot mount
> a swap partition by label or uuid, so it is not possible to completely
> isolate yourself from the problems of disk device renumbering.

True.  Let's mark for 2.5 ToDO list: magic number for swap...

Just because it does not work universally it doesn't have to be a bad idea...

	Christoph

-- 
Of course it doesn't work. We've performed a software upgrade.

Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[Stephen C. Tweedie]

Hi,

On Wed, Mar 14, 2001 at 02:11:57PM -0500, Lars Kellogg-Stedman wrote:
> > Put LABEL=<label set with e2label> in you fstab in place of the device name.
> 
> Which is great, for filesystems that support labels.  Unfortunately,
> this isn't universally available -- for instance, you cannot mount
> a swap partition by label or uuid, so it is not possible to completely
> isolate yourself from the problems of disk device renumbering.

It's not convenient, but it is certainly possible: use a
single-partition raid0 logical device with raid autostart, and you get
a logical /dev/md* device which corresponds to a single partition and
which has a fixed name which is detected by the kernel at runtime and
mapped to the correct disk, wherever the disk may be.

The IBM EVMS folks are looking to generalise this sort of probing, but
for now there is at least one solution to this problem.  LVM works too
to some extent, but it currently lacks the automatic boot-time/
device-detect-time kernel probing step that the software raid code
has.

Cheers,
 Stephen

Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[Andreas Dilger]

Christoph writes:
> In article <Pine.SGI.4.31L.02.0103141026460.532128-100000@irix2.gl.umbc.edu> you wrote:
> > drivers change their detection schemes; and changes in the kernel can
> > change the order in which devices are assigned names.
> >
> > For example, the DAC960(?) drivers changed their order of
> > detecting controllers, and I did _not_ have fun, given that the machine in
> > question had about 40 disks to deal with, spread across two controllers.
> 
> Put LABEL=<label set with e2label> in you fstab in place of the device name.
> P.S. UUID= work, too - but I prefer a human-readable label...

Works OK for ext2 only.  I'm still waiting on the reiserfs folks to add a
UUID and LABEL to their superblock.

However, for raw partitions, you will need to use LVM to get rename-safe
device labels.  You probably want LVM anyways, if you have 40 disks...

Cheers, Andreas
-- 
Andreas Dilger  \ "If a man ate a pound of pasta and a pound of antipasto,
                 \  would they cancel out, leaving him still hungry?"
http://www-mddsp.enel.ucalgary.ca/People/adilger/               -- Dogbert

Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[John Jasen]

On Wed, 14 Mar 2001, Christoph Hellwig wrote:

> In article <Pine.SGI.4.31L.02.0103141026460.532128-100000@irix2.gl.umbc.edu> you wrote:
>
> > The problem:
>
> > drivers change their detection schemes; and changes in the kernel can
> > change the order in which devices are assigned names.
> >
> > For example, the DAC960(?) drivers changed their order of
> > detecting controllers, and I did _not_ have fun, given that the machine in
> > question had about 40 disks to deal with, spread across two controllers.
>
> Put LABEL=<label set with e2label> in you fstab in place of the device name.

It solves the example, but not necessarily the problem.

We're still left with partitions that don't do labels, attached tape
devices, scsi controllers, NICs, and so forth.

--
-- John E. Jasen (jjasen1@umbc.edu)
-- In theory, theory and practise are the same. In practise, they aren't.

Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[Stephen Degler]

Hi,

The solution is not to go down the path2inst road, that is full of 
its own traps.  You want volume labels via a volume manager (do lvm and raid
already do this?) and/or filesystem labels (see e2fslabel).  This won't solve
all of the ills associated with device instance changes, but it will certainly
address the biggest one.

skd

On Wed, Mar 14, 2001 at 10:36:40AM -0500, John Jasen wrote:
> 
> The problem:
> 
> drivers change their detection schemes; and changes in the kernel can
> change the order in which devices are assigned names.
> 
> For example, the DAC960(?) drivers changed their order of
> detecting controllers, and I did _not_ have fun, given that the machine in
> question had about 40 disks to deal with, spread across two controllers.
> 
> This can create a lot of problems for people upgrading large, production
> quality systems -- as, in the worst case, the system won't complete the
> boot cycle; or in middle cases, the user/sysadmin is stuck rewriting X
> amount of files and trying again; or in small cases, you find out that
> your SMC and Intel ethernet cards are reversed, and have to go fix things
> ...
> 
> Possible solutions(?):
> 
> Solaris uses an /etc/path_to_inst file, to keep track of device ordering,
> et al.
> 
> Maybe we should consider something similar, where a physical device to
> logical device map is kept and used to keep things consistent on
> kernel/driver changes; device addition/removal, and so forth ...
> 
> I am, of course, open to better solutions.
> 
> --
> -- John E. Jasen (jjasen1@umbc.edu)
> -- In theory, theory and practise are the same. In practise, they aren't.
> 
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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Re: magic device renumbering was -- Re: Linux 2.4.2ac20

[Mikael Pettersson]

On Wed, 14 Mar 2001 12:34:06 -0700 (MST), Andreas Dilger wrote in LKML:

>Lars writes:
>> > Put LABEL=<label set with e2label> in you fstab in place of the device name.
>> 
>> Which is great, for filesystems that support labels.  Unfortunately,
>> this isn't universally available -- for instance, you cannot mount
>> a swap partition by label or uuid, so it is not possible to completely
>> isolate yourself from the problems of disk device renumbering.
>
>There is room for a LABEL and/or UUID in the swap superblock, if you
>would want to implement support for this.

Despair no more! I've implemented a patch for util-linux-2.11a
which adds LABEL support to mkswap(8) and swapon/swapoff(8).

- I shrunk the padding field in the new-style swap_header to make
  room for 16 bytes worth of volume label (same as ext2)
- mkswap -L label also sets the volume label
- swapon -L label looks for a swap partition with the given label
  (using a clone of mount(8)'s LABEL/UUID= support code)
- swapon/swapoff -a also handles swap fstab entries where the
  device is specified as LABEL=<label>

The patch is available at http://www.csd.uu.se/~mikpe/linux/swap-label/

/Mikael