Linux software RAID vs. udev

Linux software RAID vs. udev

[Kevin P. Fleming]

I'm switching my boxes over to udev and fully hotplug-driven /dev 
directories.

However, that means by default I only have /dev/md0 from the MD driver, 
and if I want to "mdadm --assemble" any other array I have to manually 
create the device node for it first.

Is there any chance that mdadm is going to learn how to assemble an 
array without having the device node for it already present? If so, then 
the starting of the array would get the device node created (since it 
would be a new block device) and everything would be happy...

(Please keep my address in the CC list, I'm not subscribed)

Re: Linux software RAID vs. udev

[Neil Brown]

On Friday April 30, kpfleming@backtobasicsmgmt.com wrote:
> I'm switching my boxes over to udev and fully hotplug-driven /dev 
> directories.
> 
> However, that means by default I only have /dev/md0 from the MD driver, 
> and if I want to "mdadm --assemble" any other array I have to manually 
> create the device node for it first.
> 
> Is there any chance that mdadm is going to learn how to assemble an 
> array without having the device node for it already present? If so, then 
> the starting of the array would get the device node created (since it 
> would be a new block device) and everything would be happy...

catch-22.  mdadm needed the device to pre-exist.  That is how the md
driver works.

Maybe udev needs to be taught to be more open minded about these
things.

NeilBrown

Re: Linux software RAID vs. udev

[Kevin P. Fleming]

Neil Brown wrote:

> catch-22.  mdadm needed the device to pre-exist.  That is how the md
> driver works.
> 
> Maybe udev needs to be taught to be more open minded about these
> things.

OK, it appears that the root cause of this problem is that udev only 
creates device nodes for gendisks that have been created/registered, 
because that's the only way it can learn about their existence. After 
looking through the md driver, I don't even see how the first unit (md0) 
is getting registered by default, unless for some reason the block layer 
is calling md_probe for unit 0 without even being asked to.

Regardless, this is a big issue in the hotplug world, although it's in 
no way the md driver's fault :-) In the loop driver, it just allocates 
all its gendisks at init() time, which is fine when it only supports 8 
(or a smallish number) of devices. However, if the md driver did that it 
would allocate 512 gendisks and all the attendant md structures which 
would be extremely wasteful.

Hypothetically speaking, how would you feel about a solution where the 
md driver also exposed a single "control" device (character device 
presumably) where mdadm could do all its machinations? Something like 
the device-mapper control interface is what I'm envisioning here, 
although this starts to get into the dm/md overlap area...

I'll move this discussion to the linux-hotplug list; there has to be a 
solution to this problem somewhere :-)

Re: Linux software RAID vs. udev

[Luca Berra]

On Mon, May 03, 2004 at 08:06:44PM -0700, Kevin P. Fleming wrote:
>Hypothetically speaking, how would you feel about a solution where the 
>md driver also exposed a single "control" device (character device 
>presumably) where mdadm could do all its machinations? Something like 
>the device-mapper control interface is what I'm envisioning here, 
>although this starts to get into the dm/md overlap area...
this seems like a nice idea to me, i never liked using md0 as a control
device.

regards,
L.

-- 
Luca Berra -- bluca@comedia.it
        Communication Media & Services S.r.l.
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