Re: [PATCH] enable auto=yes by default when using udev

[Michael Tokarev <mjt@tls.msk.ru>]

Neil Brown wrote:
> On Monday July 3, bluca@comedia.it wrote:
>> Hello,
>> the following patch aims at solving an issue that is confusing a lot of
>> users.
>> when using udev, device files are created only when devices are
>> registered with the kernel, and md devices are registered only when
>> started.
>> mdadm needs the device file _before_ starting the array.
>> so when using udev you must add --auto=yes to the mdadm commandline or
>> to the ARRAY line in mdadm.conf
>>
>> following patch makes auto=yes the default when using udev
> 
> The principle I'm reasonably happy with, though you can now make this
> the default with a line like
> 
>   CREATE auto=yes
> in mdadm.conf.
> 
> However....
> 
>> +
>> +	/* if we are using udev and auto is not set, mdadm will almost
>> +	 * certainly fail, so we force it here.
>> +	 */
>> +	if (autof == 0 && access("/dev/.udevdb",F_OK) == 0)
>> +		autof=2;
>> +
> 
> I'm worried that this test is not very robust.
> On my Debian/unstable system running used, there is no
>  /dev/.udevdb
> though there is a
>  /dev/.udev/db
> 
> I guess I could test for both, but then udev might change
> again.... I'd really like a more robust check.

Why to test for udev at all?  If the device does not exist, regardless
if udev is running or not, it might be a good idea to try to create it.
Because IT IS NEEDED, period.  Whenever the operation fails or not, and
whenever we fail if it fails or not - it's another question, and I think
that w/o explicit auto=yes, we may ignore create error and try to continue,
and with auto=yes, we fail on create error.

Note that /dev might be managed by some other tool as well, like mudev
from busybox, or just a tiny shell /sbin/hotplug script.

Note also that the whole root filesystem might be on tmpfs (like in
initramfs), so /dev will not be a mountpoint.

Also, I think mdadm should stop creating strange temporary nodes somewhere
as it does now.  If /dev/whatever exist, use it. If not, create it (unless,
perhaps, auto=no is specified) directly with proper mknod("/dev/mdX"), but
don't try to use some temporary names in /dev or elsewhere.

In case of nfs-mounted read-only root filesystem, if someone will ever need
to assemble raid arrays in that case.. well, he can either prepare proper
/dev on the nfs server, or use tmpfs-based /dev, or just specify /tmp/mdXX
instead of /dev/mdXX - whatever suits their needs better.

/mjt