dracut: qestions about mdraid module

dracut: qestions about mdraid module

[Francis Moreau]

Hello,

I'm looking at the module 'mdraid' of dracut (0.38) and was wondering
the purpose of rd.md.uuid= parameter.

I understand it can be used to selectively activate the array matching
the UUID passed through the parameter but I'm wondering in which cases
it can be useful...

Also I don't really understand why dracut try to do its own mechanism to
achieve that (by providing its own udev rules which are modified at boot
time) whereas mdadm already provide the infrastructure (through
mdadm.conf) in a cleaner way (IMHO).

Another question, hopefully you don't mind: dracut provide an udev set
of rules in a file named 59-persistent-storage-md.rules. I still don't
really see the point since most of the stuff in it seems to already be
done by the rule fileudev-md-raid-arrays.rules, shipped by mdadm (3.3.2).

Ah my last question not related to mdraid: why dracut uses stderr to
print out all its messages: info, debug, error. Stderr is usually used
for error/warning messages only.

Thanks !

Re: dracut: qestions about mdraid module

[Harald Hoyer]

On 27.09.2014 17:21, Francis Moreau wrote:
> Hello,
> 
> I'm looking at the module 'mdraid' of dracut (0.38) and was wondering
> the purpose of rd.md.uuid= parameter.
> 
> I understand it can be used to selectively activate the array matching
> the UUID passed through the parameter but I'm wondering in which cases
> it can be useful...
> 
> Also I don't really understand why dracut try to do its own mechanism to
> achieve that (by providing its own udev rules which are modified at boot
> time) whereas mdadm already provide the infrastructure (through
> mdadm.conf) in a cleaner way (IMHO).


If mdadm.conf is not included in the initramfs, you want to have a way to
prevent automatic assembly of all raids found. With big machines, lots of disk,
you only want to assemble those, which you really need.

> 
> Another question, hopefully you don't mind: dracut provide an udev set
> of rules in a file named 59-persistent-storage-md.rules. I still don't
> really see the point since most of the stuff in it seems to already be
> done by the rule fileudev-md-raid-arrays.rules, shipped by mdadm (3.3.2).

That wasn't always the case in all distributions. Nowadays can probably be removed.

> 
> Ah my last question not related to mdraid: why dracut uses stderr to
> print out all its messages: info, debug, error. Stderr is usually used
> for error/warning messages only.
> 
> Thanks !

Hmm, not true.

    warn() {
        echo "Warning: $*" >&2
    }

    info() {
        echo "$*"
    }


see.. warn() goes to error, info() to stdout.

This is in the initramfs, though.
Or do you mean dracut the tool?

Re: dracut: qestions about mdraid module

[Francis Moreau]

On 10/20/2014 01:02 PM, Harald Hoyer wrote:
> On 27.09.2014 17:21, Francis Moreau wrote:
>> Hello,
>>
>> I'm looking at the module 'mdraid' of dracut (0.38) and was wondering
>> the purpose of rd.md.uuid= parameter.
>>
>> I understand it can be used to selectively activate the array matching
>> the UUID passed through the parameter but I'm wondering in which cases
>> it can be useful...
>>
>> Also I don't really understand why dracut try to do its own mechanism to
>> achieve that (by providing its own udev rules which are modified at boot
>> time) whereas mdadm already provide the infrastructure (through
>> mdadm.conf) in a cleaner way (IMHO).
> 
> 
> If mdadm.conf is not included in the initramfs, you want to have a way to
> prevent automatic assembly of all raids found. With big machines, lots of disk,
> you only want to assemble those, which you really need.
> 

Ok but why mdadm.conf wouldn't be included in the initramfs ? If one
doesn't want to start all arrays during the boot process, it's very
likely he doesn't start all arrays automatically while the system is
running, no ?

>>
>> Another question, hopefully you don't mind: dracut provide an udev set
>> of rules in a file named 59-persistent-storage-md.rules. I still don't
>> really see the point since most of the stuff in it seems to already be
>> done by the rule fileudev-md-raid-arrays.rules, shipped by mdadm (3.3.2).
> 
> That wasn't always the case in all distributions. Nowadays can probably be removed.
> 
>>
>> Ah my last question not related to mdraid: why dracut uses stderr to
>> print out all its messages: info, debug, error. Stderr is usually used
>> for error/warning messages only.
>>
>> Thanks !
> 
> Hmm, not true.
> 
>     warn() {
>         echo "Warning: $*" >&2
>     }
> 
>     info() {
>         echo "$*"
>     }
> 
> 
> see.. warn() goes to error, info() to stdout.
> 
> This is in the initramfs, though.
> Or do you mean dracut the tool?
> 

Yes I meant dracut tool used to generate initramfs.

Thanks.

Re: dracut: qestions about mdraid module

[Andrei Borzenkov]

В Tue, 21 Oct 2014 14:35:58 +0200
Francis Moreau <francis.moro-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> пишет:

> On 10/20/2014 01:02 PM, Harald Hoyer wrote:
> > On 27.09.2014 17:21, Francis Moreau wrote:
> >> Hello,
> >>
> >> I'm looking at the module 'mdraid' of dracut (0.38) and was wondering
> >> the purpose of rd.md.uuid= parameter.
> >>
> >> I understand it can be used to selectively activate the array matching
> >> the UUID passed through the parameter but I'm wondering in which cases
> >> it can be useful...
> >>
> >> Also I don't really understand why dracut try to do its own mechanism to
> >> achieve that (by providing its own udev rules which are modified at boot
> >> time) whereas mdadm already provide the infrastructure (through
> >> mdadm.conf) in a cleaner way (IMHO).
> > 
> > 
> > If mdadm.conf is not included in the initramfs, you want to have a way to
> > prevent automatic assembly of all raids found. With big machines, lots of disk,
> > you only want to assemble those, which you really need.
> > 
> 
> Ok but why mdadm.conf wouldn't be included in the initramfs ? If one
> doesn't want to start all arrays during the boot process, it's very
> likely he doesn't start all arrays automatically while the system is
> running, no ?
> 

This could be generic initrd that does not include any host-specific
mdadm.conf (or any other host specific configuration for that matter).