raiddetect support for Linux software RAID?

raiddetect support for Linux software RAID?

[Kevin P. Fleming]

I'm wondering if anyone has planned on putting Linux software RAID (md) 
detection into the raiddetect tool. Currently I use mdadm to scan all 
devices to look for my RAID(s), but it's got some problems:

- it's not hotplug driven, it has to be run over all devices
- it's noisy because it wants to scan everything in /dev
- i can't see what's there without trying to activate it

raiddetect would be a nice addition, I would be able to tell that a 
complete array has appeared and just use mdadm to start it up. Thoughts?



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Re: raiddetect support for Linux software RAID?

[Mike Snitzer]

On Sat, May 01 2004 at 09:54,
Kevin P. Fleming <kpfleming@backtobasicsmgmt.com> wrote:

> I'm wondering if anyone has planned on putting Linux software RAID (md) 
> detection into the raiddetect tool. Currently I use mdadm to scan all 
> devices to look for my RAID(s), but it's got some problems:
> 
> - it's not hotplug driven, it has to be run over all devices
> - it's noisy because it wants to scan everything in /dev
> - i can't see what's there without trying to activate it
> 
> raiddetect would be a nice addition, I would be able to tell that a 
> complete array has appeared and just use mdadm to start it up. Thoughts?

The notion of a "complete array" is somewhat difficult to define in terms
of hotplug.  Traditionally a raid array is intialized _after_ all block
devices have been detected and made available.  The piece-wise
availability of hotplug detected block devices makes it hard to know when
its safe/right to initialize the raid array.

E.g. raid initialization special case: What if a raid array spans two
disk controllers; and the first controller happens to have enough disks
for the array to opperate in degraded mode? Once the other controller
makes its disks available the raid could already be online and the disks
that got left behind would need to re-sync.  This is a problem common to
all software raid and hotplug; regardless of whether your using Linux's MD
or DM (EVMS).

If anyone has great ideas on ways to implement a barrier for the raid
initialization special case (and any others I may be overlooking) let me
know.

Mike




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Re: raiddetect support for Linux software RAID?

[Kevin P. Fleming]

Mike Snitzer wrote:

> E.g. raid initialization special case: What if a raid array spans two
> disk controllers; and the first controller happens to have enough disks
> for the array to opperate in degraded mode? Once the other controller
> makes its disks available the raid could already be online and the disks
> that got left behind would need to re-sync.  This is a problem common to
> all software raid and hotplug; regardless of whether your using Linux's MD
> or DM (EVMS).

That is a very valid point; my initial attempts at this always allow 
mdadm to make the final choice. If the array is missing disks (even a 
single disk), and I don't pass --force and --run, it will stop at 
assembling the array but will not try to start it. A subsequent 
--assemble with more disks should add the additional needed disks, 
although I have not tried that.

I think this can be handled suitably in a hotplug script; each disk that 
appears could be "assembled" into whatever array it belongs in, if 
possible. An attempt could be made to start the array, but only if it 
will run in non-degraded mode.


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