Re: howto know if raid is really working?

[Joao Clemente <jpcl@rnl.ist.utl.pt>]

Thanks for the reply, but I still haven't cleared my doubt.
I'll cut the relevant part of the reply (relevant for what I'm trying to 
understand) and comment at the end:


Alvin Oga wrote:
> On Thu, 16 Dec 2004, Joao Clemente wrote:
> > Image you're using software raid and 1 disk fails. You somehow get 
> > alerted and , AFAIK, you
> > 1 - shutdown the machine (ok, this if you don't have a hot-swap system)
> > 2 - remove the failed disk
> > 3 - insert a new, "fresh-from-the-store" disk
>
> for sanity ...  i always fdisk the new disk to be the same as the
> remaining disk
>
> > 4 - power-up
>
> and sw raid will mirror the good disk onto the new disk
[snip]
> > Now, if this was a hardware raid solution, yes I believe the array will 
> > self-contruct again.
>
> sw raid, when PROPERLY created will also resysnc/self-construct
> again all by itself
>
> > My question is if, with these steps you'll have a 
> > software RAID system resync'ing the array... or you need to do extra 
> > steps like:
>
> no extra step is supposed to be needed except to take the old
> disk out and plug in a new one
[snip]
> > 5 - partition the disk with same partition layout as the removed one
>
> probably a good idea ... to keep it the same as before
> even if your enw disk is bigger than before
> 	- use the xtra (unused) space for something else


Ok, Alvin, as far as I understand when you said at the top "for sanity 
... i always fdisk the new disk to be the same as the remaining disk" 
you mean limiting it's size, as you say here at last few lines, right?

My doubt is: If you DONT do this (and, following my steps, you CANT 
fdisk unless you power the machine first :-) what will happen then?
Supose you have your disks with 3 partitions each, {hd?1, hd?2, hda?3} 
from wich you have your 3 software raid partitions {md0, md1, md2}.
One disk fails. You put a new one. New as "out-of-the-shop", no 
partitions, no filesystem. What happens? Will the partitions be 
generated? Or you need do setup {hdx1, hdx2, hdx3} on the new disk, 
before software raid resyncs the disks?

Or this is not the way to do it?

From what I remember reading, HW RAID handles "disks", SW RAID handles 
"partitions". If you replace a disk in a HW RAID, the new disk will be 
copied and be equal to the older ones. Mapping this to SW RAID makes the 
sentence like this: "If you replace a PARTITION in a SW RAID, the new 
PARTITION will be copied and be equal to the older ones". So what 
happens if the disk has no partitions?

Thanks
Joao Clemente