Greg Oster wrote:
> Mark Cullen writes:
> 
>>Just a few questions to clarify really.
>>
>>I have a simple RAID-1, configuration as follows:
>>
>>---
>>START array
>>1 2 0
>>
>>START disks
>>/dev/wd1a
>>/dev/wd3a
>>
>>START layout
>>128 1 1 1
>>
>>START queue
>>fifo 100
>>---
>>
>>It all works great, I can fail either disk and rebuild it using -R 
>>/dev/wdXa. I am, however, a little confused over how I might go about 
>>removing a disk from the array, should I ever want to (I have no idea if 
>>I ever will, but I like to try to be semi-prepared at least).
> 
> 
> If the array won't be used again, then just make sure you do a 
> 'raidctl -A no raid0' so that the components don't get auto-configured.
> You can then do a 'raidctl -u raid0' to unconfigure.  If you want to 
> nuke the labels (something that raidctl doesn't have an option for)
> you can use something like:  
> 
>  dd if=/dev/zero of=/dev/wd1a bs=512 count=1 seek=32

You mean, if I just do a `raidctl -u raid0`, if I were to reboot the 
array would infact reappear and 'auto-configure' itself?

Under what situation(s) might I want to nuke the labels? A little 
confused there :)

> 
> 
>>I've figured out that I can actually unconfigure the array, by doing a 
>>`raidctl -u raid0`, and then I can force a reconfiguration by then doing 
>>a `raidctl -C /root/raid.conf raid0`. I'm not sure if I have to do a 
>>`raidctl -I <serial> raid0` and `raidctl -i raid0` after this, but it 
>>doesn't seem to hurt anything, so I do these too. Is this correct?
> 
> 
> If you reconfigure with -C, then you *must* do both -I and -i.

Ok ta.

> 
> 
>>Right, so now I unconfigure the array, and change my configuration to 
>>remove one of the disks:
>>
>>---
>>START array
>>1 2 0
>>
>>START disks
>>/dev/wd1a
>>absent
>>#/dev/wd3a
>>
>>START layout
>>128 1 1 1
>>
>>START queue
>>fifo 100
>>---
>>
>>After recreating the configuration, it seems to work ok (I can still 
>>mount the device, and my test data is still there). I can't, however, do 
>>a `raidctl -i raid0`. Doing such a thing, I get an error on the console:
>>
>>"raid0: Error re-writing parity!"
>>
>>Does this matter?
> 
> 
> There's only one component, so it has no where to write the "parity" 
> (i.e. "mirror", in this case).

Makes sense :-) Just making sure it was normal behaviour!

>  
> 
>>Right, so now I have just one disk on the array. I decide to copy some 
>>more test data on to it while the second disk is missing. Works fine. I 
>>add the missing disk back and reconfigure:
>>
>>---
>>START array
>>1 2 0
>>
>>START disks
>>/dev/wd1a
>>#absent
>>/dev/wd3a
>>
>>START layout
>>128 1 1 1
>>
>>START queue
>>fifo 100
>>---
>>
>>Suprisingly, even though the data on these two disks now differs, the 
>>status of the array is ok on both disks. I assume this is because I 
>>forced a reconfiguration, and there's really no easy way to tell that 
>>the data differs.
> 
> 
> Yes...  "-C" is to be used for new arrays with nothing on them, or
> when you want to say "I know better, trust me"...  

So, this would indeed be the correct way of taking a disk (not a spare) 
out of the array configuration, as I have in the examples?

> 
> 
>>A `raidctl -i raid0` succeeds (admittedly I have no 
>>idea what it does, however),
> 
> 
> For a RAID 1 configuration, it will verify the that the data bits 
> are in sync... 

Now this is interesting... see below

> 
> 
>>but the data on the two disks is still 
>>different, that is, /dev/wd3a still has the old copy of the data. 
> 
> 
> Um... after the 'raidctl -i raid0' completes, the data parts of wd1a 
> and wd3a had better be *exactly* the same!

They weren't the same afterwards! If I mounted just the disk that was 
missing when I added the new data, after doing an -i, the data was *not* 
there on that disk. I had to force a rebuild of the disk in order to get 
the data on to it.

However, I may know why? I took wd1 out, and put the new data on wd3. If 
I issue a -i, does it use the first disk in the array config to rebuild 
the parity? This would have meant the old copy of data on wd1 would have 
been used?!

> 
> 
>>In 
>>such a situation then, am I to force the re-added disk to "failed" and 
>>rebuild it?
> 
> 
> Yes.  With raid0 in operation with just wd1a, you could have done:
> 
>  raidctl -a /dev/wd3a raid0
>  raidctl -vF absent raid0
> 
> and watched it rebuild :)

Ah! Would that have put /dev/wd3a in to the 'absent' slot, and removed 
it from the 'spares'? If so, that's quite neat!!

> 
> 
>>Other than that, I seem to be able to get along with it just fine :-)
> 
> 
> Good! :)
> 
> Later...
> 
> Greg Oster
> 
> 
> 

Thanks for your reply Greg :)