Re: Auto Rebuild on hot-plug

[John Robinson <john.robinson@anonymous.org.uk>]

On 30/03/2010 00:35, Doug Ledford wrote:
> On 03/29/2010 06:36 PM, John Robinson wrote:
>> On 29/03/2010 19:57, Doug Ledford wrote:
>>> On 03/29/2010 02:36 PM, John Robinson wrote:
>> [...]
>>>> Yes, but do create the partition(s), boot sector, etc and set up the
>>>> spare(s).
>>> Really, we should never have to do this in the situation I listed: aka
>>> no degraded arrays exist.  This implies that if you had a raid1 /boot
>>> array, that it's still intact.  So partitioning and setting up boot
>>> loaders doesn't make sense as the new disk isn't going in to replace
>>> anything.  You *might* want to add it to the raid1 /boot, but we don't
>>> know that so doing things automatically doesn't make sense.
>> Actually I've just recently had the scenario where it would have made
>> perfect sense. I hooked up the RAID chassis SATA[0-4] ports to the RAID
>> chassis and put 3 drives in the first 3 slots. Actually it turned out
>> I'd wired it up R-L not L-R so if I'd added a new drive in one of the
>> two right-hand slots it would have turned up as sda on the next boot.
> 
> Yes, but how do you want to fix that situation?  Would you want to make
> the new drives be new boot drives, or would you prefer to shut down,
> move all the previous drives over two slots, and then put the new drive
> into the fourth slot that you previously thought was the second slot?  I
> understand your situation, but were I in that position I'd just shuffle
> my drives to correct my original mistake and go on with things, I
> wouldn't make the new drives be boot drives.  So I'm still not sure I
> see the point to making a new drive that isn't replacing an existing
> drive automatically get set up for boot duty.

I wouldn't want to take the server down to shuffle the drives or cables. 
But my point really is that if I have decided that I would want all the 
drives in my chassis to have identical partition tables and carry an 
active mirror of an array - in my example /boot - I would like to be 
able to configure the hotplug arrangement to make it so, rather than 
leaving me to have to manually regenerate the partition table, install 
grub, add the spare and perhaps even grow the array.

Of course this is a per-installation policy decision of what to do when 
an extra drive is added to a non-degraded array, I'm certainly not 
suggesting this should be the default action, though I think it would be 
nice if it were possible to configure an action in this case.

>> OK, to some extent that's me being stupid, but at the same time I
>> correctly hooked up the first 5 SATA ports to the hot-swap chassis and
>> would want them considered the same group etc.
> 
> I understand wanting them in the same group, but unless something is
> degraded, just being in the same group doesn't tell us if you want to
> keep it as a spare or use it to grow things.

I quite agree. All I'm getting at is that I'd like to be able to say 
something in my mdadm.conf or wherever to say what I'd like done. This 
might mean that I end up something like the following:
DOMAIN path=pci-0000:00:1f.2-scsi-[0-4]:0:0:0       action=include
DOMAIN path=pci-0000:00:1f.2-scsi-[0-4]:0:0:0-part1 action=grow
DOMAIN path=pci-0000:00:1f.2-scsi-[0-4]:0:0:0-part2 action=replace
DOMAIN path=pci-0000:00:1f.2-scsi-[0-4]:0:0:0-part3 action=include

The first line gets the partition table and grub boot code regenerated 
even when nothing's degraded. This in turn may trigger the other lines. 
In the second line my action=grow means fix up my /boot if it's degraded 
and both --add and --grow so it gets mirrored onto a fresh disc. The 
third lines says fix up my swap array if it's degraded, but leave alone 
otherwise. The fourth line says fix up my data array if it's degraded, 
and add as a spare if it's a fresh disc. This last lets me decide later 
what (if any) kind of --grow I want to do - make it larger or reshape 
from RAID-5 to RAID-6.

But as you say, the default should be
DOMAIN path=* action=incremental

and the installer (automated or human) probably wants to edit that to 
include at least
DOMAIN path=something action=replace
to take advantage of this auto-rebuild on hot-plug feature.

Sorry if I'm being long-winded, but hopefully you can see how I'd like 
to be able to configure things. In the first instance, though, just 
getting as far as the replace option would be great.

Cheers,

John.