Re: direction criterium for synchronisation raid1

[Christina Braun <braun@mail.informatik.uni-essen.de>]

Thank you very much for your quick and detailed elaboration! That is my 
searched answer and more.
I have made a short test for quick looking at the event-counters with my 
testpartitions. I  failed and removed the local mirror-leg  and keep the 
remaining active iscsi-leg on the other server (I use raid1 with iscsi 
on 2 servers),  copied  some  files to the md, added then the removed 
local partition again.  I watched the counters in all phases  and can 
see the differences and after syncing the same counters. The criterion 
event-counter  removes also my little doubt about using iscsi-raid1.  
Also I am very glad for your advice in case of mounting md-devices  as 
plain-filesystems and remaining older data in relationship of this 
counter. I will now pay great attention to this event-counter :-)

Best regards,
Christina


Mario 'BitKoenig' Holbe schrieb:
> Christina Braun <braun@mail.informatik.uni-essen.de> wrote:
>   
>> which is in raid1 the criterion for the direction of synchronisation? In 
>>     
>
> The event counter :)
>
>   
>> How can I tell the system which mirrored partition is  now the 
>> data-source without
>> making the raid1 new or zero the superblock?  Is the  destination in 
>>     
>
> Usually you don't need to do this. md manages a per-mirror event counter
> which always gets increased when relevant events occur like assembling
> or stopping an array, adding or removing mirrors etc.
> Due to this, whenever you remove a mirror off an raid1, the event
> counter of the remaining mirrors gets increased. When you shut down your
> machine, plug a disk off and turn the machine on again, once the raid
> gets assembled, the event counter of the remaining mirrors gets
> increased.
>
> Thus, as long as you access those devices through md only (and don't
> mount the device of one of the mirrors as plain filesystem, for
> example), the remaining mirrors will always be newer than removed ones
> and thus md knows the sync-direction (when they are equally "old", they
> are in sync per definition).
>
> There are some exceptions to this:
> 1. When you like to use the older mirror as source of synchronization,
> you have to take care and better zero the superblock of the newer one
> before (make sure your raid device did not get assembled, probably based
> on the wrong mirror).
> 2. When you plug a foreign mirror into the system which - however -
> refers to the same raid-device (especially having the same UUID etc.)
> as your own mirrors but has a bigger event counter, you have to take
> care a lot :) This should usually not happen accidentially, as long as
> you avoid to assign UUIDs to new raids manually.
>
>   
>> every case the device
>> in mdadm manage after the add ? Can I see the source or destination by a 
>> info like mdstat or superblock?
>>     
>
> Have a look at mdadm -E. This shows you the superblocks of single
> mirrors and within them their respective event counters.
>
>
> regards
>    Mario
>   


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