[CentOS] Using raid 1 for rollback purposes

[Antonio da Silva Martins Junior]

----- "James Olin Oden" <james.oden at gmail.com> escreveu:
> For quite some time I've used raid 1 as a means of providing a
> rollback mechanism for an upgrade (which I learned from others long
> ago).  So essentially, before an upgrade you split the mirrors and
> upgrade one side or the other.  If your upgrade goes well you sync
> one way, if your upgrade does not you sync the other (much hand waving
> and chanting going on, as its more complicated than that, but that is the
> essence of the solution).

 Nice, I didn't think on this before... Will make a try :)

> Recently, I was asked to do the same thing but with a raid 1+0
> solution.  Its easy, enough to break the raid 1 volumes underneath, 
> but then how do I use the broke off volumes to form the duplicate strip.  
> Pictures may help.
>  We start off looking like:
> 
>   /----------- Raid 0 Volume ----------\
>   |    [disk 0]<---R 1--->[disk 2]    |
>   |                                            |
>   |    [disk 1]<---R 1--->[disk 3]    |
>   \--------------------------------------------/
> 
> 
> What we want to go to is:
> 
>   /--- Raid 0 ---\       /--- Raid 0 ---\
>   |    [disk 0]   |       |    [disk 2]   |
>   |                  |       |                  |
>   |    [disk 1]   |       |    [disk 3]   |
>   \-----------------/       \------------------/
>      Old System       New System
> 
> Is this possible with the current set of mdadm tools?

 Humm... didn't know if it's possible, but IMHO it'll be much easier to
do if you use a 0+1 RAID instead of a 1+0 schema :)

   Antonio.

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