[CentOS] Using raid 1 for rollback purposes

[Ross S. W. Walker]

Antonio da Silva Martins Junior wrote:
> 
> ----- "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 :)

I would start looking at LVM and mirroring the LV to another,
break it, upgrade the LV and if that doesn't work switching
over to the working LV.

Or try a solution with snapshots...

-Ross

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