Re: Rotating RAID 1

[NeilBrown <neilb@suse.de>]

On Mon, 15 Aug 2011 22:42:06 +0200 Pavel Hofman <pavel.hofman@ivitera.com>
wrote:

> 
> Dne 15.8.2011 22:25, Jérôme Poulin napsal(a):
> > On Mon, Aug 15, 2011 at 4:21 PM, Pavel Hofman <pavel.hofman@ivitera.com> wrote:
> >> Does your scenario involve using two "external" drives, being swapped
> >> each time?
> > 
> > Yes, exactly, 3 or more drive, one stays in place, and the others get
> > rotated off-site.
> > 
> >> I am using such setup, but in order to gain the bitmap
> >> performance effects, I have to run two mirrored RAID1s, i.e. two
> >> bitmaps, each for its corresponding external disk. This setup has been
> >> working OK for a few years now.
> > 
> > Did you script something that stops the RAID and re-assemble it? The
> > RAID must stay mounted in my case as there is live data (incremential
> > backups, so even if the last file is incomplete it is not a problem.)
> 
> I am working on wiki description of our backup solution. The priorities
> got re-organized recently, looks like I should finish it soon :-)
> 
> Yes, I have a script automatically re-assembling the array corresponding
> to the added drive and starting synchronization. There is another script
> checking synchronization status, run periodically from cron. When the
> arrays are synced, it waits until the currently running backup job
> finishes, shuts down the backup software (backuppc), unmounts the
> filesystem to flush, removes the external drives from the array (we run
> several external drives in raid0), does a few basic checks on the
> external copy (mounting read-only, reading a directory) and puts the
> external drives to sleep (hdparm -Y) for storing them outside of company
> premises.
> 
> Give me a few days, I will finish the wiki page and send you a link.
> 

I'm not sure from you description whether the following describes exactly
what you are doing or not, but this is how I would do it.
As you say, you need two bitmaps.

So if there are 3 drives A, X, Y where A is permanent and X and Y are rotated
off-site, then I create two RAID1s like this:


mdadm -C /dev/md0 -l1 -n2 --bitmap=internal /dev/A /dev/X
mdadm -C /dev/md1 -l1 -n2 --bitmap=internal /dev/md0 /dev/Y

mkfs /dev/md1; mount /dev/md1 ...


Then you can remove either or both of X and Y and which each is re-added it
will recover just the blocks that it needs.  X from the bitmap of md0, Y from
the bitmap of md1.

NeilBrown
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