mdadm, bitmap and split-brain

mdadm, bitmap and split-brain

[Ronal Andadinata]

Hi,

I'm wondering, in the case of raid1 array with internal bitmap, it goes 
like this :
1. Machine A has 1 raid1 array with Disk1 dan Disk2
2. Dettach Disk2 from Machine A, put it on Machine B
3. Machine A runs normally, Machine B runs on normally on different place.
4. Detach Disk2 from Machine B, put it back on Machine A, re-add Disk2 
to md1

If this were raid1 without bitmap, Disk2 will be overwritten from 
scratch with contents of Disk1, what happen if the array has bitmap ? 
Would there be partial sync ? Would the two disk be identical with only 
using partial sync ?

Re: mdadm, bitmap and split-brain

[NeilBrown]

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On Sun, 27 Nov 2011 03:28:31 +0700 Ronal Andadinata <randadinata@gmail.com>
wrote:

> Hi,
> 
> I'm wondering, in the case of raid1 array with internal bitmap, it goes 
> like this :
> 1. Machine A has 1 raid1 array with Disk1 dan Disk2
> 2. Dettach Disk2 from Machine A, put it on Machine B
> 3. Machine A runs normally, Machine B runs on normally on different place.
> 4. Detach Disk2 from Machine B, put it back on Machine A, re-add Disk2 
> to md1
> 
> If this were raid1 without bitmap, Disk2 will be overwritten from 
> scratch with contents of Disk1, what happen if the array has bitmap ? 
> Would there be partial sync ? Would the two disk be identical with only 
> using partial sync ?

It might fail to re-add, or it might do a partial resync and probably leave
the two devices different so corruption would likely appear.

Just don't do this.  If you want to use disk2 on a different machine,
assemble it with 
   --update=uuid
the first time so it gets a different uuid and can never become confused with
the old array.

NeilBrown


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