Re: question about bitmap for raid6

[NeilBrown <neilb@suse.de>]

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On Mon, 10 Sep 2012 16:43:14 +0000 Ming Lei <Ming.Lei@riverbed.com> wrote:

> Neil,
> 
> How about the other scenario: I plug out a drive and put aside for a while and put it back to the same machine with bitmap enabled for the raid6 array? Does it do full-recovery or fast resync?

If the array has been degraded that whole time, it will do a fast resync.

> 
> I noticed either md superblock or bitmap superblock has events recorded and I guess it may be used to identify the up-to-date drive or the old drive put back again. 

Correct.

NeilBrown


> 
> Thanks
> Ming
> 
> -----Original Message-----
> From: NeilBrown [mailto:neilb@suse.de] 
> Sent: Sunday, September 09, 2012 3:51 PM
> To: Ming Lei
> Subject: Re: question about bitmap for raid6
> 
> On Sat, 8 Sep 2012 21:07:46 +0000 Ming Lei <Ming.Lei@riverbed.com> wrote:
> 
> > Hi Neil,
> > 
> > We use 2.6.32 in house and most recently we found something bad happen when we turn on bitmap for raid6 array. Let me ask a question to help me to understand our situation.
> > 
> > Say I have two hardware identical PCs(name them A and B) running with the same linux distribution. Both box has an raid6 array assmeblying 7 drives with internal bitmap. If I yank out one drive from PC B to make this drive slot empty, and then yank out a drive from PC A and move it to PC B, would md driver force the raid6 array on PC B rebuild(take a long time) or just do resync(very short time)? What's the condition on bitmap code to check if it is really the drive I just pull out from the same machine or it is the drive pulled out from different machine?
> > 
> > Thanks
> > Ming
> 
> PC B would not do anything to drive that you moved from PC A until you add it as a spare.  Then it will perform a complete rebuild of the missing device to the new device.  It will not do a partial recovery.
> 
> 'md' knows it is a device from a different array because the 'uuid' stored in the metadata is different.
> If you managed to create two arrays on two different PCs which both had the same UUID, then you could  definitely get strange data corruption happening, but that is very unlikely.
> 
> NeilBrown
> 
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