* Re: question about bitmap for raid6
2012-09-10 16:43 ` FW: question about bitmap for raid6 Ming Lei
@ 2012-09-10 23:07 ` NeilBrown
0 siblings, 0 replies; 2+ messages in thread
From: NeilBrown @ 2012-09-10 23:07 UTC (permalink / raw)
To: Ming Lei; +Cc: linux-raid@vger.kernel.org
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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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