On Tue, 23 Sep 2008, Peter A. Castro wrote: > > > My understanding of RAID5 is that once you configure the set of disks to > > > the array, you can't just add an additional disk to it. Attempting to do > > > so does more than just "reshape" the array, it changes the logical layout > > > of the sectors completely. It's no wonder that LVM can't find anything, > > > since it's data (if it survived the reorg) is no longer where it should > > > be. And, once you've done this, there's really no going back to recover. > > > This is assuming my understanding of RAID5 is correct. > > > > I disagree, because assuming the md reshape operates intuitively, the old > > 320 GB of data should still be visible as the first 320 GB of data on the > > new VIRTUAL 640GB device. And LVM should know nothing about the underlying > > RAIDing, so it should be happy to look there. However, I have no idea what > > "bumped the values in /proc/sys/dev/raid/" means, or if it causes LVM to > > look in the wrong places for things. > > I don't believe the MD/RAID5 operates in this way. He started with a > 2-disk RAID5 (effectively having the data logically interleaved between > the two drives), then he added an additional disk. The way RAID works, > it doesn't just add the disk space to the end of the array (that would > not create a recoverable scenario should it or one of the others fail), > it re-integrates and effectively re-interleaves the logical sectors to > re-spread the data across all three drives. The trouble is, I don't > think you can actually do this with current MD tech (I could be wrong). > > Perhaps a better question would be to ask how, exactly, Bob create his > array initially (what commands) and what steps he took to add the new > drive (again, what commands did he issue). That would give us a better > idea of how the array looked before and after. > > All of this is orthogonal to LVM, however. I suspect a RAID/MD email > list would be a better forum to discuss this. > Current linux MD-raid can reshape raid-5 arrays, the operation just takes really long time and there is a critical section at the start, which means if your computer crashes during the first second or two at the start all data is pretty much lost. The raid-5 reshape is quite new feature in md-raid and I don't know how stable it is. _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
