On Thu, Sep 09, 2010 at 01:30:15AM +0200, Michael Monnerie wrote: > On Mittwoch, 8. September 2010 Dave Chinner wrote: > > Dynamically changing the RAID array geometry is a Bad Idea. Yes, > > you can do it, but if you've got a filesystem full of data and > > metadata aligned to the old geometry then after the modification > > it won't be aligned anymore. > > > > If you want to do this, then either don't bother about geomtry hints > > in the first place, or dump, rebuild the array, mkfs and restore so > > everything is properly aligned with the new world order. Hell, > > dump/mkfs/restore might even be faster than reshaping a large > > array... > > You're right. But there are some customers who don't want to spend the > money for a 2nd array, and can't afford the downtime of backup, rebuild > raid (takes 8-48 hours), restore. So an online upgrade is needed. We're > not in an ideal world. If you can't afford downtime, then I'd seriously question using reshaping to expand storage because it is one of the highest risk methods of increasing storage capacity you can use. That means you've still got to do the backup before you reshape your raid device - if reshaping fails, and then you need to rebuild + restore. Reshaping is a dangerous operation - you can't go back once it has started, and failures while reshaping can cause data loss. That is, the risk of catastrophic failure goes up significantly while a reshape is in progress. This is the same increase in risk of failures occuring during rebuild after losing a disk - the next disk failure is most likely to occur while the rebuild is in progress, simply because of the sustained inrease in load on the drives. That is, if you have SATA drives then running them for 3 or 4 days at 100% duty cycle while a reshape takes place is putting them far outside their design limits. SATA drives are generally designed for a 20-30% duty cycle for sustained operation. Put disks that are a couple of years old under this sort of load.... Of even more concern is that reshaping a multi-terabyte array requires moving the same order of magnitude of bits around as the BER of the drives. Hence there's every chance of introducing silent bit errors into your data by reshaping unless you further slow the reshape down by having it read back all the data to verify it was reshaped correctly. IMO, reshaping is not a practise you should be designing your capacity upgrade processes around, especially if you have uptime and perforamnce SLA guarantees. It's a very risky operation, and not something I would suggest anyone uses in production unless they have absolutely no other option. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
