Re: enabling device-class crush rule moves all data

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On Thu, 12 Apr 2018, Dan Van Der Ster wrote:
> Hi all,
> We noticed that changing existing pools from being class-agnostic to 
> choose a class (e.g. hdd [1]) then a lot, perhaps all, data will move.
> I think this is because the crush IDs in the shadow tree are too 
> different, or maybe just a different order, from the original bucket 
> IDs.
> But I wonder if we could use `ceph osd crush swap-bucket` (or a more 
> elaborate version of swap bucket) to move the shadow buckets into the 
> correct place in the tree, thereby preventing data movement.
> Did anyone already ponder on this topic?

We could do a one-time transition that swaps the "real" ids with the 
shadow ids for one of the classes, and at the same time change the crush 
rule for the pool.

This will only really work/help if the majority of the devices are one 
class.  That's probably usually the case?

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