Migrating large RGW buckets to erasure coding

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We're planning the move of our production Ceph cluster to Luminous (from Jewel), and thinking about migrating the main S3/radosgw pool to erasure-coding.

It's quite large (1.4PB / 440 M objects).

I've seen http://cephnotes.ksperis.com/blog/2015/04/15/ceph-pool-migration/ which seems to be the standard set of options:

1) rados cppool

This would require a lot of downtime (since it just copies objects one-by-one without checking if the object is already in the target pool), and I don't know if the warning means this would break things anyway:

WARNING: pool copy does not preserve user_version, which some apps may rely on.

2) making a new pool as a cache tier

This looks pretty risky to me (and easy to get wrong), and I imagine the cache-flush-evict-all stage is very IO intensive. Has anyone tried it with a large pool?

3) Rados export/import

I think this has the same problem as 1) above - having to do it as a big bang, and needing a lot of temporary storage

4) make a new pool, make it the default for rgw [not in above link]

This would, I think, mean that new uploads would go into the new pool, and would be non-disruptive. But, AFAICS, there is no way to:
i) look at what % of a users' buckets/objects are in which pool
ii) migrate objects from old to new pool

The second is the kicker, really - it'd be very useful to be able to move objects without having to download, remove, re-upload, but I don't think there's an API to do this? Presumably you could hack something up based on the internals of the radosgw itself, but that seems ... brave?

Surely we're not the only people to have wanted to do this; what have others done?



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