On 03/07/2014 07:32, Kyle Bader wrote: >> I was wondering, having a cache pool in front of an RBD pool is all fine >> and dandy, but imagine you want to pull backups of all your VMs (or one >> of them, or multiple...). Going to the cache for all those reads isn't >> only pointless, it'll also potentially fill up the cache and possibly >> evict actually frequently used data. Which got me thinking... wouldn't >> it be nifty if there was a special way of doing specific backup reads >> where you'd bypass the cache, ensuring the dirty cache contents get >> written to cold pool first? Or at least doing special reads where a >> cache-miss won't actually cache the requested data? >> >> AFAIK the backup routine for an RBD-backed KVM usually involves creating >> a snapshot of the RBD and putting that into a backup storage/tape, all >> done via librbd/API. >> >> Maybe something like that even already exists? > When used in the context of OpenStack Cinder, it does: > > http://ceph.com/docs/next/rbd/rbd-openstack/#configuring-cinder-backup > > You can have the backup pool use the default crush rules, assuming the > default isn't your hot pool. Another option might be to put backups on > an erasure coded pool, I'm not sure if that has been tested, but in > principle should work since objects composing a snapshot should be > immutable. > Hm... considering that the RBDs are accessed via the default crush rule due to the overlaying, they dont actually use the cache ruleset for anything I wouldnt think. Also the whole idea of backups is to have them on a separate medium. Storing RBD-backups on Ceph is no better than just taking snapshots and keeping them around. Thanks for the input though!
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