Thank you very much for the quick answer. I supose cache tier works the same way for object storage aswell!?
How is a delete of a cinder volume handled. I ask you this because after the volume got flushed to the cold storage, i then deleted it from cinder. It got deleted from the cache pool aswell but on the HDD pool,when issuing rbd -p ls the volumes were gone but the space was still used (probably rados data) untill i manually made a flush command on the cache pool (i didn't wait too long to see if the space would be cleared in time). It is probably a missconfiguration from my end though. In you opinion is cache tier ready for production? I have read that bcache (flashcache?) is used in favor of cache tier, but is not that simple to setup and there are disadvantages there aswell. Also is there a problem if i add a cache tier to an already existing pool that has data on it? Or should the pool be empty prior to adding the cache tier? 2016-01-12 16:30 GMT+02:00 Nick Fisk <[email protected]>: > > -----Original Message----- > > From: ceph-users [mailto:[email protected]] On Behalf Of > > Mihai Gheorghe > > Sent: 12 January 2016 14:25 > > To: [email protected] > > Subject: [ceph-users] Ceph cache tier and rbd volumes/SSD primary, HDD > > replica crush rule! > > > > Hello, > > > > I have a question about how cache tier works with rbd volumes!? > > > > So i created a pool of SSD's for cache and a pool on HDD's for cold > storage > > that acts as backend for cinder volumes. I create a volume in cinder > from an > > image and spawn an instance. The volume is created in the cache pool as > > expected and it will be flushed to the cold storage after a period of > inactivity > > or after the cache pool reaches 40% full as i understand. > > Cache won't be flushed after inactivity the cache agent only works on % > full (either # of objects or bytes) > > > > > Now after the volume is flushed to the HDD and i make a read or write > > request in the guest OS, how does ceph handle it. Does it upload the > whole > > rbd volume from the cold storage to the cache pool or only a chunk of it > > where the request is made from the guest OS? > > The cache works on hot objects, so particular objects (normally 4MB) of > the RBD will be promoted/demoted over time depending on access patterns. > > > > > Also, is the replication in ceph syncronious or async? If i set a crush > rule to use > > as primary host the SSD one and for replication the HDD one, would the > > writes and reads on the SSD;s be slowed down by the replication on the > > mechanical drive? > > Would this configuration be viable? (i ask this because i don't have the > > number of SSD to make a pool of size 3 on them) > > Its sync replication. If you have a very heavy read workload, you can do > what you suggest and set the SSD OSD to be the primary copy for each PG, > writes will still be limited to the speed of the spinning disks, but reads > will be serviced from the SSD's. However there is a risk in degraded > scenarios that your performance could dramatically drop if more IO is > diverted to spinning disks. > > > > > Thank you! > >
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