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. 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? 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) Thank you!
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