I have a SSD writeback cache in front of a HDD, set up through lvmcache (so a dm-cache). When the cache LV is not full (Data% column in lvs < 100.00%), writes go to the cache device (monitored via dstat). However, when the cache LV is full (Data% = 100.00%), writes go directly to the HDD, essentially becoming a writethrough cache. Blocks do not get evicted from the SSD cache, even after some time, and performance drops. When I try reading recently read data from the cached LV, reads are from the SSD, so I assume the entire SSD has now become a read cache. Is this expected behavior for dm-cache's write cache, even in writeback mode? Is there no reserved space for writes? This seems like quite a poor design as essentially users can only write one cache LV's worth of data before the cache becomes a writethrough cache.
My understanding is that dm-cache uses the mq eviction algorithm, but that only applies to read caching and thus is irrelevant to the write caching issue I am observing.
Is there a way to reserve space for a write cache, or use both a dm-writecache (which I understand will not do any read caching) and a dm-cache at the same time?