On Tue, Dec 30, 2014 at 7:56 AM, Erik Logtenberg <erik@xxxxxxxxxxxxx> wrote: > > Hi, > > I use a cache tier on SSD's in front of the data pool on HDD's. > > I don't understand the logic behind the flushing of the cache however. > If I start writing data to the pool, it all ends up in the cache pool at > first. So far so good, this was what I expected. However ceph never > starts actually flushing the objects to the data pool. This was not what > I expected. Why not use some of the idle time now and then to flush some > data? > > When I continue to write data, eventually the cache pool fills up > completely (up to target_max_bytes or target_max_objects at least), and > then finally it starts to evict objects to make room for the new writes. > But this evicting is expensive, because the data hasn't been flushed yet > -- so it's a flush+evict. Now the write performance is no better than > without cache pool, because it is limited to the speed at which objects > can be flushed+evicted to the HDD pool. > > So why doesn't ceph flush stuff when it has the time? > > I noticed I can tune some settings like cache_min_flush_age and > cache_min_evict_age but... no -max- age, just -min-. Again I don't > understand, why would I want to force ceph nót to flush objects that are > "too young"? If the cache pool is under heavy write load, I can only > imagine that ceph might run out of objects old enough to be allowed to > flush. Anyway these settings don't seem to matter since ceph doesn't > seem to flush at all until it really has to. > > By the way my cache_target_dirty_ratio and cache_target_full_ratio > settings are set to the default values. Maybe those need tuning? > > Any insight you could provide would be appreciated. > > Thanks, > > Erik. Hi Erik, I have tiering working on a couple test clusters. It seems to be working with Ceph v0.90 when I set: ceph osd pool set POOL hit_set_type bloom ceph osd pool set POOL hit_set_count 1 ceph osd pool set POOL hit_set_period 3600 ceph osd pool set POOL cache_target_dirty_ratio .5 ceph osd pool set POOL cache_target_full_ratio .9 Eric _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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