On 07/07/2014 09:17 AM, Lazuardi Nasution wrote:
Hi,
I'm thinking of using SSD for cache on CEPH where the SSDs are on the
same OSD nodes with HDDs. My options are using CEPH Cache Tiering or
using another cache software like bCache or FlashCache. On the second
option, the SSDs will only cache data related to HDDs on the same node only.
Any experience of both options? Actually, I don't have idea how CEPH
Cache Tiering is working. What data is cached? Is there any inter-nodes
cache transfer?
It really depends on your use case. If you are doing RBD, I suspect you
will need a very skewed hot/cold distribution and a very specific
workload for it to be worth it. If you very seldomly do writes, and do
just enough hot reads that buffer cache isn't doing enough, you may see
some benefit. Reducing the RBD object size may help in some cases, but
it'll take some tweaking/testing.
If you are using RGW, the cache tier may perform somewhat better but
will still likely need a pretty skewed distribution.
Basically the take-away for right now should be that cache tiering is
useful in some cases, but isn't really a general solution (yet?). You
need to be careful to make sure it makes sense for your workload. Ceph
may be able to make much better use of the hardware in other ways (Using
the SSDs for write journals, bcache/flashcache, etc).
Mark
Best regards,
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