Hi, Gregory Farnum wrote:
3) The cost of a cache miss is pretty high, so they should only be used when the active set fits within the cache and doesn't change too frequently.
Can you roughly quantify how long a cache miss would take? Naively I'd assume it would turn one read into a read from the backing pool, a write into the cache pool, then the read from the cache. Is that right?
So, Ceph will not automatically redirect to the base pool in case of failures; in the general case it*can't*, but you could set up monitoring to remove a read-only pool if that happens. But in general, I would only explore cache pools if you expect to periodically pull in working data sets out of much larger sets of cold data (e.g., jobs run against a particular bit of scientific data out of your entire archive).
That's a pity. What would be your hesitation about using WB caching with RBD images, assuming the cache pool is sized large enough to match the working set.
Cheers, Dan
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