On 05/07/2014 10:38 AM, Gregory Farnum wrote:
On Wed, May 7, 2014 at 8:13 AM, Dan van der Ster
<[email protected]> wrote:
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?
Yes, that's roughly it. The part you're leaving out is that a write
may also require promotion, and if it does and the cache is full then
it requires an eviction, and that requires writes to the backing
pool...
Also, doubling the latency on a read can cross a lot of "I don't
notice it" boundaries.
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.
Just a general lack of data indicating it performs well. It will
certainly function, and if you have e.g. 1/4 of your RBD volumes in
use at a time according to time of day, I would expect it to do just
fine.
From what we've seen so far, there are definitely some tradeoffs with
tiering. Using it in the wrong way (IE for pools that have little hot
data) can actually decrease overall performance. We're working on doing
some RBD tests with different skewed distributions now.
-Greg
Software Engineer #42 @ http://inktank.com | http://ceph.com
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