Hi,
The cache doesn't give you any additional storage capacity as the cache
can never store data, thats not on the tier below it (or store more
writes than the underlying storage has room for).
As for how much you should go for... thats very much up to your use
case. Try to come up with an estimate of how much data is frequently
being accessed (this is the data most likely to remain in the cache).
Then double that estimate - ALWAYS double your estimates ;) (this isn't
Ceph-specific).
There might be additional magic, but in general the cache will store all
the data that is being read from the underlying storage (in hopes of it
being required again later) as well as any writes that may occur (if you
don't configure the cache to be read-only that is). Do note that this
also means that currently (afaik this is being worked on) pulling a
backup of your RBDs will completely flush the cache. This only means
that the files you'd want cached will have to be pulled back in after
that and you may lose the performance advantage for a little while after
each backup.
Hope that helps, dont hesitate with further inquiries!
Marc
On 05/05/2015 11:05 AM, Götz Reinicke - IT Koordinator wrote:
Hi folks,
one more question:
after some more interanal discussions, I'm faced with the question how a
SSD Cache Pool Tiering is calculated in the "overall" usable storage space.
And how "big" do I calculate an SSD Cache Pool?
From my understanding, the cache pool is not calculated into the overall
usable space. It is a "cache".
E.g. The slow pool is 100 TB, the SSD Cache 10 TB, I dont have 110TB all
in all?
True? I'm wrong?
As always thanks a lot and regards! Götz
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