Thanks Marc Nick, that makes things much more clear!
/Götz
Am 05.05.15 um 11:36 schrieb Nick Fisk:
Just to add, the caching promote/demotes whole objects, so if you have
lots of small random IO’s you will need a lot more cache than compared
to the actual amount of hot data. Reducing the RBD object size can help
with this, but YMMV
Also don’t try and compare Ceph Tiering to a generic cache. With a
generic cache you tend to get a benefit even when the cache is too
small, however due to the way Ceph promote/demotes, cache misses are
very expensive and I have found that unless the bulk of you’re working
set fits in the cache tier, then performance can actually be worse than
without the cache.
*From:*ceph-users [mailto:ceph-users-boun...@lists.ceph.com] *On Behalf
Of *Marc
*Sent:* 05 May 2015 10:25
*To:* ceph-users@lists.ceph.com; goetz.reini...@filmakademie.de
*Subject:* Re: [ceph-users] capacity planing with SSD Cache Pool Tiering
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
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Götz Reinicke
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