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-bounces@xxxxxxxxxxxxxx] *On Behalf > Of *Marc > *Sent:* 05 May 2015 10:25 > *To:* ceph-users@xxxxxxxxxxxxxx; goetz.reinicke@xxxxxxxxxxxxxxx > *Subject:* Re: 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 -- Götz Reinicke IT-Koordinator Tel. +49 7141 969 82 420 E-Mail goetz.reinicke@xxxxxxxxxxxxxxx Filmakademie Baden-Württemberg GmbH Akademiehof 10 71638 Ludwigsburg www.filmakademie.de Eintragung Amtsgericht Stuttgart HRB 205016 Vorsitzender des Aufsichtsrats: Jürgen Walter MdL Staatssekretär im Ministerium für Wissenschaft, Forschung und Kunst Baden-Württemberg Geschäftsführer: Prof. Thomas Schadt
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