Hello, On Wed, 20 Jul 2016 11:44:15 +0200 Mateusz Skała wrote: [snip] > > > > > > > > > There are a number of other options to control things, especially with > > Jewel. > > > > Also setting your cache mode to readforward might be a good idea > > > > depending on your use case. > > > > > > > I'm considering this move, especially we are also using SSD Journal. > > Journals are for writes, they don't affect reads, which would come from the > > HDD base backing pool. > > I know that journals are for write, but if I good understand, cache-tier in writeback mode also is used for writes, so each write goes Journal SSD -> Cache Tier ->(after some time) cold storage. Yes, writeback is caching writes (as well), as the name implies. And no, journals are part of OSDs not something separate. So a write to a pool with a cache-tier goes: cache-pool (journal) and within split seconds to cache-pool (OSD). It may then eventually go to the backing-pool (journal, then OSD), but if your object is hot all the time in may NEVER wind up on the backing pool (cold storage in your terms). > > > > > >Please confirm, can I use cache tire readforward with pool size 1? It > > >is > > safe? Then I will have 3 times more space for cache tier. > > > > > Definitely not. > > Even with the best, most trusted SSDs you want a replication size of 2, so you > > can survive an OSD or node failure, etc. > I thought that in readforward mode for cache tier, failure on ssd is not affective on backing storage, and then ceph should re-read objects from this storage. What is workflow for ceph, if fails OSD from cache-tier pool in readforward mode. > I think you're confusing this with readonly, which as the Ceph documentation points out is a mode you're most likely don't want to use anyway. In theory a readonly cache-tier setup could survive the loss of the cache pool, but in practice I think it would break horribly, at least until you removed the broken cache pool manually. The readforward and readproxy modes will cache writes (and thus reads for objects that have been written to and are still in the cache). And as such they contain your most valuable data and can't be allowed to fail, ever. Most people will want writes to be cached, as more applications are allergic to slow writes than reads. Christian [snap] -- Christian Balzer Network/Systems Engineer chibi@xxxxxxx Global OnLine Japan/Rakuten Communications http://www.gol.com/ _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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