On 12/22/2013 1:57 AM, shacky wrote: > > Replication is set on a per pool basis. You can set some, or all, > pools to replica size of 2 instead of 3. > > > Thank you very much. I saw this is to be setted in the global > configuration (osd pool default size). > So it's up to me to configure Ceph to be rendundant and fault tolerant?
The default size is already 2, so it will be redundant and somewhat fault tolerant by default. You can learn quite a bit more about pools by reviewing the information in the docs here: http://ceph.com/docs/master/rados/operations/pools/ Just know that replication does not have to be the same across all pools. > If I set "osd pool default size" to 2, I will be sure that if a > cluster node goes down my data will be safe? That is just the default value. Another default is replication being done so that we never place a copy of the data on the same host. Depending on your infrastructure you may want to expand this to racks or rows. In any case losing a single node generally means that your data will be safe (according to the defaults). The potential for data loss would be there if you lost two drives in different hosts BEFORE the cluster finished recovery from the first drive. The documentation regarding the CRUSH map is your friend when it comes to understanding all of this: http://ceph.com/docs/master/rados/operations/crush-map/ > > > Ceph uses replication not erasure coding (unlike RAID). So data is > completely duplicated in multiple copies. Erasure coding is > scheduled for the Firefly release, according to the roadmap. > > > Ad I said, I expect having something similar to RAID5: if one hard > drive per cluster node fails, my data will be safe. If an entire > cluster node fails, my data will be safe. Could you help me to > understand the correct configuration for this situation? Losing an entire cluster node, by default, your data would be safe. However if you were to lose one drive on each node, quickly enough (before recovery completes), it would be possible to have lost some data. To avoid that you could setup RAID sets behind each OSD but that will drive up your cost per gigabyte and, depending on the RAID configuration, could mean having to replicate larger amounts of data when you do lose an OSD. Also, that sort of setup could have performance implications involved. I prefer to see drives setup in JBOD, pools with a replication level 3, a properly implemented infrastructure in the CRUSH map, all sitting on (at least) a 10Gbps cluster network. It might be a bit daunting at first, as there is a lot to learn when it comes to Ceph, but the documentation really is going to be worth the read. From your questions I would suggest going through the Architecture documentation which explains RADOS and how data is stored. Understanding how Ceph stores data will give you a better idea of how replication and failures are handled. http://ceph.com/docs/master/architecture/ > > Thank you very much for your help! > Bye. Good luck! -- JuanJose "JJ" Galvez Professional Services Inktank Storage, Inc. LinkedIn: http://www.linkedin.com/in/jjgalvez
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