On 11/06/2012 08:30 PM, Josh Durgin wrote: > On 11/05/2012 06:49 PM, Dennis Jacobfeuerborn wrote: >> On 11/06/2012 01:14 AM, Josh Durgin wrote: >>> On 11/05/2012 09:13 AM, Dennis Jacobfeuerborn wrote: >>>> Hi, >>>> I'm thinking about building a ceph cluster and I'm wondering what a good >>>> configuration would look like for 4-8 (and maybe more) 2HU 8-disk or 3HU >>>> 16-disk systems. >>>> Would it make sense to make each disk an individual OSD or should I >>>> perhaps >>>> create several raid-0 and create OSDs from those? >>> >>> This mainly depends on your ratio of disks to cpu/ram. Generally we >>> recommend 1GB ram and 1Ghz per OSD. If you've got enough cpu/ram, >>> running 1 OSD/disk is pretty common. It makes recovering from a >>> single disk failure faster. >> >> So basically a 2Ghz quad-core CPU and 8GB RAM would be sufficient for 8 >> OSDs? > > Yes, although more RAM will be better (providing more page cache). > >>>> Also what is the best setup for the journal? If I understand it correctly >>>> then each OSD needs its own journal and that should be a separate disk but >>>> that would be quite wasteful it seems. Would it make sense to put in two >>>> small SSD disks in a raid-1 configuration and create a filesystem for each >>>> OSD journal on it? >>> >>> This is certainly possible. It's a bit less overhead if you give each >>> osd it's own partition of the ssd(s) instead of going through another >>> filesystem. >>> >>> I suspect it would be better to not use raid-1, since these ssds will be >>> receiving all the data the osds write as well. If they're in raid-1 instead >>> of being used independently, their lifetimes might be much >>> shorter. >> >> My primary concern here is fault tolerance. What happens when the journal >> disk dies? Can ceph cope with that and write directly to the OSDs or would >> that mean that with a single shared disk for all OSDs a failure would mean >> the entire system is effectively offline for ceph? > > I'm going to point to some messages in the archives to avoid repetition: > > http://permalink.gmane.org/gmane.comp.file-systems.ceph.devel/6377 > >>>> How does the number of OSDs/Nodes affect the performance of say a >>>> single dd >>>> operation? Will blocks be distributed over the cluster and written/read in >>>> parallel or does the number only improve concurrency rather than benefit >>>> single threaded workloads? >>> >>> In cephfs and rbd, objects are distributed over the cluster, but the >>> OSDs/node ratio doesn't really affect the performance. It's more >>> dependent on the workload and striping policy. For example, with >>> a small stripe size, small sequential writes will benefit from more >>> osds, but the number per node isn't particularly important. >> >> By OSDs/Nodes I really meant "OSDs or nodes" and not the ratio. What I'm >> trying to understand is if a) the number of nodes plays a significant role >> when it comes to performance (e.g. a 4 node cluster with large disks vs. a >> 16 node cluster with smaller disks) and b) how much of an impact the number >> of OSDs has on the cluster e.g. an 8 node cluster with each node being a >> single OSD (with all disks as raid-0) vs. an 8 node cluster with say 64 >> OSDs (each node with 8 disks as individual OSDs). > > Generally more smaller nodes will recover faster from a node or disk > failure than a few larger node, since the remaining OSDs recover in > parallel. There are some other advantages of many small nodes. Wido and > Stefan covered this well in this thread: > > http://comments.gmane.org/gmane.comp.file-systems.ceph.devel/10212 > So that sound like a raid-1 (or potentially a raid-10) is pretty much a must when using a shared ssd disk for the journals for more than one OSD. Without redundancy the failure of a single disk (the journal one) would take down all OSDs on that node making a multi OSD per node setup pointless. Regards, Dennis -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
