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? >> 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? >> 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). What I'm trying to find is a good baseline hardware configuration that works well with the algorithms and assumptions made by cephs design i.e. if cepth works better with many smaller OSDs rather than a few larger ones then that would obviously influence the overall design of the box. 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
