Hello, I've been doing a lot of reading and am looking at the following design for a storage cluster based on Ceph. I will address all the likely knee-jerk reactions and reasoning below, so hold your guns until you've read it all. I also have a number of questions I've not yet found the answer to or determined it by experimentation. Hardware: 2x 4U (can you say Supermicro? ^.^) servers with 24 3.5" hotswap bays, 2 internal OS (journal?) drives, probably Opteron 4300 CPUs (see below), Areca 1882 controller with 4GB cache, 2 or 3 2-port Infiniband HCAs. 24 3TB HDs (30% of the price of a 4TB one!) in one or two RAID6, 2 of them hotspares, giving us 60TB per node and thus with a replication factor of 2 that's also the usable space. Space for 2 more identical servers if need be. Network: Infiniband QDR, 2x 18port switches (interconnected of course), redundant paths everywhere, including to the clients (compute nodes). Ceph configuration: Additional server with mon, mons also on the 2 storage nodes, at least 2 OSDs per node (see below) This is for a private cloud with about 500 VMs at most. There will 2 types of VMs, the majority writing a small amount of log chatter to their volumes, the other type (a few dozen) writing a more substantial data stream. I estimate less than 100MB/s of read/writes at full build out, which should be well within the abilities of this setup. Now for the rationale of this design that goes contrary to anything normal Ceph layouts suggest: 1. Idiot (aka NOC monkey) proof hotswap of disks. This will be deployed in a remote data center, meaning that qualified people will not be available locally and thus would have to travel there each time a disk or two fails. In short, telling somebody to pull the disk tray with the red flashing LED and put a new one from the spare pile in there is a lot more likely to result in success than telling them to pull the 3rd row, 4th column disk in server 2. ^o^ 2. Density, TCO Ideally I would love to deploy something like this: http://www.mbx.com/60-drive-4u-storage-server/ but they seem to not really have a complete product description, price list, etc. ^o^ With a monster like that, I'd be willing to reconsider local raids and just overspec things in a way that a LOT disks can fail before somebody (with a clue) needs to visit that DC. However failing that, the typical approach to use many smaller servers for OSDs increases the costs and/or reduces density. Replacing the 4U servers with 2U ones (that hold 12 disks) would require some sort of controller (to satisfy my #1 requirement) and similar amounts of HCAs per node, clearly driving the TCO up. 1U servers with typically 4 disk would be even worse. 3. Increased reliability/stability Failure of a single disk has no impact on the whole cluster, no need for any CPU/network intensive rebalancing. Questions/remarks: Due to the fact that there will be redundancy, reliability on the disk level and that there will be only 2 storage nodes initially, I'm planning to disable rebalancing. Or will Ceph realize that making replicas on the same server won't really save the day and refrain from doing so? If more nodes are added later, I will likely set an appropriate full ratio and activate rebalancing on a permanent basis again (except for planned maintenances of course). My experience tells me that an actual node failure will be due to: 1. Software bugs, kernel or otherwise. 2. Marginal hardware (CPU/memory/mainboard hairline cracks, I've seen it all) Actual total loss of power in the DC doesn't worry me, because if that happens I'm likely under a ton of rubble, this being Japan. ^_^ Given that a RAID6 with just 7 disk connected to an Areca 1882 controller in a different cluster I'm running here gives me about 800MB/s writes and 1GB/s reads I have a feeling that putting the journal on SSDs (Intel DC S3700) would be a waste, if not outright harmful. But I guess I shall determine that by testing, maybe the higher IOPS rate will still be beneficial. Since the expected performance of this RAID will be at least double the bandwidth available on a single IB interface, I'm thinking of splitting it in half and have an OSD for each half and bound to a different interface. One hopes that nothing in the OSD design stops it from dealing with these speeds/bandwidths. The plan is to use Ceph only for RBD, so would "filestore xattr use omap" really be needed in case tests determine ext4 to be faster than xfs in my setup? Given the above configuration, I'm wondering how many CPU cores would be sufficient in the storage nodes. Somewhere in the documentation http://ceph.com/docs/master/start/hardware-recommendations/ is a recommendation for 1GB RAM per 1TB of storage, but later on the same page we see a storage server example with 36TB and 16GB RAM. Ideally I would love to use just one 6 or 8 core Opteron 4300 with 32GB of memory, thus having only one NUMA domain and keeping all the processes dealing with I/O and interrupts (and there will be lots of them) on the same CPU. (I very much agree with Kyle Bader's mail about NUMA last week, I've seen this happen myself) According to this: http://ceph.com/docs/master/rados/configuration/network-config-ref/ a monitor doesn't need to be on the cluster network, only the OSDs do. However my initial tests seemed to tell me otherwise. If the mons don't need to be on the cluster network, I'd consider for the initial deployment a direct IB interconnect between the 2 storage nodes. This actually brings me to my next question, should the cluster network fail, would the OSDs still continue to function and use the public network instead? I hope this wasn't tl;dr. ^o^ Regards, Christian -- Christian Balzer Network/Systems Engineer chibi@xxxxxxx Global OnLine Japan/Fusion 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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