Hello Mike, On Tue, 17 Dec 2013 12:32:35 -0500 Mike Dawson wrote: > Christian, > > I think you are going to suffer the effects of spindle contention with > this type of setup. Based on your email and my assumptions, I will use > the following inputs: > > - 4 OSDs, each backed by a 12-disk RAID 6 set > - 75iops for each 7200rpm 3TB drive > - RAID 6 write penalty of 6 > - OSD Journal co-located with OSD As I wrote, would be on SSDs if proven to be beneficial, but lets continue with this assumption. > - Ceph replication size of 2 > > 4osds * 12disks * 75iops / 6(RAID6WritePenalty) / 2(OsdJournalHit) / > 2(CephReplication) = 150 Writes/second max > I have a gut feeling that this can't be quite right given my experience with DRBD based clusters (but on those I keep the closest analogy to the OSD journal, the activity log on SSDs). For starters it would seem to ignore the ability of the RAID controller to merge I/O, especially with a 4GB cache. For example this iostat output right now, sda being the backing device for drbd0: Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await r_await w_await svctm %util sda 0.00 354.00 1.00 766.00 4.00 4558.67 11.90 0.79 1.03 6.67 1.03 0.03 2.27 drbd0 0.00 0.00 1.00 1120.00 4.00 4558.67 8.14 16.64 14.84 6.67 14.85 0.05 5.47 > 4osds * 12disks * 75iops / 2xCephReplication = 1800 Reads/second max > > My guess is 150 writes/second is far lower than your 500 VMs will > require. After all, this setup will likely give you lower writes/second > than a single 15K SAS drive. Further, if you need to replace a drive, I > suspect this setup would grind to a halt as the RAID6 set set attempts > to repair. > 150 IOPs is would indeed be a downer. Though the 500VMs will be the end state, something about 2 years in the future. Thus also my plan to leave space for more storage nodes. As for RAID recovery, it is the usual trade off between impact to the RAID performance and the time you're willing to wait for full restoration of redundancy. Not unlikely to what is configurable with Ceph for OSD rebalancing after a failure. Usually that means low impact, but also low resync speed during busy times, but a quicker recovery during off peak hours. > On the other hand, if you planned for 48 individual drives with OSD > journals on SSDs in a typical setup of perhaps 5:1 or lower ratio of > SSDs:HDs, the calculation would look like: > > 48osds * 75iops / 2xCephReplication = 1800 Writes/second max > > 48osds * 75iops / 2xCephReplication = 1800 Reads/second max > > As you can see, I estimate 12x more random writes without RAID6 (6x) and > co-located osd journals (2x). > Alas at this point we are comparing one round fruit to a different one. To get the same resilience and redundancy as my design the "Ceph" way I think the result would be: 7x 2U nodes with 12drives, 2 SSDs, 10 storage, 3 way replication. 6 nodes needed to get the same storage capacity, 7th to avoid hitting full ratio when a node goes down. 14U instead of 8, 70 disks instead of 48, etc. > Plus you'll be able to configure 12x more placement groups in your CRUSH > rules by going from 4 osds to 48 osds. That will allow Ceph's > psuedo-random placement rules to significantly improve the distribution > of data and io load across the cluster to decrease the risk of hot-spots. > > A few other notes: > > - You'll certainly want QEMU 1.4.2 or later to get asynchronous io for > RBD. > > - You'll likely want to enable RBD writeback cache. It helps coalesce > small writes before hitting the disks. > Sage advice that I would love to follow but likely won't be able to for initial deployment, as native QEMU RBD support is just being added to ganiti. And no, Openstack is not an option at this point in time (no CPU pinning, manual node failure recovery), especially given that this is supposed to be up and running in 3 months tops. Thanks, Christian > > Cheers, > Mike > > > > On 12/17/2013 2:44 AM, Christian Balzer wrote: > > > > 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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