Hi Simon, we are using something similar for ceph-fs. For a backup system your setup can work, depending on how you back up. While HDD pools have poor IOP/s performance, they are very good for streaming workloads. If you are using something like Borg backup that writes huge files sequentially, a HDD back-end should be OK.
Here some things to consider and try out: 1. You really need to get a bunch of enterprise SSDs with power loss protection for the FS meta data pool (disable write cache if enabled, this will disable volatile write cache and switch to protected caching). We are using (formerly Intel) 1.8T SATA drives that we subdivide into 4 OSDs each to raise performance. Place the meta-data pool and the primary data pool on these disks. Create a secondary data pool on the HDDs and assign it to the root *before* creating anything on the FS (see the recommended 3-pool layout for ceph file systems in the docs). I would not even consider running this without SSDs. 1 such SSD per host is the minimum, 2 is better. If Borg or whatever can make use of a small fast storage directory, assign a sub-dir of the root to the primary data pool. 2. Calculate with sufficient extra disk space. As long as utilization stays below 60-70% bluestore will try to make large object writes sequential, which is really important for HDDs. On our cluster we currently have 40% utilization and I get full HDD bandwidth out for large sequential reads/writes. Make sure your backup application makes large sequential IO requests. 3. As Anthony said, add RAM. You should go for 512G on 50 HDD-nodes. You can run the MDS daemons on the OSD nodes. Set a reasonable cache limit and use ephemeral pinning. Depending on the CPUs you are using, 48 cores can be plenty. The latest generation Intel Xeon Scalable Processors is so efficient with ceph that 1HT per HDD is more than enough. 4. 3 MON+MGR nodes are sufficient. You can do something else with the remaining 2 nodes. Of course, you can use them as additional MON+MGR nodes. We also use 5 and it improves maintainability a lot. Something more exotic if you have time: 5. To improve sequential performance further, you can experiment with larger min_alloc_sizes for OSDs (on creation time, you will need to scrap and re-deploy the cluster to test different values). Every HDD has a preferred IO-size for which random IO achieves nearly the same band-with as sequential writes. (But see 7.) 6. On your set-up you will probably go for a 4+2 EC data pool on HDD. With object size 4M the max. chunk size per OSD will be 1M. For many HDDs this is the preferred IO size (usually between 256K-1M). (But see 7.) 7. Important: large min_alloc_sizes are only good if your workload *never* modifies files, but only replaces them. A bit like a pool without EC overwrite enabled. The implementation of EC overwrites has a "feature" that can lead to massive allocation amplification. If your backup workload does modifications to files instead of adding new+deleting old, do *not* experiment with options 5.-7. Instead, use the default and make sure you have sufficient unused capacity to increase the chances for large bluestore writes (keep utilization below 60-70% and just buy extra disks). A workload with large min_alloc_sizes has to be S3-like, only upload, download and delete are allowed. Best regards, ================= Frank Schilder AIT Risø Campus Bygning 109, rum S14 ________________________________________ From: Anthony D'Atri <anthony.da...@gmail.com> Sent: Saturday, November 18, 2023 3:24 PM To: Simon Kepp Cc: Albert Shih; ceph-users@ceph.io Subject: [ceph-users] Re: How to use hardware Common motivations for this strategy include the lure of unit economics and RUs. Often ultra dense servers can’t fill racks anyway due to power and weight limits. Here the osd_memory_target would have to be severely reduced to avoid oomkilling. Assuming the OSDs are top load LFF HDDs with expanders, the HBA will be a bottleck as well. I’ve suffered similar systems for RGW. All the clever juggling in the world could not override the math, and the solution was QLC. “We can lose 4 servers” Do you realize that your data would then be unavailable ? When you lose even one, you will not be able to restore redundancy and your OSDs likely will oomkill. If you’re running CephFS, how are you provisioning fast OSDs for the metadata pool? Are the CPUs high-clock for MDS responsiveness? Even given the caveats this seems like a recipe for at best disappointment. At the very least add RAM. 8GB per OSD plus ample for other daemons. Better would be 3x normal additional hosts for the others. > On Nov 17, 2023, at 8:33 PM, Simon Kepp <si...@kepp.tech> wrote: > > I know that your question is regarding the service servers, but may I ask, > why you are planning to place so many OSDs ( 300) on so few OSD hosts( 6) > (= 50 OSDs per node)? > This is possible to do, but sounds like the nodes were designed for > scale-up rather than a scale-out architecture like ceph. Going with such > "fat nodes" is doable, but will significantly limit performance, > reliability and availability, compared to distributing the same OSDs > on more thinner nodes. > > Best regards, > Simon Kepp > > Founder/CEO > Kepp Technologies > >> On Fri, Nov 17, 2023 at 10:59 AM Albert Shih <albert.s...@obspm.fr> wrote: >> >> Hi everyone, >> >> In the purpose to deploy a medium size of ceph cluster (300 OSD) we have 6 >> bare-metal server for the OSD, and 5 bare-metal server for the service >> (MDS, Mon, etc.) >> >> Those 5 bare-metal server have each 48 cores and 256 Gb. >> >> What would be the smartest way to use those 5 server, I see two way : >> >> first : >> >> Server 1 : MDS,MON, grafana, prometheus, webui >> Server 2: MON >> Server 3: MON >> Server 4 : MDS >> Server 5 : MDS >> >> so 3 MDS, 3 MON. and we can loose 2 servers. >> >> Second >> >> KVM on each server >> Server 1 : 3 VM : One for grafana & CIe, and 1 MDS, 2 MON >> other server : 1 MDS, 1 MON >> >> in total : 5 MDS, 5 MON and we can loose 4 servers. >> >> So on paper it's seem the second are smarter, but it's also more complex, >> so my question are «is it worth the complexity to have 5 MDS/MON for 300 >> OSD». >> >> Important : The main goal of this ceph cluster are not to get the maximum >> I/O speed, I would not say the speed is not a factor, but it's not the main >> point. >> >> Regards. >> >> >> -- >> Albert SHIH 🦫 🐸 >> Observatoire de Paris >> France >> Heure locale/Local time: >> ven. 17 nov. 2023 10:49:27 CET >> _______________________________________________ >> ceph-users mailing list -- ceph-users@ceph.io >> To unsubscribe send an email to ceph-users-le...@ceph.io >> > _______________________________________________ > ceph-users mailing list -- ceph-users@ceph.io > To unsubscribe send an email to ceph-users-le...@ceph.io _______________________________________________ ceph-users mailing list -- ceph-users@ceph.io To unsubscribe send an email to ceph-users-le...@ceph.io _______________________________________________ ceph-users mailing list -- ceph-users@ceph.io To unsubscribe send an email to ceph-users-le...@ceph.io
