Ceph Nested fault domains?

[TSAN]

Broadcom is here. I began exploring Proxmox + Ceph yesterday. Got a nested 3 node spun up and now diving into Ceph.

Does Ceph offer a protection scheme similar to VMware VSAN 2 host mirroring (Nested fault domains) ?

If not familiar - with VSAN 2/3 node cluster having 3 disk groups within a storage policy you can mirror data across 2 separate disks/groups + 2 hosts. Usable space does get a 3x overhead (100GB = 400GB) but makes data very available. Total host failure + disk group failure on remaining host. Its primarily for VSAN 2 nodes but know that Ceph wants minimum of 3 hosts.

Watching few videos thus far is appears Ceph model / crush rules is OSD or Host as protection strategy. But not both together.
I have not done much digging into Ceph yet but thought I'd ask if this is possible. Thanks.

 

[aaron]

Ceph is quite flexible in how it stores data in a redundant way, but doing it in a two node cluster goes against any design principles.

If you are looking into getting some HCI storage into a 2-node cluster, look into local ZFS storage + Replication. It can be combined with HA.

We recommend ZFS pools that are using mirrored VDEVs for a VM workload. You can add multiple to a pool to get more space and performance (zpool add {pool} mirror /dev/disk/by-id/{disk1} /dev/disk/by-id/{disk2}). The result is similar to a RAID-10.

If you need more protection against disk failures and can stomach the additional cost, no one is saying that a mirror must be only 2 disks ;-)
You can do 3 or 4 way mirrors as well.

The only downside is that the VM replication is async. The shortest possible interval between replication runs is currently 1 minute. So in a worst-case scenario, should a node die and the HA guests be recovered by the second node, you might have a little bit of data loss: since the last successful replication.

Overall, for 2-node clusters, keep in mind that Proxmox VE clusters work by quorum (majority). Therefore, the smallest number of votes should be 3. The QDevice mechanism can be used to add another vote to 2-node clusters. The external part can serve multiple Proxmox VE clusters with a vote, and the latency requirements aren't as tight as with Corosync between the Proxmox VE nodes.

 

[TSAN]

Thanks for info and links. Yes I'm also late to ZFS party. ZFS on the agenda along with Proxmox + Ceph and then also visiting HyperV + Starwind.

I'll likely stick with 3 node cluster minimum for Proxmox. 2 node VSAN with witness became interesting to me after the introduction of nested fault domain. (Otherwise not an ideal IMO unless combined with a replica solution to like veeam.) Thought maybe Ceph might support 3 host w/ 2 OSD redundancy kinda like the nested fault domain.

Aside from the ZFS 1 min. async. My other concern would be that its only a isolated replica for failover. So cluster only gets throughput of the primary ZFS pool.
With VSAN one could disable "site read locality" setting and regardless of which host a VM is running on IO reads would happen across all disk groups / hosts. I don't know how Ceph does any load balancing yet but that's why I first figured I would explore Ceph as a HCI solution prior to ZFS. (Along with it being sync)

The other concern (learning topic) I have for Proxmox is vdisk resiliency against corruption during power / server failures when HBA disk cache enabled. VMFS6 / vmdks have proven themselves to me over the years. Also backup a big topic as well - yet to explore Proxmox backup solution. DB crash consistency / VSS / Snapshots. I read Veeam has or is considering investigating support for Proxmox. I hope this develops.

 

[aaron]

Thought maybe Ceph might support 3 host w/ 2 OSD redundancy kinda like the nested fault domain.

I could imagine that building something like this could be done, but you would definitely be customizing your setup a lot. And that is something you probably want to avoid as it makes it harder to predict the behavior in case something is failing. Also, keep in mind that Ceph will be actively recovering a lost replica if it has options to store it on the remaining nodes. Therefore, in a small 3-node cluster, consider to have smaller but more OSDs. At least 4 would be good. Otherwise, the loss of a single disk/OSD could fill up the remaining ones.

From a power-loss resiliency perspective, ZFS is copy on write. The way data is written in ZFS means that the final commit is atomic. Therefore, chances for corruption are very low. The slides in this blog post show how it works.
Ceph with the 3 replicas will let you know if one replica is problematic (checksum doesn't match).

Regarding performance, between Ceph & ZFS, keep in mind that in the default settings, Ceph might also read data from a different node -> higher latency (can be changed to read from the closest replica), while ZFS is local. With mirrored VDEVs, reading is a lot faster than writing, as reading can be spread over all the disks in the mirror.
I cannot say right now what will be faster for reading without benchmarking it. But my guess is, it will be ZFS, especially if you use mirrors with more than 2 disks.

when HBA disk cache enabled

Does it have a battery backup unit (BBU)? Sync writes should only be ACKed by the storage layer once the data is stored in a way where immediate power loss will not lead to data loss.
Ceph writes pretty much all data in sync mode. On ZFS it depends on how the application writes its data. Since DBs will issue sync writes most of the time, their data should be safe as well once ACKed.

For the sake of being thorough: Use good datacenter/enterprise SSDs with power loss protection (PLP) to get good performance with sync writes. No consumer SSDs

 

[TSAN]

Thanks. I agree, no interest in deviating from the Ceph common/supported deployments.

I have yet to dig into ZFS or Ceph how it works in regards to disk spans (+ Ceph fault domains). RAID10 / RAID 60 equivalents.
And I don't know how ZFS replication works if the exact same disk layout needs to be present on replica host. Or if its only capacity requirement.

If it is the same disk layout on ZFS replica, its interesting you feel ZFS is the higher performant. Say RAID10 6 disk spans - 12 disks on both ZFS primary + Replica the ZFS would perform better only using the local ZFS 12 disk pool as opposed to Ceph having access to the 24 disks. It makes sense that Ceph writes would take a penalty but I would (ignorantly) think reads would be better on Ceph provided its public / cluster network is on dedicate 10+ gbps links.

My understanding of most RAID cards / HBAs is that BBU is typically used on the hardware raid side. Write-back caching on virtual disk which then uses memory of RAID card with is protected by BBU. But typically one would have disk cache itself is disabled. (HDDs)

But when using a HBA / RAID card in passthrough mode the BBU is not in use. One chooses to enable/disable disk controller caching for HDDs which then really only protected by the servers' UPSs. SSDs having PLP changed some of this concern.

So for example a subject I want to dig into is the topic of ZFS + Ceph DB/ WAL caching journal disks on high performing SSDs with PLP. So HDDs with disk cache enabled and power loss (shouldn't happen) the data integrity is still protected that the journal disk didn't get a ACK from the HDD yet when power was lost. Upon coming online the PLP SSD journal disk confirms HDD has the data prior to deleting/releasing it.

All these software defined storage solution only want direct disk HBA and not HW RAID. So HBA BBU still valuable?

I've primarily been hardware raid, physical SAN, VMware VSAN guy. And for last 15+ years exclusive VMware. So its time to dig into other software defined storage + Hypervisors. All on the agenda dive head first this year and I have a rather large VMware home environment to learn on nested deployments.

First thing I attempted in this nested lab was installing Ceph dashboard to help expedite my understanding of Ceph disk / fault domains concepts but it appears their is a fix in progress for the dashboard. Fingers crossed soon.
"Module 'dashboard' has failed dependency: PyO3 modules may only be initialized once per interpreter process"
https://forum.proxmox.com/threads/ceph-warning-post-upgrade-to-v8.129371/page-6

 

[aaron]

I have yet to dig into ZFS or Ceph how it works in regards to disk spans (+ Ceph fault domains). RAID10 / RAID 60 equivalents.

Try not to think and map everything to classical RAID levels. Especially for Ceph since it works very different in the way it provides redundancy for the stored data.

And I don't know how ZFS replication works if the exact same disk layout needs to be present on replica host. Or if its only capacity requirement.

The VM replication is using the ZFS send/recv functionality. The only requirement from Proxmox VE is that the storage and ZFS pool are called the same on the nodes. The underlying physical layout can be different, but I don't recommend it.

For Ceph NICs, 10 Gbit can quickly become a bottleneck. A 25 Gbit (lower latency than 10 Gbit) or even a 100 Gbit network will prevent the network from being the bottleneck. If you haven't seen it, we did release a new benchmark whitepaper, investigating how different network setups affect the overall performance. https://forum.proxmox.com/threads/p...eds-in-a-proxmox-ve-ceph-reef-cluster.137964/

So for example a subject I want to dig into is the topic of ZFS + Ceph DB/ WAL caching journal disks on high performing SSDs with PLP

From my experience, I would avoid a dedicated DB/WAL disk and use good SSDs with PLP for the OSD and be good with it. DB/WAL disks can be a bandaid if HDDs are used for the OSDs, but will not magically improve performance to great levels.

All these software defined storage solution only want direct disk HBA and not HW RAID. So HBA BBU still valuable?

With ZFS and Ceph, the disks should be made available to the host as directly as possible. Ideally either directly connected (NVME via PCI connections) or via an HBA. If the HBA is flashed to IT mode, you are golden.

I've primarily been hardware raid, physical SAN, VMware VSAN guy. And for last 15+ years exclusive VMware. So its time to dig into other software defined storage + Hypervisors. All on the agenda dive head first this year and I have a rather large VMware home environment to learn on nested deployments.

In order to just test functionality, setting up virtual Proxmox VE nodes/clusters with many vdisks attached is a good start. With that you can test how things work and more important, how they fail and recover

For performance comparisons, you would need real hardware to get meaningful results.

If you want to dive deeper into Ceph, take a look at the CLI tools and explore. The ceph tool is to manage the Ceph cluster itself on a higher level. The rados tool to interact with it on the object layer and the rbd tool to manage it on the RBD (block devices for disk images) layer.

The following blog article (yes, shameless self plug ) might be interesting as well to understand better how RBD is storing all the (meta)data in the rados object layer of Ceph.

 

[Azunai333]

Ceph might also read data from a different node -> higher latency (can be changed to read from the closest replica)

The latter is news to me. How do you change that?

 

[aaron]

The latter is news to me. How do you change that?

Ah, you called me out on it so I had to look into it in more detail as I only remembered reading about that config option in the docs some time ago. I did remember it somewhat wrong. It is not a setting that can be done in the Ceph config, but needs to be configured on the client when opening an RBD image. So this would need to be handled by the Proxmox VE tooling when starting up a guest with RBD images. Definitely something that could be looked into to make it an option to see if it works as expected and provides considerably faster reading speeds.

https://docs.ceph.com/en/reef/rbd/rbd-config-ref/#confval-rbd_read_from_replica_policy