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Add design doc for QOS for rbd devices mapped with both krbd and rbd-nbd closes: #521 Signed-off-by: Madhu Rajanna <[email protected]>
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| # Design Doc for RBD QoS using cgroup v2 | ||
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| ## Introduction | ||
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| The RBD QoS (Quality of Service) design aims to address the issue of IO noisy | ||
| neighbor problems encountered in early Ceph deployments catering to OpenStack | ||
| environments. These problems were effectively managed by implementing QEMU | ||
| throttling at the virtio-blk/scsi level. To further enhance this, | ||
| capacity-based IOPs were introduced, providing a more dynamic experience | ||
| similar to public cloud environments. | ||
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| The challenge arises in virtual environments, where a noisy neighbor can lead | ||
| to performance degradation for other instances sharing the same resources. | ||
| Although it's uncommon to observe noisy neighbor issues in Kubernetes | ||
| environments backed by Ceph storage, the possibility exists. The existing QoS | ||
| support with rbd-nbd doesn't apply to krbd, and as rbd-nbd isn't suitable for | ||
| container production workloads, a solution is needed for krbd. | ||
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| To mitigate resource starvation issues, setting QoS at the device level through | ||
| cgroup v2 when enabled becomes crucial. This approach guarantees that I/O | ||
| capacity isn't overcommitted and is fairly distributed among workloads. | ||
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| ## Dependency | ||
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| * cgroup v2 must be enabled on the Node | ||
| * We might have Kubernetes dependency as well | ||
| * Container runtime dependency that supports cgroupv2 | ||
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| ## Manual steps for implementing RBD QoS in a Kubernetes Cluster | ||
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| ```bash | ||
| [$] ssh root@node1 | ||
| sh-4.4# chroot /host | ||
| sh-5.1# cat /proc/partitions | ||
| major minor #blocks name | ||
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| 259 0 125829120 nvme0n1 | ||
| 259 1 1024 nvme0n1p1 | ||
| 259 2 130048 nvme0n1p2 | ||
| 259 3 393216 nvme0n1p3 | ||
| 259 4 125303791 nvme0n1p4 | ||
| 259 6 52428800 nvme2n1 | ||
| 7 0 536870912 loop0 | ||
| 259 5 536870912 nvme1n1 | ||
| 252 0 52428800 rbd0 | ||
| sh-5.1# | ||
| ``` | ||
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| Once the rbd device is mapped on the node we get the device's major and minor | ||
| number we need to set the io limit on the device but we need to find the right | ||
| cgroup file where we need to set the limit | ||
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| Kubernetes/Openshift creates a custom cgroup hierarchy for the pods it created | ||
| but start is `sys/fs/cgroup` folder | ||
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| ```bash | ||
| sh-5.1# cd /sys/fs/cgroup/ | ||
| sh-5.1# ls | ||
| cgroup.controllers cgroup.subtree_control cpuset.mems.effective io.stat memory.reclaim sys-kernel-debug.mount | ||
| cgroup.max.depth cgroup.threads dev-hugepages.mount kubepods.slice memory.stat sys-kernel-tracing.mount | ||
| cgroup.max.descendants cpu.pressure dev-mqueue.mount machine.slice misc.capacity system.slice | ||
| cgroup.procs cpu.stat init.scope memory.numa_stat sys-fs-fuse-connections.mount user.slice | ||
| cgroup.stat cpuset.cpus.effective io.pressure memory.pressure sys-kernel-config.mount | ||
| ``` | ||
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| `kubepods.slice` is the starting point and it contains multiple hierarchy | ||
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| ```bash | ||
| sh-5.1# cd kubepods.slice | ||
| sh-5.1# ls | ||
| cgroup.controllers cpuset.cpus hugetlb.2MB.rsvd.max memory.pressure | ||
| cgroup.events cpuset.cpus.effective io.bfq.weight memory.reclaim | ||
| cgroup.freeze cpuset.cpus.partition io.latency memory.stat | ||
| cgroup.kill cpuset.mems io.max memory.swap.current | ||
| cgroup.max.depth cpuset.mems.effective io.pressure memory.swap.events | ||
| cgroup.max.descendants hugetlb.1GB.current io.stat memory.swap.high | ||
| cgroup.procs hugetlb.1GB.events kubepods-besteffort.slice memory.swap.max | ||
| cgroup.stat hugetlb.1GB.events.local kubepods-burstable.slice memory.zswap.current | ||
| cgroup.subtree_control hugetlb.1GB.max kubepods-pod2b38830b_c2d6_4528_8935_b1c08511b1e3.slice memory.zswap.max | ||
| cgroup.threads hugetlb.1GB.numa_stat memory.current misc.current | ||
| cgroup.type hugetlb.1GB.rsvd.current memory.events misc.max | ||
| cpu.idle hugetlb.1GB.rsvd.max memory.events.local pids.current | ||
| cpu.max hugetlb.2MB.current memory.high pids.events | ||
| cpu.max.burst hugetlb.2MB.events memory.low pids.max | ||
| cpu.pressure hugetlb.2MB.events.local memory.max rdma.current | ||
| cpu.stat hugetlb.2MB.max memory.min rdma.max | ||
| cpu.weight hugetlb.2MB.numa_stat memory.numa_stat | ||
| cpu.weight.nice hugetlb.2MB.rsvd.current memory.oom.group | ||
| ``` | ||
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| Based on the QoS of the pod, either our application pod will end up in the | ||
| above besteffort.slice or burstable.slice cgroup or Guaranteed cgroup, The 3 | ||
| QoS class is defined | ||
| [here](https://kubernetes.io/docs/concepts/workloads/pods/pod-QoS/#quality-of-service-classes) | ||
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| To identify the right cgroup file, we need pod UUID and container UUID from the | ||
| `pod yaml` output | ||
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| ```bash | ||
| [$]kubectl get po csi-rbd-demo-pod -oyaml |grep uid | ||
| uid: cdf7b785-4eb7-44f7-99cc-ef53890f4dfd | ||
| [$]kubectl get po csi-rbd-demo-pod -oyaml |grep -i containerID | ||
| - containerID: cri-o://77e57fbbc0f0630f41f9f154f4b5fe368b6dcf7bef7dcd75a9c4b56676f10bc9 | ||
| [$]kubectl get po csi-rbd-demo-pod -oyaml |grep -i qosClass | ||
| qosClass: BestEffort | ||
| ``` | ||
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| Now check in the besteffor.slice and identify the right path using pod UID and | ||
| container UID | ||
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| Before that check `io.max` on the application pod and see if there is any limit | ||
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| ```bash | ||
| [$]kubectl exec -it csi-rbd-demo-pod -- sh | ||
| sh-4.4# cat /sys/fs/cgroup/io.max | ||
| sh-4.4# | ||
| ``` | ||
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| Come back to the Node and find the right cgroup scope | ||
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| ```bash | ||
| sh-5.1# cd kubepods-besteffort.slice/kubepods-besteffort-podcdf7b785_4eb7_44f7_99cc_ef53890f4dfd.slice/crio-77e57fbbc0f0630f41f9f154f4b5fe368b6dcf7bef7dcd75a9c4b56676f10bc9.scope/ | ||
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| sh-5.1# echo "252:0 wbps=1048576" > io.max | ||
| sh-5.1# cat io.max | ||
| 252:0 rbps=max wbps=1048576 riops=max wiops=max | ||
| ``` | ||
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| Now go back to the application pod and check if we have the right limit set | ||
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| ```bash | ||
| [$]kubectl exec -it csi-rbd-demo-pod -- sh | ||
| sh-4.4# cat /sys/fs/cgroup/io.max | ||
| 252:0 rbps=max wbps=1048576 riops=max wiops=max | ||
| sh-4.4# | ||
| ``` | ||
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| Note:- We can only support the QoS that cgroup v2 io controller supports. | ||
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| Below are the configurations that will be supported | ||
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| | Parameter | Description | | ||
| | --- | --- | | ||
| | MaxReadIOPS | Max read IO operations per second | | ||
| | MaxWriteIOPS | Max write IO operations per second | | ||
| | MaxReadBytesPerSecond | Max read bytes per second | | ||
| | MaxWriteBytesPerSecond | Max write bytes per second | | ||
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| ## Different approaches | ||
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| The above solution can be implemented using 3 different approaches. | ||
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| ### QoS using new parameters in RBD StorageClass | ||
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| ```yaml | ||
| --- | ||
| apiVersion: storage.k8s.io/v1 | ||
| kind: StorageClass | ||
| metadata: | ||
| name: csi-rbd-sc | ||
| provisioner: rbd.csi.ceph.com | ||
| parameters: | ||
| MaxReadIOPS: "" | ||
| MaxWriteIOPS: "" | ||
| MaxReadBytesPerSecond: "" | ||
| MaxWriteBytesPerSecond: "" | ||
| ``` | ||
| #### Implementation for StorageClass QoS | ||
| 1. Create new storageClass with new parameters for QoS | ||
| 1. Modify CSIDriver object to pass pod details to the NodePublishVolume CSI | ||
| procedure | ||
| 1. During NodePublishVolume CSI procedure | ||
| * Retrieve the QoS configuration from the volumeContext in NodePublishRequest | ||
| * Identify the rbd device using the NodeStageVolumePath | ||
| * Get the pod UUID from the NodeStageVolume | ||
| * Set io.max file in all the containers in the pod | ||
| #### Drawbacks of StorageClass QoS | ||
| 1. No way to update the QoS at runtime | ||
| 1. Need to take a backup and restore to New QoS StorageClass | ||
| 1. Delete and Recreate the PV object | ||
| ### Add support for VolumeAttributeClass | ||
| ```yaml | ||
| apiVersion: storage.k8s.io/v1alpha1 | ||
| kind: VolumeAttributesClass | ||
| metadata: | ||
| name: silver | ||
| parameters: | ||
| MaxReadIOPS: "" | ||
| MaxWriteIOPS: "" | ||
| MaxReadBytesPerSecond: "" | ||
| MaxWriteBytesPerSecond: "" | ||
| ``` | ||
| VolumeAttributesClassName is a new parameter in the PVC object the user can | ||
| choose from and this can also be updated or removed later. | ||
| This new VolumeAttributeClass is designed to keep storage that supports setting | ||
| QoS at the storage level which means setting some configuration at the storage | ||
| (like QoS for nbd) | ||
| #### Implementation of VolumeAttributeClass QoS | ||
| 1. Modify CSIDriver object to pass pod details to the NodePublishVolume CSI | ||
| procedure | ||
| 1. Add support in Ceph-CSI to expose ModifyVolume CSI procedure | ||
| 1. Ceph-CSI will store QoS in the rbd image metadata | ||
| 1. During NodeStage operation retrieve the image metadata and store it in | ||
| stagingPath | ||
| 1. Whenever a new pod comes in apply the QoS | ||
| #### Drawbacks of VolumeAttributeClass QoS | ||
| One problem with above is all application need to be scaled downed and scaled | ||
| up to get the new QoS value even though its changed in the PVC object, this is | ||
| sometime impossible as it will have downtime. | ||
| ### VolumeAttributeClass with NodePublish Secret | ||
| 1. Modify CSIDriver object to pass pod details to the NodePublishVolume CSI | ||
| procedure | ||
| 1. Add support in Ceph-CSI to expose ModifyVolume CSI procedure | ||
| 1. Ceph-CSI will store QoS in the rbd image metadata | ||
| 1. During NodePublishVolume operation retrieve the QoS from image metadata | ||
| 1. Whenever a new pod comes in apply the QoS | ||
| This solution addresses the aforementioned issue, but it requires a secret to | ||
| communicate with the ceph cluster. Therefore, we must create a new | ||
| PublishSecret for the storageClass, which may be beneficial when Kubernetes | ||
| eventually enables Node operations. | ||
| Both options 2 and 3 are contingent upon changes to the CSI spec and Kubernetes | ||
| support. Additionally, | ||
| [VolumeAttributeClass](https://github.com/kubernetes/enhancements/pull/3780) is | ||
| currently being developed within the Kubernetes realm and will initially be in | ||
| the Alpha stage. Consequently, it will be disabled by default, but it does | ||
| offer the following advantages. | ||
| 1. No Restore/Clone operation is required to change the QoS | ||
| 1. Easily QoS can be changed for existing PVC only with second approach not | ||
| with third as it needs new secret. | ||
| #### Hybrid Approach | ||
| Considering the advantages and drawbacks, we can use StorageClass and | ||
| VolumeAttributeClass to support QoS, with VolumeAttributeClass taking | ||
| precedence over StorageClass. | ||
| ### Conclusion | ||
| The RBD QoS design aims to mitigate IO noisy neighbor issues in Ceph | ||
| deployments by providing a way to set I/O limits on devices through cgroupv2 | ||
| for both krbd and rbd-nbd mapped devices. Different approaches involving | ||
| StorageClass and VolumeAttributesClass have been discussed, each with its own | ||
| advantages and drawbacks. The hybrid approach offers a flexible solution that | ||
| accounts for dynamic changes while addressing the challenges of existing | ||
| approaches. | ||
| ### References | ||
| Some of the useful links that helped me to understand cgroup v2 and how to set | ||
| QoS on the device. | ||
| * <https://kubernetes.io/docs/concepts/architecture/cgroups/> | ||
| * <https://andrestc.com/post/cgroups-io/> | ||
| * <https://docs.kernel.org/admin-guide/cgroup-v2.html> | ||
| * <https://blog.kintone.io/entry/2022/03/08/170206> | ||
| * <https://tracker.ceph.com/issues/36191> | ||
| * <https://facebookmicrosites.github.io/cgroup2/docs/io-controller.html> | ||
| * <https://martinheinz.dev/blog/91> | ||
| * <https://github.com/kubernetes/kubernetes/issues/92287> |