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103 lines
4.9 KiB
Markdown
103 lines
4.9 KiB
Markdown
# RBD NBD VOLUME HEALER
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- [RBD NBD VOLUME HEALER](#rbd-nbd-volume-healer)
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- [Rbd Nbd](#rbd-nbd)
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- [Advantages of userspace mounters](#advantages-of-userspace-mounters)
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- [Side effects of userspace mounters](#side-effects-of-userspace-mounters)
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- [Volume Healer](#volume-healer)
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- [More thoughts](#more-thoughts)
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## Rbd nbd
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The rbd CSI plugin will provision new rbd images and attach and mount those
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to workloads. Currently, the default mounter is krbd, which uses the kernel
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rbd driver to mount the rbd images onto the application pod. Here on
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at Ceph-CSI we will also have a userspace way of mounting the rbd images,
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via rbd-nbd.
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[Rbd-nbd](https://docs.ceph.com/en/latest/man/8/rbd-nbd/) is a client for
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RADOS block device (rbd) images like the existing rbd kernel module. It
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will map an rbd image to an nbd (Network Block Device) device, allowing
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access to it as a regular local block device.
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It’s worth making a note that the rbd-nbd processes will run on the
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client-side, which is inside the `csi-rbdplugin` node plugin.
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### Advantages of userspace mounters
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- It is easier to add features to rbd-nbd as it is released regularly with
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Ceph, and more difficult and time consuming to add features to the kernel
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rbd module as that is part of the Linux kernel release schedule.
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- Container upgrades will be independent of the host node, which means if
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there are any new features with rbd-nbd, we don’t have to reboot the node
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as the changes will be shipped inside the container.
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- Because the container upgrades are host node independent, we will be a
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better citizen in K8s by switching to the userspace model.
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- Unlike krbd, rbd-nbd uses librbd user-space library that gets most of the
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development focus, and hence rbd-nbd will be feature-rich.
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- Being entirely kernel space impacts fault-tolerance as any kernel panic
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affects a whole node not only a single pod that is using rbd storage.
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Thanks to the rbd-nbd’s userspace design, we are less bothered here, the
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krbd is a complete kernel and vendor-specific driver which needs changes
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on every feature basis, on the other hand, rbd-nbd depends on NBD generic
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driver, while all the vendor-specific logic sits in the userspace. It's
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worth taking note that NBD generic driver is mostly unchanged much from
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years and consider it to be much stable. Also given NBD is a generic
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driver there will be many eyes on it compared to the rbd driver.
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### Side effects of userspace mounters
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Since the rbd-nbd processes run per volume map on the client side i.e.
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inside the `csi-rbdplugin` node plugin, a restart of the node plugin will
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terminate all the rbd-nbd processes, and there is no way to restore
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these processes back to life currently, which could lead to IO errors
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on all the application pods.
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This is where the Volume healer could help.
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## Volume healer
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Volume healer runs on the start of rbd node plugin and runs within the
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node plugin driver context.
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Volume healer does the below,
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- Get the Volume attachment list for the current node where it is running
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- Filter the volume attachments list through matching driver name and
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status attached
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- For each volume attachment get the respective PV information and check
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the criteria of PV Bound, mounter type
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- Build the StagingPath where rbd images PVC is mounted, based on the
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KUBELET path and PV object
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- Construct the NodeStageVolume() request and send Request to CSI Driver.
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- The NodeStageVolume() has a way to identify calls received from the
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healer and when executed from the healer context, it just runs in the
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minimal required form, where it fetches the previously mapped device to
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the image, and the respective secrets and finally ensures to bringup the
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respective process back to life. Thus enabling IO to continue.
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### More thoughts
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- Currently the NodeStageVolume() call is safeguarded by the global Ceph-CSI
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level lock (per volID) that needs to be acquired before doing any of the
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NodeStage, NodeUnstage, NodePublish, NodeUnPulish operations. Hence none
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of the operations happen in parallel.
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- Any issues if the NodeUnstage is issued by kubelet?
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- This can not be a problem as we take a lock at the Ceph-CSI level
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- If the NodeUnstage success, Ceph-CSI will return StagingPath not found
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error, we can then skip
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- If the NodeUnstage fails with an operation already going on, in the
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next NodeUnstage the volume gets unmounted
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- What if the PVC is deleted?
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- If the PVC is deleted, the volume attachment list might already got
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refreshed and entry will be skipped/deleted at the healer.
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- For any reason, If the request bails out with Error NotFound, skip the
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PVC, assuming it might have deleted or the NodeUnstage might have
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already happened.
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- The Volume healer currently works with rbd-nbd, but the design can
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accommodate other userspace mounters (may be ceph-fuse).
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