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- Fixes spelling mistakes. - Grammatical error correction. - Wrapping the text at 80 line count..etc Signed-off-by: Humble Chirammal <hchiramm@redhat.com>
100 lines
4.9 KiB
Markdown
100 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 to
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workloads. Currently, the default mounter is krbd, which uses the kernel rbd
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driver to mount the rbd images onto the application pod. Here on at Ceph-CSI we
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will also have a userspace way of mounting the rbd images, via rbd-nbd.
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[Rbd-nbd](https://docs.ceph.com/en/latest/man/8/rbd-nbd/) is a client for RADOS
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block device (rbd) images like the existing rbd kernel module. It will map an
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rbd image to an nbd (Network Block Device) device, allowing access to it as a
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regular local block device.
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It’s worth making a note that the rbd-nbd processes will run on the client-side,
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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 Ceph,
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and more difficult and time consuming to add features to the kernel rbd module
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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 there
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are any new features with rbd-nbd, we don’t have to reboot the node as the
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changes will be shipped inside the container.
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- Because the container upgrades are host node independent, we will be a better
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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. Thanks
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to the rbd-nbd’s userspace design, we are less bothered here, the krbd is a
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complete kernel and vendor-specific driver which needs changes on every
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feature basis, on the other hand, rbd-nbd depends on NBD generic driver, while
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all the vendor-specific logic sits in the userspace. It's worth taking note
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that NBD generic driver is mostly unchanged much from years and consider it to
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be much stable. Also given NBD is a generic driver there will be many eyes on
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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. inside
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the `csi-rbdplugin` node plugin, a restart of the node plugin will terminate all
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the rbd-nbd processes, and there is no way to restore these processes back to
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life currently, which could lead to IO errors 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 node
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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 status
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attached
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- For each volume attachment get the respective PV information and check the
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criteria of PV Bound, mounter type
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- Build the StagingPath where rbd images PVC is mounted, based on the KUBELET
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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 healer and
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when executed from the healer context, it just runs in the minimal required
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form, where it fetches the previously mapped device to the image, and the
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respective secrets and finally ensures to bringup the respective process back
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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, NodeUnPublish operations. Hence none of
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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 next
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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 get
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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 already
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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). |