mirror of
https://github.com/ceph/ceph-csi.git
synced 2024-11-23 06:40:23 +00:00
539686329f
This commit address the issue- https://github.com/ceph/ceph-csi/issues/3448. Signed-off-by: riya-singhal31 <rsinghal@redhat.com>
101 lines
4.9 KiB
Markdown
101 lines
4.9 KiB
Markdown
# RBD NBD VOLUME HEALER
|
||
|
||
- [RBD NBD VOLUME HEALER](#rbd-nbd-volume-healer)
|
||
- [Rbd Nbd](#rbd-nbd)
|
||
- [Advantages of userspace mounters](#advantages-of-userspace-mounters)
|
||
- [Side effects of userspace mounters](#side-effects-of-userspace-mounters)
|
||
- [Volume Healer](#volume-healer)
|
||
- [More thoughts](#more-thoughts)
|
||
|
||
## Rbd nbd
|
||
|
||
The rbd CSI plugin will provision new rbd images and attach and mount those to
|
||
workloads. Currently, the default mounter is krbd, which uses the kernel rbd
|
||
driver to mount the rbd images onto the application pod. Here on at Ceph-CSI we
|
||
will also have a userspace way of mounting the rbd images, via rbd-nbd.
|
||
|
||
[Rbd-nbd](https://docs.ceph.com/en/latest/man/8/rbd-nbd/) is a client for RADOS
|
||
block device (rbd) images like the existing rbd kernel module. It will map an
|
||
rbd image to an nbd (Network Block Device) device, allowing access to it as a
|
||
regular local block device.
|
||
|
||
|
||
|
||
It’s worth making a note that the rbd-nbd processes will run on the client-side,
|
||
which is inside the `csi-rbdplugin` node plugin.
|
||
|
||
### Advantages of userspace mounters
|
||
|
||
- It is easier to add features to rbd-nbd as it is released regularly with Ceph,
|
||
and more difficult and time consuming to add features to the kernel rbd module
|
||
as that is part of the Linux kernel release schedule.
|
||
- Container upgrades will be independent of the host node, which means if there
|
||
are any new features with rbd-nbd, we don’t have to reboot the node as the
|
||
changes will be shipped inside the container.
|
||
- Because the container upgrades are host node independent, we will be a better
|
||
citizen in K8s by switching to the userspace model.
|
||
- Unlike krbd, rbd-nbd uses librbd user-space library that gets most of the
|
||
development focus, and hence rbd-nbd will be feature-rich.
|
||
- Being entirely kernel space impacts fault-tolerance as any kernel panic
|
||
affects a whole node not only a single pod that is using rbd storage. Thanks
|
||
to the rbd-nbd’s userspace design, we are less bothered here, the krbd is a
|
||
complete kernel and vendor-specific driver which needs changes on every
|
||
feature basis, on the other hand, rbd-nbd depends on NBD generic driver, while
|
||
all the vendor-specific logic sits in the userspace. It's worth taking note
|
||
that NBD generic driver is mostly unchanged much from years and consider it to
|
||
be much stable. Also given NBD is a generic driver there will be many eyes on
|
||
it compared to the rbd driver.
|
||
|
||
### Side effects of userspace mounters
|
||
|
||
Since the rbd-nbd processes run per volume map on the client side i.e. inside
|
||
the `csi-rbdplugin` node plugin, a restart of the node plugin will terminate all
|
||
the rbd-nbd processes, and there is no way to restore these processes back to
|
||
life currently, which could lead to IO errors on all the application pods.
|
||
|
||
|
||
|
||
This is where the Volume healer could help.
|
||
|
||
## Volume healer
|
||
|
||
Volume healer runs on the start of rbd node plugin and runs within the node
|
||
plugin driver context.
|
||
|
||
Volume healer does the below,
|
||
|
||
- Get the Volume attachment list for the current node where it is running
|
||
- Filter the volume attachments list through matching driver name and status
|
||
attached
|
||
- For each volume attachment get the respective PV information and check the
|
||
criteria of PV Bound, mounter type
|
||
- Build the StagingPath where rbd images PVC is mounted, based on the KUBELET
|
||
path and PV object
|
||
- Construct the NodeStageVolume() request and send Request to CSI Driver.
|
||
- The NodeStageVolume() has a way to identify calls received from the healer and
|
||
when executed from the healer context, it just runs in the minimal required
|
||
form, where it fetches the previously mapped device to the image, and the
|
||
respective secrets and finally ensures to bringup the respective process back
|
||
to life. Thus enabling IO to continue.
|
||
|
||
### More thoughts
|
||
|
||
- Currently the NodeStageVolume() call is safeguarded by the global Ceph-CSI
|
||
level lock (per volID) that needs to be acquired before doing any of the
|
||
NodeStage, NodeUnstage, NodePublish, NodeUnPublish operations. Hence none of
|
||
the operations happen in parallel.
|
||
- Any issues if the NodeUnstage is issued by kubelet?
|
||
- This can not be a problem as we take a lock at the Ceph-CSI level
|
||
- If the NodeUnstage success, Ceph-CSI will return StagingPath not found
|
||
error, we can then skip
|
||
- If the NodeUnstage fails with an operation already going on, in the next
|
||
NodeUnstage the volume gets unmounted
|
||
- What if the PVC is deleted?
|
||
- If the PVC is deleted, the volume attachment list might already get
|
||
refreshed and entry will be skipped/deleted at the healer.
|
||
- For any reason, If the request bails out with Error NotFound, skip the
|
||
PVC, assuming it might have deleted or the NodeUnstage might have already
|
||
happened.
|
||
- The Volume healer currently works with rbd-nbd, but the design can
|
||
accommodate other userspace mounters (may be ceph-fuse).
|