Native Ceph RBD SM driver for XCP-ng

olivierlambert

Food for thoughts for the @Team-Storage and @Team-XAPI-Network

benapetr

Meanwhile until I finish the RBD-native driver which is probably going to take much longer than I anticipated and is probably never going to have "ideal snapshot rollback mechanism", I also decided to create another driver called LVMoRBD which is essentially same as LVMoHBA - it builds LVM-SR on top of RBD block device and is meant to work without need for some complex hacks - https://github.com/benapetr/CephRBDSR/commit/fbde8b49b180d4de60ffea477dffe712f07a4d07

it's really rather a trivial wrapper, it's main benefits as described in the commit message are the ability to auto-mount the RBD image on reboot and to use its own LVM config that enables RBD devices (so no need to modify the default shipped with xcp-ng) and some other things that make creation of LVMSR on top of RBD far easier and natural.

Again - it's not production ready yet, I will be doing many tests on it and probably still need to fix some SCSI related false-positive errors that sometimes appear in SM log. I estimate it will be ready much sooner than RBD-native driver.

LVM on top of RBD is not ideal as there is some tiny overhead, but still probably better than CephFS when it comes to RAW performance (RBD really is something like LUN from HBA)

And yes I do plan to create SMAPIv3 equivalents later when I learn how for now I am still targetting XCP-ng 8.2 as that's what I use in production, and I haven't seen many SMAPIv3 drivers there.

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benapetr committed to benapetr/CephRBDSR

new driver LVMoRBD

This is a simple wrapper for LVM on RBD which is something I have
experience with from the past - it's actually possible to do this using
native LVMSR driver with few hacks - such as modifying the
master/lvm.conf to work with RBD devices

This SR driver is meant to streamline the access without any need for
custom hacks.

Its main benefits over using LVMSR:

* It automatically maps and unmaps rbd devices for you on reboot
* It uses some wrappers to suppress SCSI meta-calls that cause
  unnecessary false-positive warnings in SM log (rbd devices don't have
  SCSI ID)
* It automatically creates RBD device for you with compatibility
  settings known to work fine with XCP-ng dom0 kernel
* It uses its custom LVM config file, so you don't need to expose RBD
  LVM structure to hypervisor, or override the default config file

psafont

@benapetr This is driven by hacky logic from 16 years ago:

on revert, unserialize the previous state, and update the VM record with its saved values. As we do not want to modify that each time we add a field in the datamodel, use some low-level database functions to iterate over the fields of a record. Not very nice as it makes some assumptions on the database layer, but seems to work allright and I don't think that database layer will change a lot in the future.

I think it might be a good idea to add a revert rpc call to the storage interface that xapi can call to, with a backup to use the current logic if necessary; xapi should be able to clean up the database afterwards. I'll ask other maintainers about this or possible alternatives, but since SMAPIv1 is considered deprecated, I doubt it will happen.

I have to say that SMAPIv3 was finally fixed upstream on June by Xenserver (migrations were finally done!) and XCP-ng should get the update that fixes it in the coming weeks. Given this, I would encourage you to take all the learnings you've acquired while doing the driver and porting it to SMAPIv3. SMAPIv1 just simply has too many problems, some of them are architectural, so in general xenserver and xcp-ng maintainers would like to see it finally go away.

for now I am still targetting XCP-ng 8.2 as that's what I use in production, and I haven't seen many SMAPIv3 drivers there.

8.2 is out of support for xenserver, and for xcp-ng yesterday was the last day it was supported, you really should update

benapetr

@psafont thanks for the reply, but isn't that 16 year old logic part of XAPI? I mean - this same hacky logic is present in SMAPIv3 isn't it?

I was going through SMAPIv3 docs and from SM driver perspective (feature-wise) it doesn't seem much different, it looks to me more like many cosmetic changes that make packaging and modularization easier (definitely a good thing), but don't really change any fundamental SM logic - the RPCs are all same as in SMAPIv1, even porting my own driver is probably going to be pretty trivial, it's just about splitting it into multiple files and add some wrappers around it, but it still won't solve my problem - the rollback RPC is just not there, so I would need to instead support this "rollback by making another snapshot of a snapshot" logic enforced by XAPI

psafont

@benapetr You're right. Unfortunately, there's no VDI revert that allows the revert to happen '. This is shown in the documentation: https://xapi-project.github.io/new-docs/toolstack/features/snapshots/index.html (see revert section)

There's an old proposal to do add this: https://xapi-project.github.io/new-docs/design/snapshot-revert/index.html

But the effort fizzed out because currently the imports do not set the snapshot_of correctly, and the operation needs to work even if the field is not set correctly, as it is now. (falling back to the current code seems sensible) https://github.com/xapi-project/xen-api/pull/2058

This needs some effort to get fixed, I'll set up some ticketing so it can be prioritized accordingly.

djs55 opened this pull request in xapi-project/xen-api

closed VDI.revert pull request + extra bits #2058

Maelstrom96

This post is deleted!

olivierlambert

The author of the main recent effort is basically the person who posted just before you

Maelstrom96

@olivierlambert you basically replied just after that I noticed that and deleted my message...

JamesG

Nothing fruitful to add....

But...

Oooof....

This will be somewhat messy to clean up. I'm rooting for you guys though!!

olivierlambert

There's some nice progress on @psafont's work regarding improved revert. I'm confident we'll get there