CBT: the thread to centralize your feedback

rtjdamen

@StormMaster i understand, we are also not using this on these vms but it does make sense, something is breaking the cbt chain and causing a full, question is if this is caused by the backup job or something else. Thanks for your input.

andyh

I was looking to do some updates on our TrueNAS Scale device providing an NFS share to my XCP-ng hosts (8.2.1), we have CBT enabled for backups.

However when I try to move the Xen Orchestra VDI from TrueNAS to local storage I receive the following error:

{
  "id": "0m261vorl",
  "properties": {
    "method": "vdi.migrate",
    "params": {
      "id": "f91f81f2-308d-4de9-879e-c1fa84a37d27",
      "sr_id": "49822b62-3367-7e7c-76ee-1cfc91a262e9"
    },
    "name": "API call: vdi.migrate",
    "userId": "7b63bade-51f3-4916-9174-f969da17774a",
    "type": "api.call"
  },
  "start": 1728731129889,
  "status": "failure",
  "updatedAt": 1728731132752,
  "end": 1728731132752,
  "result": {
    "code": "VDI_CBT_ENABLED",
    "params": [
      "OpaqueRef:4f16cd0e-fbaf-48c3-aae4-092b9906b9e4"
    ],
    "task": {
      "uuid": "7ce61fba-d6d3-12cb-2585-79d5b69d3857",
      "name_label": "Async.VDI.pool_migrate",
      "name_description": "",
      "allowed_operations": [],
      "current_operations": {},
      "created": "20241012T11:05:31Z",
      "finished": "20241012T11:05:32Z",
      "status": "failure",
      "resident_on": "OpaqueRef:fe0440a3-4a31-44d6-8317-a0e64d0ee01e",
      "progress": 1,
      "type": "<none/>",
      "result": "",
      "error_info": [
        "VDI_CBT_ENABLED",
        "OpaqueRef:4f16cd0e-fbaf-48c3-aae4-092b9906b9e4"
      ],
      "other_config": {},
      "subtask_of": "OpaqueRef:NULL",
      "subtasks": [],
      "backtrace": "(((process xapi)(filename ocaml/xapi-client/client.ml)(line 7))((process xapi)(filename ocaml/xapi-client/client.ml)(line 19))((process xapi)(filename ocaml/xapi-client/client.ml)(line 12359))((process xapi)(filename lib/xapi-stdext-pervasives/pervasiveext.ml)(line 24))((process xapi)(filename lib/xapi-stdext-pervasives/pervasiveext.ml)(line 35))((process xapi)(filename ocaml/xapi/message_forwarding.ml)(line 134))((process xapi)(filename lib/xapi-stdext-pervasives/pervasiveext.ml)(line 24))((process xapi)(filename lib/xapi-stdext-pervasives/pervasiveext.ml)(line 35))((process xapi)(filename lib/xapi-stdext-pervasives/pervasiveext.ml)(line 24))((process xapi)(filename ocaml/xapi/rbac.ml)(line 205))((process xapi)(filename ocaml/xapi/server_helpers.ml)(line 95)))"
    },
    "message": "VDI_CBT_ENABLED(OpaqueRef:4f16cd0e-fbaf-48c3-aae4-092b9906b9e4)",
    "name": "XapiError",
    "stack": "XapiError: VDI_CBT_ENABLED(OpaqueRef:4f16cd0e-fbaf-48c3-aae4-092b9906b9e4)\n    at Function.wrap (file:///opt/xo/xo-builds/xen-orchestra-202410111017/packages/xen-api/_XapiError.mjs:16:12)\n    at default (file:///opt/xo/xo-builds/xen-orchestra-202410111017/packages/xen-api/_getTaskResult.mjs:13:29)\n    at Xapi._addRecordToCache (file:///opt/xo/xo-builds/xen-orchestra-202410111017/packages/xen-api/index.mjs:1041:24)\n    at file:///opt/xo/xo-builds/xen-orchestra-202410111017/packages/xen-api/index.mjs:1075:14\n    at Array.forEach (<anonymous>)\n    at Xapi._processEvents (file:///opt/xo/xo-builds/xen-orchestra-202410111017/packages/xen-api/index.mjs:1065:12)\n    at Xapi._watchEvents (file:///opt/xo/xo-builds/xen-orchestra-202410111017/packages/xen-api/index.mjs:1238:14)"
  }
}

I can see a task disabling CBT on the disk and looking at the UI it shows as CBT disabled.

I experience the same issue attempting to migrate other VDI's too.

rtjdamen

You need to remove all snapshots before migration and disable cbt. Storage migration is not supported when cbt is invalid. I believe xoa should do this automatically however.

andyh

@rtjdamen Thanks for the super fast response!

Just removed the existing snapshots and the task is proceeding.

Did you mean cbt is enabled as opposed to cbt is invalid?

rtjdamen

@andyh no cbt should be disabled, u can’t migrate an cbt enabled vdi.

andyh

@rtjdamen I understand now

flakpyro

After a number of XOA Updates i decided to test CBT with snapshot delete again.

Instead of " "can't create a stream from a metadata VDI, fall back to a base" i am seeing a more verbose error but the issue remains the same. In a 2 host pool with shared NFS storage if i have CBT with Snap delete enabled after a VM is migrated from host A to host B (remaining on the shared NFS SR) and a backup runs the delta backup fails and a full runs. This time the error shows " Can't do delta with this vdi, transfer will be a full"

This is with XOA Latest: 5.100.0

I have attached the backup log if this helps.

2024-11-05T17_37_58.455Z - backup NG.json.txt

rtjdamen

@flakpyro in general this error means that the cbt is not valid, we have seen this as well and i know vates is looking into this. In general we saw this problem more on nfs then on iscsi, not shure if it is nfs related but maybe if u have access to an iscsi target try your results there.

flakpyro

@rtjdamen this test pool is running on TrueNAS, so i could configure iSCSI. Our production is on NFS so i tried to keep test close to the same as far as storage is concerned. We currently use NBD + CBT without the snapshot delete function in production and it works well via NFS. Will continue to keep testing as updates roll out and look forward to when this is completely stable. If there's anything anyone at Vates needs log or test wise i'm happy to help!

rtjdamen

@flakpyro i think that would be a good test, can u check how it is behaving on iscsi on your end?

chr1st0ph9

@StormMaster Thank you - I experienced that too , 2 backup plans with same VMs. Backup failed once out of two with error message "can't create a stream from a metadata VDI, fall back to a base"

rtjdamen

@chr1st0ph9 i understand a fix is being made for this, @florent patched our proxy yesterday and since then no more fulls so far!

flakpyro

@rtjdamen Thats great news, with both block and file based SRs?

florent

this branch ( already deployed on @rtjdamen systems) add a better handing of host that took too much time to compute the changed block list :
https://github.com/vatesfr/xen-orchestra/pull/8120

it will be release in patch this week

I am still investigating an error that still occurs occasionally : XapiError: SR_BACKEND_FAILURE_460(, Failed to calculate changed blocks for given VDIs. [opterr=Source and target VDI are unrelated], )

fbeauchamp opened this pull request in vatesfr/xen-orchestra

open fix(@xen-orchestra/xapi): use callSync for VDI_listChangedBlocks #8120

rtjdamen

@florent thanks for letting me know, on our end this error seems to occur on the same vms every time, it are just a handfull. Could it be these vms are facing higher load on them what causes xapi tasks to take longer then expected?

flakpyro

@rtjdamen @florent Be very curious to test this once it hits XOA.

The lab i am testing this a two host pool with each server having a 24 Core Epyc CPU and 256 GB of ram. There are only around 4 VMs running on this test environment currently and all are low load VMs.

Is it still something that mostly occurs with file based SRs? I never did get a chance to setup ISCSI and test with it instead of NFS.

Once this becomes stable there i definitely plan to switch to this backup method in production!

rtjdamen

@flakpyro no on my end there is no difference for nfs or iscsi on XOA backups, we only had this issue with Alike Backup, my assumption was this would be the case in XOA as well, but not at this time.

What i found out is that one of my vms facing this issue has 2 disks, both on a different SR, i am moving one of the disks to the same SR and check tomorrow (tonight will be a full after migrating) if this is still the case there.

The other vm we face this issue on is has both disks on the same SR so that cannot be the case there. keep u posted on the results.

florent

@rtjdamen we found a clue with @Bastien-Nollet : there was a race condition between the timeframe allowed to enable CBT and the snapshot, leading to a snapshot taken before CBT , thus failing to compute correctly the list of changed block at the next backup

The fix is deployed, and we'll see this night. If everything goes well, this night will be a full, but the disks will keep CBT enabled. And the next night, we'll have delta

if everything is ok, it will be released in a second patch ( 5.100.2 )

rtjdamen

@florent sounds like a plan! i will keep an eye on them and let u know the results!

barwys

Hi,
I observed one issue with the backup process when the CBT is enabled. The case is showing when the VM has two disks on different SRs. When the backup tries to be taken, it fails, and the machine goes to a state where no other changes/reconfiguration is possible. It is not possible to disable CBT, move a disk to a different SR or create another backup. Sometimes after a failed backup the console access and machine are freezing. Everything works fine when all of the VM disks are on one SR.

The logs from the first backup: