Feature pages are design documents that developers have created while collaborating on oVirt.

Most of them are outdated, but provide historical design context.

They are not user documentation and should not be treated as such.

Documentation is available here.

Managed Block Storage Copy

Summary

The motivation for this feature is to allow users to copy disks from regular Storage Domains to Managed Block Storage Domains.

Owner

Benefit to oVirt

This enables migrating existing VMs to Managed Block Storage domains, as well as enabling usage of features not yet available for Managed Block Storage, like Import from Glance and Image Upload.

User Experience

At the time of writing this page it is only possible to perform the copying with the REST API, for example: POST http://engine:8080/ovirt-engine/api/disks/51b7f988-1c96-4d3c-892c-622499930e5f/copy

<action>
  <storage_domain id="33a0885e-2672-414a-8b79-c7026b0b1570"/>
  <disk>
    <name>centos83</name>
  </disk>
</action>

Eventually copying using the UI will be possible as well.

High Level Flow

The general flow of copying from a regular Storage Domain (Block/File) to Managed Block Storage Domain is:

  1. Create a Managed Block Storage disk
  2. Attach the disk to a Vdsm host that will perform the copying
  3. Create an external lease, writing relevant metadata on the lease’s LVB storage
  4. Perform the copy
  5. Detach the Managed Block disk from the host
  6. Remove the lease

This features offers similar semantics to those of SDM.copy_data, and is implemented using storage jobs. The progress of the copy is monitored using the Host.get_jobs Vdsm verb. If the connection with the host is lost and it is not longer reporting the job’s status, the lease will be used to determine the jobs status.

LVB

LVB (Lock Value Block), is a block on a sanlock lease the can be written and read from using set_lvb and get_lvb. The size of the block is the sector size of the storage, in Vdsm we use only 512 bytes which is compatible with all storage types. To write or read LVB, the lease has to be acquired with the LVB flag

Since Managed Block Storage copy aims to offer similar semantics as regular disk copy, the state of the copy_data job has to be stored on shared storage. In regular copy this is done using volume leases where the state of the job (generation) is stored on the volume’s metadata. Because Managed Block Storage disks are not managed by vdsm, so instead, the state is stored on an external lease’s LVB area.

Vdsm

CopyDataExternalEndpoint

A new endpoint CopyDataExternalEndpoint was added to represent a target volume that is not managed by vdsm. This endpoint is meant to be a general purpose endpoint for volumes that are not managed by Vdsm.

Lease.create

The Lease.create verb was modified to accept to receive a metadata parameter

Lease.status

Lease.status which was previously not implmeneted is now used, it is used to determine the job’s status when we cannot receive information from the host using Host.getJobs.

Lease.fence

A new verb Lease.fence was added to perform fencing on the lease. The only type of fencing currently implemented is job fencing, in which the job’s status is updated to FENCED and the generation is increased by 1.

Engine

A new sublcass CopyManagedBlockDiskCommand to handle copy operations involving Managed Block Storage disks. This class is responsible for coordinating the operations required for the copy:

  1. Create target disk using AddManagedBlockStorageDiskCommand
  2. Attach the disk to the host using ManagedBlockStorageCommandUtil#attachManagedBlockStorageDisk
  3. Create lease using AddExternalLease, this creates an external lease with metadata that looks like:
     {
     "generation": 0,
     "job_status": "PENDING",
     "job_id": "20e62244-19ad-46b7-afcc-a5fe2007a259",
     "type": "job",
     "host_hardware_id": "6e11cad3-2214-4e1a-aadc-e050accfe3c0",
     "created": 1614073237,
     "modified": 1614073237
     }
    

    Engine passes the job_status, job_id and type fields, the rest is added internally in Vdsm by Lease.create.

  4. Run the SDM.copy_data verb with CopyDataCommand. Similar to CopyImageGroupWithDataCommand, the status SDM.copy_data is polled using Host.get_jobs.
    • If we couldn’t get information using Host.get_jobs, the polling will use ExternalLeasePoller.
    • ExternalLeasePoller uses Lease.status to check the status of the job:
      • If the lease is owned, the job is considered still running
      • If the lease is free:
        • Job status is “SUCCEEDED”, the operation will complete successfully
        • Job status is “FAILED” or “FENCED” the operation will fail
    • If we couldn’t get the job’s status using ExternalLeasePoller (Lease.status failed), Lease.fence will be invoked, updating the job’s status and increasing the generation
  5. When SDM.copy_data completes, detach the volume using ManagedBlockStorageCommandUtil#disconnectManagedBlockStorageDeviceFromHost
  6. Remove the external lease with RemoveExternalLeaseCommand

Open Issues / Limitations

  • No UI support currently, will be added soon.
  • There is currently no proper handling when detaching the Managed Block Storage volume fails. This affects other flows, such as VM power-off and will fixed along with it.
  • Moving a disk is currently not implemented, moving is a special case of copying where the source disk is removed