[ovirt-devel] [VDSM] Correct implementation of virt-sysprep job

[Adam Litke]

On 05/12/16 11:17 +0200, Arik Hadas wrote:
>
>
>On Mon, Dec 5, 2016 at 10:05 AM, Nir Soffer <nsoffer at redhat.com> wrote:
>
>    On Sun, Dec 4, 2016 at 8:50 PM, Shmuel Melamud <smelamud at redhat.com> wrote:
>    >
>    > Hi!
>    >
>    > I'm currently working on integration of virt-sysprep into oVirt.
>    >
>    > Usually, if user creates a template from a regular VM, and then creates
>    new VMs from this template, these new VMs inherit all configuration of the
>    original VM, including SSH keys, UDEV rules, MAC addresses, system ID,
>    hostname etc. It is unfortunate, because you cannot have two network
>    devices with the same MAC address in the same network, for example.
>    >
>    > To avoid this, user must clean all machine-specific configuration from
>    the original VM before creating a template from it. You can do this
>    manually, but there is virt-sysprep utility that does this automatically.
>    >
>    > Ideally, virt-sysprep should be seamlessly integrated into template
>    creation process. But the first step is to create a simple button: user
>    selects a VM, clicks the button and oVirt executes virt-sysprep on the VM.
>    >
>    > virt-sysprep works directly on VM's filesystem. It accepts list of all
>    disks of the VM as parameters:
>    >
>    > virt-sysprep -a disk1.img -a disk2.img -a disk3.img
>    >
>    > The architecture is as follows: command on the Engine side runs a job on
>    VDSM side and tracks its success/failure. The job on VDSM side runs
>    virt-sysprep.
>    >
>    > The question is how to implement the job correctly?
>    >
>    > I thought about using storage jobs, but they are designed to work only
>    with a single volume, correct?
>
>    New storage verbs are volume based. This make it easy to manage
>    them on the engine side, and will allow parallelizing volume operations
>    on single or multiple hosts.
>
>    A storage volume job is using sanlock lease on the modified volume
>    and volume generation number. If a host running pending jobs becomes
>    non-responsive and cannot be fenced, we can detect the state of
>    the job, fence the job, and start the job on another host.
>
>    In the SPM task, if a host becomes non-responsive and cannot be
>    fenced, the whole setup is stuck, there is no way to perform any
>    storage operation.
>   
>    > Is is possible to use them with operation that is performed on multiple
>    volumes?
>    > Or, alternatively, is it possible to use some kind of 'VM jobs' - that
>    work on VM at whole?
>
>    We can do:
>
>    1. Add jobs with multiple volumes leases - can make error handling very
>        complex. How do tell a job state if you have multiple leases? which
>        volume generation you use?
>
>    2. Use volume job using one of the volumes (the boot volume?). This does
>        not protect the other volumes from modification but engine is
>    responsible
>        for this.
>
>    3. Use new "vm jobs", using a vm lease (should be available this week
>    on master).
>        This protects a vm during sysprep from starting the vm.
>        We still need a generation to detect the job state, I think we can
>    use the sanlock
>        lease generation for this.
>
>    I like the last option since sysprep is much like running a vm.
>   
>    > How v2v solves this problem?
>
>    It does not.
>
>    v2v predates storage volume jobs. It does not use volume leases and
>    generation
>    and does have any way to recover if a host running v2v becomes
>    non-responsive
>    and cannot be fenced.
>
>    It also does not use the jobs framework and does not use a thread pool for
>    v2v jobs, so it has no limit on the number of storage operations on a host.
>   
>
>
>Right, but let's be fair and present the benefits of v2v-jobs as well:
>1. it is the simplest "infrastructure" in terms of LOC

It is also deprecated.  V2V has promised to adopt the richer Host Jobs
API in the future.

>2. it is the most efficient mechanism in terms of interactions between the
>engine and VDSM (it doesn't require new verbs/call, the data is attached to
>VdsStats; probably the easiest mechanism to convert to events)

Engine is already polling the host jobs API so I am not sure I agree
with you here.

>3. it is the most efficient implementation in terms of interaction with the
>database (no date is persisted into the database, no polling is done)

Again, we're already using the Host Jobs API.  We'll gain efficiency
by migrating away from the old v2v API and having a single, unified
approach (Host Jobs).

>Currently we have 3 mechanisms to report jobs:
>1. VM jobs - that is currently used for live-merge. This requires the VM entity
>to exist in VDSM, thus not suitable for virt-sysprep.

Correct, not appropriate for this application.

>2. storage jobs - complicated infrastructure, targeted for recovering from
>failures to maintain storage consistency. Many of the things this
>infrastructure knows to handle is irrelevant for virt-sysprep flow, and the
>fact that virt-sysprep is invoked on VM rather than particular disk makes it
>less suitable.

These are more appropriately called HostJobs and the have the
following semantics:
 - They represent an external process running on a single host
 - They are not persisted.  If the host or vdsm restarts, the job is
   aborted
 - They operate on entities.  Currently storage is the first adopter
   of the infrastructure but virt was going to adopt these for the
   next-gen API.  Entities can be volumes, storage domains, vms,
   network interfaces, etc.
 - Job status and progress is reported by the Host Jobs API.  If a job
   is not present, then the underlying entitie(s) must be polled by
   engine to determine the actual state.

>3. V2V jobs - no mechanism is provided to resume failed jobs, no leases, etc

This is the old infra upon which Host Jobs are built.  v2v has
promised to move to Host Jobs in the future so we should not add new
dependencies to this code.

>I have some arguments for using V2V-like jobs [1]:
>1. creating template from vm is rarely done - if host goes unresponsive or any
>other failure is detected we can just remove the template and report the error

We can chose this error handling with Host Jobs as well.

>2. the phase of virt-sysprep is, unlike typical storage operation, short -
>reducing the risk of failures during the process 

Reduced risk of failures is never an excuse to have lax error
handling.  The storage flavored host jobs provide tons of utilities
for making error handling standardized, easy to implement, and
correct.

>3. during the operation the VM is down - by locking the VM/template and its
>disks on the engine side, we render leases-like mechanism redundant

Eventually we want to protect all operations on storage with sanlock
leases.  This is safer and allows for a more distributed approach to
management.  Again, the use of leases correctly in host jobs requires
about 5 lines of code.  The benefits of standardization far outweigh
any perceived simplification resulting from omitting it.

>4. in the worst case - the disk will not be corrupted (only some of the data
>might be removed).

Again, the way engine chooses to handle job failures is independent of
the mechanism.  Let's separate that from this discussion.

>So I think that the mechanism for storage jobs is an over-kill for this case.
>We can keep it simple by generalise the V2V-job for other virt-tools jobs, like
>virt-sysprep.

I think we ought to standardize on the Host Jobs framework where we
can collaborate on unit tests, standardized locking and error
handling, abort logic, etc.  When v2v moves to host jobs then we will
have a unified method of handling ephemeral jobs that are tied to
entities.

-- 
Adam Litke