[ovirt-devel] Storage in VDSM questions

[Liron Aravot]

>
> ---------- Forwarded message ----------
> From: Shmuel Melamud <smelamud at redhat.com>
> Date: Wed, Jun 22, 2016 at 4:51 PM
> Subject: [ovirt-devel] Storage in VDSM questions
> To: devel <devel at ovirt.org>
>
>
> Hi!
>
> I have a bunch of questions related to storage in VDSM. I would be happy
> if anybody familiar with the subject can help me with them.
>
> 1. What is HSM? IRS?
>

IRS = SPM
HSM=non SPM host


> 2. What is SDM?
>
SDM= Storage Domain Management
A feature currently under development that provides alternate method to SPM
to manage shared storage.


> 3. What hsm.spmSchedule() method does?
>
Schedules a task to be executed as SPM (used to perform operation that alter
the shared storage or data operations currently).


> 4. What does domainMonitor.getHostId(vol_info.sd_id) method return?
>
I'd recomment to do some SANLock reading (see below).
Basically, its the id the host uses to acquire locks on the shared storage.

http://linux.die.net/man/8/sanlock
http://old.ovirt.org/SANLock

>
> Storage domain
>
> 5. What is StorageDomainManifest? Why there are separate StorageDomain and
> StorageDomainManifest classes? (And the same with Volume and
> VolumeManifest.)
>
Those are changes made in order to share "SPM" related code with "SDM"
related code (as not all of the code is relevant for both).
alitke/nsoffer could share more information on the decisions behind adding
those.

> 6. What does sd_manifest.domain_lock(host_id) guard? Why do we need the
> host_id parameter?
>
see answer to question 4.


> 7. Why in HSM we are using vars.task.getSharedLock(STORAGE, sdUUID)
> sometimes instead of domain lock?
>
The task lock (which uses the resource manager) is used to acquire in
memory logical lock within the current host.
Theoretical example- you wouldn't want to attempt to delete a domain while
you create a volume on it.

8. What does sd_manifest.get_volume_artifacts(img_id, vol_id) return?
>
> Resource manager
>
> 9. What is ResourceManager?
>
Resource manager is basically a in memory lock manager in vdsm.

> 10. What rmanager.acquireResource(image_res_ns, img_id, lockType) does?
> What types of resources it may be used with?
>

Well, the best answer would be the simple one - its used to acquire a
resource, you can specify a namespace and id, the lock will be taken on
namespace_ID
so basically you can acquire lock on the same id under different namespaces.

>
> Images and volumes
>
> 11. What methods dom.linkBCImage(imgPath, imgUUID) and
> dom.unlinkBCImage(imgUUID) do?
>
Creates/Removes a symlink to a given image.


> 12. What methods dom.activateVolumes(imgUUID, imgVolumes) and
> dom.deactivateImage(imgUUID) do?
>
Can you be a bit more specific in the question? I don't have much to add
over the method names.

>
>
13. What method dom.getVolumeLease(imgUUID, volUUID) does?
>
see related SANLock question before.

> 14. What methods volume.prepare() and volume.teardown() do?
>
> Thanks in advance for your answers!
>
> Shmuel
>
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