[ovirt-devel] Re: Access to images in block domain from more VMs

On Tue, Jul 2, 2019 at 1:41 PM Tomáš Golembiovský <tgolembi(a)redhat.com&gt; wrote: > Hi, > > we have a large gap in handling ISO files in data domains and especially > on block domains. None of the flows that work with ISOs works (well, one > sort of does): > > 1) Change CD for running VM [1] > Broken because VM.changeCD accepts a path instead of PDIV. This used to work with ISO domain since file based storage does not need to be prepared, but with block storage someone must prepare the ISO image before using it and tear down the image when done. If you send PDIV to vdsm vdsm will prepare the image and tear it down and use the correct path to the image. > 2) Attaching boot disk in Run Once -- this works but I wonder if it's by > accident, because engine issues spurious teardown on images [2] > Can you point to more info about "spurious teardown"?

> 3) Guest tools during VM import [3] > Looks like [2] and [3] are the same. We don't know what "Guest tools during VM import" means.

> 4) Automatic attach of guest tools to Windows VMs > I don't have any idea what is this automatic attach.

Before virt can fix all those we need input from storage on how we > should handle the images on block domains. You should handle ISO images in the same way for all domains: - never assume any path on the host, you should not care about the path, and it is different for iso/data file/data block. - If you start a vm with a ISO, or plug/unplug, always use PDIV and let vdsm prepare and teardown the image for you, and update the xml with the correct path - if you need to access the ISO externally (e.g. pass it to some virt tool) you probably want to prepare the image and get the path to the image on the host from the results of Image.prepare(). In this case you are responsible for tearing down the image. This how image transfer prepare and teardown images. > The specialty of ISO image is > that it makes sense to have it attached to multiple VMs. There should be no issue to share ISO image as read only image. > And that also > means something should control whether image needs attaching or whether > the image is already attached to the host. All images need to be prepared and teardown. You should not care about the type. Vdsm will do the right thing. > Similarly the image should be > (attempted to) detached only when there is no longer any VM on the host > that uses it. > Usually engine is responsible for preparing and tearing down images. I know we have some holes in this area. Hence my question to storage -- is there already some framework from > storage for handling this? Does it make more sense to do this on engine > side or should each host handle it on it's own? We considered moving management of prepared images to vdsm, in the same way it handles managed block storage, but there are no concrete plans yet. > To me it seems > preferable to handle this on engine side as that is where the requests > for attach/detach normally come from (disks in VM import are an > exception). It could possibly mean less race conditions (but I may be > totally wrong here). Also having it on VDSM side would probably mean > changes to the API for 1) and 2). The issue is not where request to prepare and teardown come from, but conflicting requests. For example: - Flow 1 prepare volume V1, V2, V3 - Flow 1 start to download volume V3 - Flow 2 prepare volumes V1, V2 - Flow 2 start to download volume V2 - Flow 1 finish download and tear down volumes V1, V2, V3 if you are lucky, volumes V1 and V2 fail to teardown, if you are not, Flow 2 download fails - Flow 2 finish and tear down volumes V1, V2 We have several requirements: - If you prepare a volume, you must teardown - volumes should be tore down only when there are no users This can be implemented in engine or vdsm, but since this is about host internal state, I don't see why engine should care about it.

I hope it helps. Nir Any input appreciated. > > > Tomas > > [1] https://bugzilla.redhat.com/show_bug.cgi?id=1589763 > [2] https://bugzilla.redhat.com/show_bug.cgi?id=1721455 > [3] https://bugzilla.redhat.com/show_bug.cgi?id=1721455 > > -- > Tomáš Golembiovský <tgolembi(a)redhat.com&gt; >