GitHub user wido added a comment to the discussion: Shareable RAW disk
> Hi, > > To weigh in here, this is a very dangerous feature and very hard to implement > correctly. > > The use cases I've seen require specific SCSI support from the underlying > storage for either "persistent reservations" (Microsoft's cluster shared > volumes) or atomics (VMWare VMFS), or other guarantees. > > Take, for example, the following use case: two VMs (v1 and v2) on two hosts > (h1 and h2) have the same disk and use some kind of communication (which is > not via the storage) on who does what. Then, h2 loses connectivity to the > network and storage but has some operations in flight that are in the queue. > > There's no mechanism that v2 can use to "cancel" these operations. There's > also no way for v1 to know if there are such operations or where they write. > So if v1 takes over and the connectivity of h2/v2 is restored, this pretty > much guarantees data corruption because the queued old operations will be > executed on top of the things v1 has already done. > > This is solvable with storage or host fencing but would have to be exposed to > VMs as an API, which has its own problems. > > Looking at the oVirt documentation, there's the cop-out "it's the > responsibility of the VM to take care of locking," but to reiterate, to be > able to do this in a useful way, the guests will need access to low-level > SCSI storage functions (or their emulation, like via qemu-pr-helper) or > fencing. I do not disagree that this is a dangerous feature, but having it *can* be beneficial. Implementing it from the CloudStack side isn't that difficult. - Only shared storage would be supported: NFS, Ceph, StorPool - Caching needs to be forced to 'none' on any shared disk - Mark the disk as shareable in libvirt: https://libvirt.org/formatdomain.html#elementsDisks ``` If present, this indicates the device is expected to be shared between domains (assuming the hypervisor and OS support this), which means that caching should be deactivated for that device. ``` It's up to the user on how to use this, not CloudStack. GitHub link: https://github.com/apache/cloudstack/discussions/9976#discussioncomment-11396439 ---- This is an automatically sent email for users@cloudstack.apache.org. To unsubscribe, please send an email to: users-unsubscr...@cloudstack.apache.org
