Ubuntu on IBM z Systems

  1. Bug #1893775
  2. Comment #12

Comment 12 for bug 1893775

Revision history for this message
bugproxy (bugproxy) wrote : Comment bridged from LTC Bugzilla

------- Comment From <email address hidden> 2020-09-07 11:05 EDT-------
> Request to always force ms-dos partition table in KVM was requested before.

Can you expain the context? The KVM team certainly did not request ms-dos partition table.

> However, surely it is virtualization/layering violation, when a physical
> host device which is not virtio-scsi compatible is passed through to qemu
> which emulates it as scsi, despite the underlying device on the host not
> able to create GPT, extended partitions, etc.
>
> Can we please prohibit passing through /dev/dasda to qemu as virtio-scsi
> provider on qemu and/or virtio layer?
>
> If one passes /dev/dasda to qemu, it must show up as /dev/dasd* inside the
> guest too. As there is no way for the guest to know or figure out that the
> passthrough device is actually masquerading a dasd drive.
>
> Can you please explain again, why is it correct and desired to provide
> /dev/vda in the guest with /dev/dasda on the host, when the two types are
> incompatible with each other?

This is virtio-blk and virtio-blk has in QEMU special code to detect DASDs and will then also pass through geometry and block size.
This then allows the partition detection code and parted to detect this as a DASD. This did work in the past for all previous Ubuntu versions including 20.04, it does work with Redhat, Fedora and SUSE. parted has this detection and zipl has this detection. So it is far from being incompatible.

It is an important use case, because the majority of customers that also have z/OS will usually use DASDs also for Linux. This allows them to setup HA/failover solutions where the DASD is accessible from multiple locations and the failover is orchestrasted by z/OS. This also avoids the need for a shared filesystem.

Again: the old installer was able to handle this, the new one is not. It was certainly expected that we get regressions with a new installer. The right fix is not to discuss away useful features, instead the right solution is certainly to fix things that are regressions. no?