[doc][rfe] Need to call out Generic Driver limitations

[Goutham Pacha Ravi]

The "Generic" backend/driver in Manila is a community maintained software defined storage backend/driver. It provisions and uses OpenStack compute (Nova) VMs as NFS/CIFS share servers and creates backing storage via Cinder volumes.

There are some limitations that need to be acknowledged for considering this driver in a production deployment of OpenStack Manila. Currently, there is no documentation on what these limitations are, besides the note that some Nova backends can only support 26 shares due to a limitation on the number of PCI devices [1]

[1] https://docs.openstack.org/manila/latest/admin/generic_driver.html#known-restrictions

[Goutham Pacha Ravi (gouthamr)]

One of the limitations that severely affects availability in the cloud is the Single Point of Failure (SPOF) issue. The driver uses a Nova VM as its NAS (NFS/CIFS) server. If/When the server goes down, there is no way to continue serving data.

[Goutham Pacha Ravi (gouthamr)]

Some more notes on the SPOF issue can be found on this specification: https://review.opendev.org/#/c/504987/

[Tom Barron (tpb)]

The generic driver assumes the manila-share service is running on a node where there is an integration bridge where it can plug in the service VM (nova instance in this case). This condition does not hold in a common deployment topology where manila-share is run on a controller node and networking services are run on a separate dedicated node.