Support multi-attach

[embercsi]

Support multi-attach

[Akrog]

Multi-attach mount volumes would be problematic even on the single writer multiple readers scenario.

There are filesystems (like NTFS, and Ext4) that would corrupt the volume when we mount them on multiple hosts, even if they are read only.

We'll still have issues when we use other filesystems that don't have that issue, because Linux clients cache blocks locally, and they do not invalidate that cache. So we could experience application crashes when accessing data structures that were partially cached while being changed on the original.

Ext4 replays the journal even on readonly mounts. So a readonly mount will still write to the underlying block device. It would be unsafe even if both the mounts were readonly. We could try attaching the volume as readonly (or setting it to RO with blockdev --setro) before mounting and see it if works for the all readers (and no writers) case.