> A better approach is to export not an GNBD but an iSCSI
> device from DRBD.
how about exporting each single HD as iSCSI instead of GNBD in the first place then? Is the iSCSI subsystem more robust in that sense? Then on the softwareRaid master instead of exporting a filesystem, export a raid'ed iSCSI device. If the RAID meta-data were spread out onto multiple disks (which actually would mean multiple machines) then if the master went down any of the others should be able to rebuild the RAID set or at least run in degraded mode.
back to more reading and tinkering...