When ldiskfs hit critical errors it will remount the filesystem in read-only mode, the OST or MDT should provide read-only service properly in such situation, so that user can backup important files before further damage.

That feature requires Lustre is able to handle -EROFS gracefully, so there could be lot of code changes in following code path:

• Server start; (remount-ro could happen when mount ldiskfs)
• connection handler; (it currently requires client data updating)