If a hard drive fails which is used as a backing block device for DRBD on one of the nodes, DRBD may either pass on the I/O error to the upper layer (usually the file system) or it can mask I/O errors from upper layers.
Passing on I/O errors. If DRBD is configured to pass on I/O errors, any such errors occurring on the lower-level device are transparently passed to upper I/O layers. Thus, it is left to upper layers to deal with such errors (this may result in a file system being remounted read-only, for example). This strategy does not ensure service continuity, and is hence not recommended for most users.
Masking I/O errors. If DRBD is configured to detach on lower-level I/O error, DRBD will do so, automatically, upon occurrence of the first lower-level I/O error. The I/O error is masked from upper layers while DRBD transparently fetches the affected block from a peer node, over the network. From then onwards, DRBD is said to operate in diskless mode, and carries out all subsequent I/O operations, read and write, on the peer node(s) only. Performance in this mode will be reduced, but the service continues without interruption, and can be moved to the peer node in a deliberate fashion at a convenient time.
See Section 5.13, “Configuring I/O error handling strategies” for information on configuring I/O error handling strategies for DRBD.