Barriers and Device-mapper (1)

• Things are not so simple for device-mapper:

• • A table may use multiple devices (a mirror, for example). The barrier needs to reach them all.
  • Each device we need to send a barrier to needs a bio and this must be allocated from somewhere.

• Firstly, we refactored the core device-mapper code so that, on receipt of a barrier, it now waits for all outstanding I/O to complete (any further I/O is queued), sends a barrier to any targets that support it, then releases queued I/O. (It can only process one barrier at once.)

• This code is upstream.