Staging: dst: transactions.

Staging: dst: transactions.

DST uses transaction model, when each store has to be explicitly acked
from the remote node to be considered as successfully written. There
may be lots of in-flight transactions. When remote host does not ack
the transaction it will be resent predefined number of times with specified
timeouts between them. All those parameters are configurable. Transactions
are marked as failed after all resends completed unsuccessfully, having
long enough resend timeout and/or large number of resends allows not to
return error to the higher (FS usually) layer in case of short network
problems or remote node outages. In case of network RAID setup this means
that storage will not degrade until transactions are marked as failed, and
thus will not force checksum recalculation and data rebuild. In case of
connection failure DST will try to reconnect to the remote node automatically.
DST sends ping commands at idle time to detect if remote node is alive.

Because of transactional model it is possible to use zero-copy sending
without worry of data corruption (which in turn could be detected by the
strong checksums though).

Transactions are handled in this patch: allocation/freeing/completion,
scanning for stall and to-be-resent transactions and overall management
of the storing tree.

Signed-off-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>