DASD Fast-Write Buffering

DASD Fast-Write Buffering

      A method for using a non-volatile storage in a high write
environment while minimizing channel usage is disclosed.  The method
eliminates the thrashing problems that result when data is being
added to the non-volatile storage faster than it can be destaged to
the DAS devices.  The non-volatile storage is still used as a buffer
until the data is written to the device but the destage is scheduled
immediately.  Since the buffer is non-volatile, ending status can be
presented after the data is in the non-volatile storage but the
logical device will be unavailable until the destage completes.  The
advantage is that the ending status is received before the data is on
the device and the non-volatile storage full condition is avoided.
Channel usage is minimized because the control unit does not need to
reconnect to the channel after the data is written to the device.
This processing method is referred to DASD Fast Write Buffering.

      The Figure illustrates the advantages of DASD Fast Write
Buffering.  Without DASD Fast Write Buffering (10), when a write
request is received the data will be transferred from the channel to
the cache.  The control unit then disconnects from the channel (50)
and initiates the processing of selecting and orienting the DAS
device and writing the data from the cache to the device.  The
control unit then reconnects to the channel (30) and presents ending
status.

      With DASD Fast-Write Buffering (20), when a write request is
received the data will again be transferred from the channel to the
cache but now the control unit will present ending status (60).  The
device is marked unavailable to the channel and the data is written
to the device as before.  After the write completes (40) the device
is marked available.  The control does not need to reconnect to the
channel as required without DASD Fast-Write Buffering (30).

      Write operations with and without DASD Fast-Write Buffering
show that the device is not available f...