Product manual

CDA 7 Input/Output Operations
77 A1 69UP Rev00 3-11
3.2.2.2 Read Miss
In a read miss operation, the requested data is not in cache and must be retrieved
from a disk device. While the channel director creates space in the cache, the disk
director reads the data from the disk device. The disk director stores the data in
cache as the channel director simultaneously transfers the cached data to the host.
If the requested data is in the process of being pre-fetched (sequential read ahead),
the miss is considered to be a "short miss." If the requested data is not in the
process of being fetched, it goes on a list to be fetched and the miss is considered to
be a "long miss."
CONNECT TIME
OVERHEAD
TOTAL SERVICE TIME
DISCONNECT TIME
Figure 3-9. Read Miss
Since the data is not in cache, CDA 7 must search for the data on disk and then
transfer it to the channel. This adds seek and latency times to the operation.
During the disconnect time, other commands can be executed on other devices on
the bus, or commands can queue to the same device.
3.2.3 Write Operations
For write operations, the channel director sends a device end and channel end (or a
SCSI good ending status) to the channel as soon as the data is in cache and verified
as error-free so the host system can continue processing without having to wait for
CDA 7 to write the data to disk. The disk director asynchronously "destages" the
data to disk.
There are two types of write operations:
• Fast Write
• Delayed Fast Write