Product manual

CDA 7 5630 Product Manual
3-8 77 A1 69UP Rev00
In addition to the four types of I/O operation mentioned, CDA 7 performs a
background operation that "destages" "written-to" blocks back to disk. The
following figure illustrates this "destaging" operation.
1 Destage Block(s)
2 Up date Directory
1
2
CHANNEL
CHANNEL
DIRECTOR
DISK
DIRECTOR
CACHE
DISKS
DIRECTORY
Figure 3-6. Destaging Operation
CDA 7 performs read operations from cache. CDA 7 always caches write
operations. This cache operation is transparent to the host operating system. A
read operation causes the channel director to scan the cache directory for the
requested data. If the requested data is in cache, the channel director transfers this
data immediately to the channel with a channel end and device end (or a SCSI
good ending status). If the requested data is not in cache, the disk director transfers
the data from the disk device to the cache and the channel director transfers the
requested data from the cache to the channel.