The I/O subsystem is attached to
the server using controllers. These dedicated storage devices are sometimes grouped
into storage area networks (SANs) to denote their logical organization as a separate
set of networked devices. The disk arrays are defined and managed within the dedicated
I/O subsystem, and the resulting logical ???disks??? are seen by the operating
system as physical disks.
This type of disk-volume management is completely transparent to the database
server and offers many benefits:
??? The database server does not spend CPU resources managing the disk arrays.
??? The I/O subsystem uses memory for an I/O cache, so the performance of Oracle
I/O can improve significantly (for example, from an average I/O time of 10??“12
milliseconds to 3??“5 milliseconds).
??? Write I/O is completed as soon as the data has been written to the subsystem??™s
cache.
??? The I/O subsystem will destage the data from cache to actual disk later.
??? Read I/O can be satisfied from the cache. The subsystem can employ some type
of algorithm to sense I/O patterns and preload the cache in anticipation of pending
read activity.
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