Background Scrubbing
Background scrubbing is provided by Internal Disk to further reduce the probability
of customer loss of data.
There is a small possibility that after a data failure occurs, the redundancy data is
also found to be unreadable, resulting in loss of data to the customer. Data
scrubbing is provided to help ensure that all the data in the disk subsystem is
periodically proven readable so that if a failure occurs, the redundancy exists to
fully restore the original data.
In the background, the SSA adapter card initiates reads of all the data on all the
HDDs in the subsystem. No data transfer to the system is involved. Problems with
the data are promptly corrected by use of the redundancy data within the RAID
array.
Predictive Failure Analysis
Internal Disk is notified of impending hardware failures by the disk drive, which is
continuously performing predictive failure analysis. This process predicts device
failure conditions before failure occurs by measuring various parameters of the
head and disk as well as device electronics. When one of the predictive functions
detects an anomaly, the disk drive notifies the attachment of the potential problem.
The attachment in turn notifies the CPC service element, which initiates a service
action request.
Media Maintenance
The 18GB UltraStar 18ZX HDDs used within Internal Disk incorporate their own
internal media maintenance capabilities. Internal Disk media maintenance strategy
is to capitalize on the HDD capabilities wherever possible. Media maintenance is
performed internal to the storage subsystem. Therefore, defective sectors are
reassigned internally and require no customer interaction.
Search Assist
The Internal Disk emulates a 3990-2 control unit. Many of the channel programs
that were written for the 3990 find a target record by repetitively executing a
Search, TIC (Transfer in Channel) loop at every record until either the search is
satisfied or an exception such as End of Cylinder or No Record Found occurs. An
example of such a channel program is the PDS Directory Search channel program
which has the following form:
Seek
Define Extent
Search ID Equal
TIC *-1
(To the previous command)
Read Count
Search Key Equal or High
TIC *-2
(To the previous Read Count command)
Read Data or other command
The above program has two loops in it. Internal Disk detects both loops. Each of
the CCWs is executed in sequence, however, unlike a channel attached outboard
subsystem, Internal Disk is able to detect the TIC command along with its
parameters. The address to which the TIC command is pointing reveals a
Search/TIC loop. The parameters from the Search commands are preserved within
the subsystem, so that a search algorithm to find the desired record can be
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