Ultrastar 7K2
HGST Hard Disk Drive
Technical Reference Manual
29
A typical read can return data at a rate as defined in our performance section without additional drive
delay. This capability is based upon the LDPC (Low Density Parity Check) Channel technology which
provides data with ECC On-The-Fly data correction capability.
Beyond this on-the-fly capability, read errors can occur and are defined as follows:
Recoverable – whereby the drives error recovery procedure is required to correctly return the data after an
initial error condition was encountered.
Unrecoverable – whereby the drives error recovery procedures are unable to correctly return the
data requested; this data should be allocated to a new area of the drive.
Mis-corrected – as specified in the error rate above the frequency for this type of occurrence is
extremely rare. This type of event can occur as it relates to the tradeoffs of the channel
technology engine against the quantity, lengths, and patterns of data errors which may occur
within a sector. As mentioned above the LDPC channel is required to enable the recoverable and
unrecoverable error rates as specified above.
Before measuring read error rates, ensure that:
1. The data that is being used for measurement of read error rates must be verified that it is written
correctly on the media.
2. All media defect induced errors must be excluded from error rate calculations.
An interface error is defined as when the drive receiver detects errors of the incoming data whereby the drive
in unable to recover the data as transmitted to the receiver. These errors can include any of: running
disparity errors, illegal code, loss of word sync, or CRC errors.
Environmental Interference
When evaluating systems under conditions of EMI, the performance of the drive within the system shall
be considered acceptable if the drive does not generate an unrecoverable condition. This unrecoverable
condition is defined as one that:
1. Is not detected and corrected by the drive itself.
2. Is not capable of being detected from the error or fault status provided through the drive or its
interface.
3. Is not capable of being recovered by normal drives or system recovery methods without operator
intervention.
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