Maintaining data integrity
177
Data Integrity, Availability, and Protection
These bytes contain the record ID and a double Longitude
Redundancy Code (LRC) check byte, as shown in Figure 41 on
page 177. The Symmetrix system uses these bytes to check that the
data is from the specified record and alarms the host if it is not. This
second level of protection further ensures data integrity by
preventing incorrect data from being transferred.
Figure 41 Symmetrix data record format
Symmetrix systems assure the highest level of data integrity by
checking data validity through the various levels of the data transfer
in and out of global memory. Should an error be undetected at one
level, it will be detected at one of the other levels.
Disk error correction
and error
verification
The disk directors use idle time to read data and check the
polynomial correction bits for validity. All data and command words
passed between the disk directors and the disk drives include
frame-based CRC used to check integrity at each data transfer. If a
disk read error occurs, the disk director reads all data on that track to
Symmetrix global memory. The disk director writes several
worst-case patterns to that track, searching for media errors.
When the test completes, the disk director rewrites the data from
global memory to the disk drive, verifying the write operation. The
disk microprocessor maps around any bad block (or blocks) detected
during the worst-case write operation, thus skipping defects in the
media. If necessary, the disk microprocessor can reallocate up to 32
blocks of data on that track. To further safeguard the data, each disk
drive has several spare cylinders available. If the number of bad
blocks per track exceeds 32 blocks, the disk director rewrites the data
to an available spare cylinder. This entire process is called error
verification. The disk director increments a soft error counter with
each bad block detected. When the internal soft error threshold is
reached, the Symmetrix service processor automatically dials the
EMC Customer Support Center and notifies the host system of errors
through sense data. The Symmetrix system also invokes sparing. This
feature maximizes data availability by diagnosing marginal media
errors before data becomes unreadable.
Data Record
ECC
Bytes
LRC
Bytes
Embedded
ID
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