RAID-5 ADAPTER LEVEL PROTECTION
At the adapter level, RAID-5 has become an industry standard method to provide increased
availability for servers. RAID-5 and RAID-1 implementations allow servers to continue
operation even if there is a 'catastrophic' failure of a hard drive.
Normal Operations
During normal operations in a RAID 5 environment, redundant information is calculated and
written out to the drives as shown above. In a 'n' disk environment, 'n-1' disks of data are
provided with 1 disk of space dedicated to redundant 'check sum' or 'parity' information. As
pictured above, three 2GB drives will provide 4GB of data space and 2GB of redundancy.
Catastrophic Disk Failure
If a drive that is a member of a RAID-5 array fails, the remaining members of the array can use
their redundant information to recalculate the lost data - either to respond to user requests for
data or to rebuild the data stored on the lost drive when it is replaced with a new one.
Grown Sector Media Errors
Let's assume there is a read request for a file. As the drive attempts to read this data, it
determines there is a bad sector within Record 1 of Disk 1, as pictured below. If the media error
is minor, the information is corrected or remapped by the drive using the drive ECC information,
all of which is transparent to the RAID array. However, what if the disk can not recreate the
information from the ECC information on the drive? Is data still lost, as it was before without
RAID support ? In this case, IBM RAID adapters provide the additional capability to recognize
the fault and re-create the data from redundant information stored on other drives. For example,
Record 1 in the diagram will be corrected from data stored in Record 2 on Drive 2 and check sum
information on Drive 3. The RAID adapter then requests that Record 1 be rewritten, the drive
will remap the bad sector elsewhere on the drive and Record 1 will have good data.
In this case, RAID 5 has increased the availability of the information by re-creating data that
otherwise would have been lost. However, the initial assumption is that this process has been
initiated by accessing this data on the drive. If this data is not accessed, this error will not be
detected. This can become a real problem if a catastrophic failure occurs before the data is
corrected.
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