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In the past when drive capacities were smaller, this type of problem was less likely to occur because small arrays
typically contain fewer media defects than large arrays. Medium errors are specified as x number of defects per
y number of bits. Therefore, larger drives are prone to more defects because they contain a greater number of
bits and the disks are written with a higher density. Today, hard drive capacities have increased remarkably, and
the likelihood has grown that one or more media defects will occur over the lifespan of the drive. In addition,
large arrays take longer to rebuild than small arrays, thus increasing the amount of time the array is not
redundant.
WHAT IS THE NEED FOR RAID-6?
RAID-5 implementations use single parity to protect against single disk failure. Single disk failure is the issue.
Traditional single-parity RAID offers adequate protection against a single failure event. The caveat is that no
other disk failure or uncorrectable media error can occur while reconstruction is still in progress. During normal
operation, if there is an unrecoverable error and the event is a read error, then recreating data from parity
occurs almost instantaneously, and the array remains online. However, if a disk in a RAID-5 group fails, then all
its data has to be recreated, and the array remains in a vulnerable degraded mode (without parity protection)
until data has been reconstructed onto a spare disk. For a high-density or slow disk, that could be a very long
time.
RAID-6 with a dual-parity configuration offers a greatly enhanced data protection against any two disk failure
events occurring in the same RAID group.
OVERVIEW OF STEPS THAT CAN BE TAKEN IN KEEPING WITH RAID BEST PRACTICES
Perform all recommended driver, controller firmware, and Storage Management application (Adaptec
Storage Manager) updates.
Install Adaptec Storage Manager:
Adaptec Storage Manager helps you to monitor and maintain Adaptec RAID controllers, enclosures, and
disk drives in your storage space from a single location. When Adaptec Storage Manager is installed on a
system, the Adaptec Storage Manager Agent is also installed automatically as a service. It’s designed to
run in the background, without user intervention, and its job is to monitor and manage system health,
event notifications, tasks schedules, and other on-going processes on that system. It sends notices when
tasks are completed successfully, and sounds an alarm when errors or failures occur on that system.
Please see the
Adaptec Storage Manager User’s Guide
for more details.
Run regular consistency checks on the system:
Verification is designed to proactively detect hard disk media defects while the array is online and
redundant. A RAID-5 or RAID-6 array is inconsistent when the data and parity do not match. Likewise, a
RAID-1 array is inconsistent when the data and mirror do not match.
The verification process issues commands to each drive in the array to test all sectors. When a bad
sector is found, the RAID controller instructs the hard drive to reassign the bad sector, and then
reconstructs the data using the other drives. The affected hard drive then writes data to the newly
assigned good sector. These operations continue so that all sectors of each configured drive are
checked, including hot spares. As a result, bad sectors can be remapped before data loss occurs.
