Legacy Smart Disk Cloning


S.M.A.R.T. Implementation

S.M.A.R.T. stands for Self-Monitoring, Analysis and Reporting Technology. It is a technology that allow disk drives to predict near-term failure. Disk drives equipped with S.M.A.R.T. will make a status report to the host if it detects degradation with predetermined drive attributes. Not all failures are predictable. S.M.A.R.T. predictability is limited to the attributes that the drive can monitor.  Typically, these include:

Head Flying Height 
Data Throughput Performance 
Spin-Up Time 
Re-Allocated Sector Count 
Seek Error Rate 
Seek Time Performance 
Spin Try Recount 
Drive Calibration Retry Count 

For SCSI drives, an industry standard specification is used as defined in the ANSI-SCSI Informational Exception Control (IEC) document X3T10/94-190. Normally, SCSI drives with S.M.A.R.T. capability communicate a reliability condition as either good or failing. The specification provides for a sense bit to be flagged if a reliability issue exists. The host may then alert the user.

Legacy’s Implementation

Legacy is using ANSI-SCSI Informational Exception Control (IEC) document X3T10/94-190 standard as well.  There are four settings relating to S.M.A.R.T. function in the firmware setup:
1. Disable - S.M.A.R.T. function not activated
2. Detect - S.M.A.R.T. function enabled. RAID Controller(s) will send command to enable all the drives' S.M.A.R.T. function. If a drive detects a problem, the RAID Controller(s) will send an event log.
3. Perpetual Clone - S.M.A.R.T. function enabled. RAID Controller(s) will send command to enable all drives' S.M.A.R.T.  function. If a drive detects a problem, the Raid Controller(s) will send an event log. The Raid Controller(s) will clone the drive if there is a hot-spare drive. The faulty drive will not be taken off-line, and the cloned drive still continues to operate as a spare drive. In case the faulty drive stops working, the spare drive will take over immediately. If the faulty drive continues to function and another drive fails instead, the spare drive will become active data rebuild into it.
4.  Clone + Replace - S.M.A.R.T. function enabled.  RAID Controller(s) will send command to enable all drives' S.M.A.R.T. function. If a drive detects a problem, Raid Controller(s) will send an event log.  The Raid Controller(s) will clone the drive to the spare drive and take the failed drive off-line. 

Legacy Disk Cloning Benefits

All RAID 5 Subsystems can only protect against a single disk drive failure. Recovery from a failed disk is accomplished with the help of the parity information stored on the remaining disks. During a single drive failure, not only is there peformance loss from having to recalculate from parity for all read requests, the entire system is at risk of a complete failure should a second disk fail before a rebuild is completed.

The more disks there are within a RAID Subsystem, the more likelihood of a second disk failure. For our example below, it's easy to see why a hot spare disk is commonly used as another level of protection. Rather than waiting for a failed disk to be replaced and then rebuilt, simply having a Hot Spare Disk within the RAID Subsystem can reduce the risk of a second disk failure by 4 times!
  • 1:10,000 Chance of a 2nd Disk Failure if no Hot Spare Disk is installed, assuming it takes 24 hours to replace and rebuild the failed disk.
  • 1:40,000 Chance of a 2nd Disk Failrue if a Hot Spare Disk is installed, assuming it takes 6 hours to rebuild a 36GB failed Disk.


Because the average size of SCSI Disk Drives double each year, the time it will take to rebuild the average disk also continues to double. Therefore, the customer's exposure to the fatal 2nd disk failure will also increase over time. Legacy's Disk Cloning Technology reduces the risk of a 2nd Disk Failure even further by preventing the first disk failure. Rather than taking an average of 360 minutes to rebuild a 36GB Disk Drive, the process of cloning a disk is merely to copy the blocks of data from one live drive to another. No re-calculations are required. Plus if the disk is not full, it will not need to copy 36GB of data either. Assume the time required is reduced from 360 minutes to merely 36 minutes. The risk of a 2nd disk drive is then further reduced by 10 times. That's 40 times better than the original configuration without a Hot Spare Disk.
  • 1:40,000 Chance of a 2nd Disk Failure if a Hot Spare Disk is installed, assuming it takes 6 hours to rebuild a 36GB Failed Disk.
  • 1:400,000 Chance of a 2nd Disk Failure if a Hot Spare Disk is installed and automatically cloned within 36 minutes.