Hard Disk Recovery

  In my last installment I covered two of the three reasons why we do a parity check.  First we want to make sure that we do not have a stale hard drive in the array.  Although I did not cover how one determines if in fact a stale hard drive exists, I will mention it now. 

  In reference to stale drives there is a trick to identifying if in fact the parity is failing because of a stale hard drive or if there is another reason.  When a drive goes stale it will have a mix of current data, and old data.  As time passes the disparity between the current data and old data grows larger.  However, the fact remains that there is some data on a clients system that no matter how long the drive has been stale it will never change.  For example, all the operating system files, those hardly ever get updated.  Another example is if a client has many picture files, or music files, these files almost never change because they are put on the media once, only to be used for viewing, not for editing.  There are other examples, however these are the most prevalent.

  In conjunction with this fact, most file system handlers will try to fill the media from sector zero to sector X contiguously.  Like filling a glass of water.  So the operating system files and older data that has not been deleted will remain at the beginning of the drive and the more recent data towards the end of the drive.  There are factors that effect this, the main one being deleted data.  If data has been deleted then the void left by the deleted file will quickly be filled by another file or files.  Internet cache is notorious for this as they are usually smaller files that can be stuffed into areas vacated by deleted data.  So if your clients RAID has gone down and they have been deleting and adding files it will affect the way the parity check looks when scanning with a stale drive.

  Next installment I will finish my parity check example and how to see if there is in fact a stale drive, and hopefully explain the staggered offsets which plagued me in one of my most recent recovery attempts.

  Until next time…

Links to Previous RAID Data Recovery Articles In This Series

• Check RAID Hard Drive Consistency BEFORE a Rebuild - the three most important steps to take prior to rebuilding a RAID array.
• Analyzing RAID Parity - How to use our free RAID Diagnostic Toolkit to analyze RAID parity.
• Hard Drive Recovery - More information about hard drive data recovery services by dtidata.com.