  Causes of SCSI Disk Corruptions? 
 The Question is:
 
Dear Wizard,
 
I have a MicroVAX 3100-95 system with two SCSI channels attached to two BA356's
 containing both wide and narrow disks. Recently, the system failed to boot
 from the System disk reporting an illegal disk structure. On booting the
 system from a backup copy,
I discovered several of the disks on both channels reported "primary home block
 is bad". I repaired these using Analayze/Disk but am a little concerned that a
 system disk that looses the primary home block fails to boot. Do you haver any
 ideas why this wo
uld have happened?
 
Thanks in Advance.
 
 The Answer is:
 
  It is possible that this can be caused by a system or storage hardware
  problem, or by (privileged) software scribbling on the disk, or by a
  power failure at an inopportune moment during a multi-block transfer,
  or by configuration and/or latent problems with (privileged) layered
  products acting as adjuncts to the OpenVMS file system.  (eg: disk
  defragmentation or I/O caching tools.)  No specific cause can likely
  be identified.
 
  OpenVMS can and does use alternate disk home blocks when necessary.
 
  The OpenVMS Wizard will assume that you have also reviewed your system
  data archiving policy and have investigated the validity and particularly
  the restorability of any archival mechanism(s) in use.  The OpenVMS Wizard
  will also assume that volume shadowing (RAID) has been considered -- while
  RAID will not prevent privileged applications from scribbling, shadowing
  can be useful in avoiding disk-related (hardware-level) errors.
 
