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Shadow Copies and HBMM, Volumes, File Structures

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The Question is:

We've used volume shadowing for OpenVMS V7.3-2 for cross-site mirroring of 18GB
 disk. It usually takes 3-4 hours to shadow copy the 18GB disk set and smaller
 disk such as 9GB or even 18GB is not available in the market. Actually, only
 half of the 18GB dis
k has been used but the volume shadowing simply just shadow copy the entire
 18GB. Could we simply use the command below provided in V7.3-2 to initialize
 the total block size of shadowed 18GB disk to 9GB such that it would only
 shadow copy 9GB data instead
 of 18GB?
On node A
$init/size=18000000 $1$dkb100 18GDSK
On node B
$init/size=18000000 $2$dkb100 18GDSK
$mount dsa1/shadow=($1$dkb100,$2$dkb100)18GDSK

The Answer is :

  Host-based minimerge (HBMM) would be the approach considered by the
  OpenVMS Wizard.  HBMM support is available via ECO kit for V7.3-2.
  A secondary approach would involve a controller capable of provding
  partitioned-device support (and hardware-level RAID) operating
  with volume shadowing.  Or yes, INITIALIZE/SIZE.
  Volume shadowing operates at the volume level and largely outside the
  file structure; that written, INITIALIZE/SIZE and the associated
  Dynamic Volume Expansion (DVE) support very confusingly also
  establishes the effective device size for shadowing and other
  volume-level operations, and will provide the desired effect of
  reducing the aggregate amount of disk data requiring shadowing.
  While shadowing operates at the volume level, the INITIALIZE/SIZE
  command operates outside the normal command expectations and
  configures both the file structure layered on the disk volume,
  and the effective volume size.  (It might be more obvious to
  consider the SET VOLUME/LIMIT and /SIZE mechanism, than the
  Smaller SCSI devices are also readily available on the used-equipment
  market, and storage controllers are available that will present
  the appearance of smaller disks; that support hardware-level disk
  partitioning.  These would operate with volume shadowing of course,
  and would reduce the volume of data transfered for a shadow copy.

answer written or last revised on ( 9-NOV-2004 )

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