Figure A.4 State Transition Diagram for Dummy SnapOPC+
Table A.1 Relationship Between Session Operation and Express Function
Session Operation Express Function
Start normal SnapOPC+ acsnap start
Start dummy SnapOPC+ acsnap start -estimate
Stop acsnap cancel
Start
When the SnapOPC+ session is started, the copy-on-write process is started. The maximum number of copy sessions that can be
executed with Express is as follows.
Device Name Maximum Number of Copy Sessions
ETERNUS DX60/DX60 S2/DX80/DX80 S2/DX90/DX90 S2 8
ETERNUS DX100 S3 1024
ETERNUS DX200 S3
ETERNUS DX200F
2048
When a write I/O is issued to the source volume, the copy-on-write process copies the old-data to the copy destination before writing
the new data to the source volume. By doing so the copy-on-write process maintains the Point in Time image. This process is an
internal background system in the ETERNUS Disk storage system and it is transparent to the host server. The copy destination area
becomes accessible after SnapOPC+ session has been started.
- Unlike the Clone methods (OPC, Quick OPC and EC) the physical capacity for the destination need to be large enough to contain
updated areas. So the size requirement is typically small.
- When the target volume is being read it needs to reference the original source volume to obtain the unmodified data. This
dependence to the source volume means that SnapOPC+ is not suited for persistent or long-term data retention or as a backup
data for recovery in case of physical loss of source media.
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