Specifications

ES/OL
Evaluator Series On-Line™
Storage Systems
Product Analysis
May, 2011
Page 23 of 30
Analysis of Fujitsu ETERNUS DX400 S2
Models DX410 and DX440
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One Point Copy (OPC)
The copy is a “clone” copy where an exact original copy is made to another physical area. Entire volumes
are copied using OPC.
OPC has two copy phases; the logical phase and the physical phase. The logical phase starts after the
OPC command is initiated. A bitmap that represents the copy is created in cache memory. That process
takes a few seconds and after completion, the copy is available for use. The physical phase then starts,
which is the process of the storage system copying the data to the copy location. The physical copy is
completed in the background. During the physical phase, host access to data is available. The copied
volume is available almost instantly after OPC initiates, upon completion of the logical phase. This is
accomplished as follows:
• Read to Target – If the requested block has been previously copied to the target, a normal read is
done to the target volume. If the requested block has not been copied, the system will copy the
block from the source to the target and then honor the read from the target volume.
• Write to Source – If the requested block has been copied to the target, a normal write operation is
done to the source. If the requested block has not been copied yet, the original block is copied to
the target and the write is then done to the source.
• Write to Target – If the requested block has been copied to the target, a normal write operation is
done. If the block has not been copied, the bitmap is changed to indicate the copy has been done
and the normal write is done.
The process of the physical copying persists through a power down or reboot.
There are three speed settings (pacing parameters) to control the amount of storage system resources
consumed for the copy operation. The speed or priority settings are controlled via the management
software, ETERNUSmgr. The three speed options are low, high and automatic. The automatic setting
changes the priority dynamically in response to the system workload.
One Point Copy consumes cache memory for the bitmap and the additional data being copied.
QuickOPC
The first copy is the same as One Point Copy, except a bitmap is used to track all changed blocks from the
time the logical copy completed. The design leverages the use of a bitmap, which tracks changed blocks
for the volume. Subsequent copies are optimized by only copying the blocks changed since the previous
copy.
SnapOPC
SnapOPC operates similar to other Copy on Write point in time copy operations. Rather than copying the
new data to the copy volume, the old data is copied to the duplicate volume. Space is saved by only
saving the original blocks of data, when new data is written to the SnapOPC volume. By copying only a
portion of the source volume, it minimizes copy volume capacity.