Hitachi Copy-on Write SnapShot, User Manual

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Planning and Design
2–5
Hitachi Simple Modular Storage Copy-on-Write SnapShot User’s Guide
Establishing Data Pool Size
The data pool holds data from the P-VOL that is being replaced. By holding
this original data in the data pool, the mirror image of the V-VOL is
maintained at the time of the snapshot.
You determine the size of the data pool that your system requires by:
• Measuring the amount of write workload that passes from the host
application to the P-VOL. Write workload is the megabytes per second
that are written to the primary volume over a specific time.
• Calculating the amount of data that would accumulate during the
lifespan of your V-VOL
• Multiplying times the number of V-VOLs
•
Figure 2-2: Write workload and data pool size
Measuring Workload Data
To set up SnapShot, you must measure the amount of data that changes in
your production system. The amount of data written to the primary volume
indicates how large the data pool must be.
Workload data is collected using performance monitoring software on your
operating system—preferably during the busiest time of month, quarter,
and year. The goal is to collect data that shows your system’s actual
workloads during high peaks and spikes, when more is changing and the
demands on the system are greatest.
To collect workload data
1. Using your operating system’s performance monitoring software, collect
the following:
• Disk-write bytes/second for every physical volume that will be
replicated.
• Collect this data at 10 minute intervals.
• Collect this data over a 4-6 week period that includes high peaks
and spikes, and when the demands on the system are greatest.
•