Storage Area Networks For Dummies®

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Part III: Using Advanced SAN Features
The neat thing about metadata copies is that you don’t have to make a complete
bit-for-bit initial copy. A metadata PiT copy is instantaneous because
you don’t have to read and write the original to another area to make a
second copy of it. You just make all the PiT pointers point to the original
blocks of data.
The other benefit of metadata copies is that they use only as much disk
space as is required by the amount of data that changes in the original.
Different implementations of metadata PiT exist, so check with your vendor
on how this feature is implemented. Some vendors require that you preallocate
the same amount of space as the disks you want to copy, just in case
you change all the source data. Some vendors use a reserved area of disk to
dynamically size the area for the changed data to be written to. As long as
you don’t run out of reserved space, you can keep making as many copies as
you want.
Okay, now remove those photographs of your eyes and mouth from the
mirror; otherwise people will think you’re weird.
Which PiT Type Should You Use?
The decision to use a complete copy or a metadata copy for PiT snapshots of
your data depends on a few things. Basically, ask yourself what the PiT copy
will be used for and whether you can tolerate any performance impact on
your source production disk drives.
Here are some guidelines to follow. Use a complete data PiT copy when
✓ The second copy of your data must be 100-percent reliable.
✓ The initial copy time is not an issue.
✓ You can bear the expense of an additional 100 percent of the original
disk space for the second copy.
✓ The use of the copy cannot affect the original source’s performance.
Use a metadata PiT copy when
✓ The copy must be available immediately after it is snapped.
✓ You can’t spare the space for a complete duplicate copy.
✓ The performance impact on the original data is not important.