You can have a mix of space-efficient and fully allocated
VDisks in FlashCopy® mappings.
One common combination is a fully allocated source with a space-efficient
target, which allows the target to consume a smaller amount of real
storage than the source.
For best performance, the grain size of the space-efficient
VDisk must
match the grain size of the FlashCopy mapping.
However, if the grain sizes are different, the mapping still proceeds.
Consider the following information when you create your FlashCopy mappings:
- If you are using a fully allocated source with a space-efficient
target, disable background copy and cleaning mode on the FlashCopy map
by setting both the background copy rate and cleaning rate to zero.
Otherwise, if these features are enabled, all the source is copied
onto the target VDisk. This causes the space-efficient
VDisk to
either go offline or to grow as large as the source.
- If you are using only space-efficient source, only the space that
is used on the source VDisk is copied to the target VDisk. For example,
if the source VDisk has a virtual size of 800 GB and a real size of
100 GB of which 50 GB have been used, only the used 50 GB are copied.
- A FlashCopy bitmap
contains one bit for every grain on a VDisk. For example, if you have
a space-efficient
VDisk with
1 TB virtual size (100 MB real capacity), you must have a FlashCopy bitmap
to cover the 1 TB virtual size even though only 100 MB of real capacity
is allocated.