The impact on Space and Fractional Reserve by increasing a LUN's size is the same as if the LUN would have been created at the expanded size originally. But (there's always a "but") increasing the size of a LUN does not increase the size of the volume.
Technically, space "usage" in a LUN is always the same - space is used when a client server writes to the LUN. Space "required" depends on the combination of a lot of settings, including volume space guarantee, lun space reservation, snapshot reserve, and fractional reserve. If you understand how all those come together when a LUN is first created, then you will understand how they affect a LUN which is expanded. The answer is that they all interact in the same way. You just use the larger size as the base of all the storage decisions.
For example - a fully space guaranteed LUN with fractional reserve of 100% will need a volume of double the size of the LUN to create a snapshot. Remember that fractional reserve is used to "hold" space to ensure a LUN can be overwritten up to the fractional reserve percent (in cDoT either all or none). So in this example, if the LUN in question has never had a snapshot and was created on a volume with exactly double space, then increasing the LUN size will leave the volume with less free space than the fractional reserve and a snapshot creation couuld fail.
When you consider increasing a LUN, consider all the standard settings in your environment. How would you have sized the underlying volume if the LUN had the new size originally? Adjust your volume size at the same time you increase the LUN size as appropriate for your storage allocation patterns and you should be good.
I hope this helps you.
Bob Greenwald
Lead Storage Engineer
Huron Legal | Huron Consulting Group
NCDA, NCIE - SAN Clustered, Data Protection
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