With the combination of Data ONTAP 7.2 and flexible volumes we have some new options for managing space in volumes that contain LUNs. Obviously, we still have to provide a free block when the host wants to write, but now we have a new block pool in the aggregate we can pull from. We also have the option of automatically deleting snapshots as a source of free blocks.
If we set the fractional reserve to 0% and then set the autosize option on a volume, the volume can grow when it needs more space by pulling it from the aggregate. Effectively the aggregate is supplying the pool of available blocks.
Effectively the aggregate is providing a pool that can be shared across all the volumes it contains. It is also very unlikely that all the LUNs in all the volumes will suddenly become extremely active at the same time. Therefore it may make sense to set aside a free space pool that is less than what we would have provided to support a 100% space reserve for every LUN in the aggregate.
This is especially true when we consider that the autosize option is typically combined with automatic deletion of snapshots.
The following command configures vol2’s autosize options:
In this case I have configured vol2 to grow to a max size of 1 Gigabyte in 100 megabyte steps. I can limit the maximum size the volume can achieve and I can configure the size of the increments it uses to grow.
Automatically growing volumes is generally combined with automatic deletion of snapshots. You can control which policy you want to implement first with the following option:
In this example vol2 will first try growing the volume before deleting snapshots. Once the volume has reached maximum size it will start deleting snapshots to free blocks. Remember, we are not changing the size of the LUN, we are adding fresh blocks to the volume which can be used to as provide the LUN substitute blocks for block updates or overwrites.
Which is better depends on your storage environment. You can control this behavior individually for each volume. If you tend to keep a lot of snapshots, perhaps some longer than necessary just to be safe, then it might be better to delete snapshots first. If you have some extra space in your aggregate, perhaps for performance reasons, then it might be better to auto-grow.
Next we’ll take a look at how to control which snapshots are deleted.
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