NETAPP'S APPROACH. NetApp uses a slightly different approach for its snapshot technology. It first specifies a snap reserve, a percentage of the primary data volume set aside for snapshots. The snap reserve is used only for space accounting on a NetApp device. Next, the snapshot is created. As with the copy-on-write approach, the snapshot view points to the existing data blocks in the primary data set. As changes are made to the primary data set, the blocks of data affected by the changes remain in place. This is the key difference between the copy-on-write approach and NetApp's. The new information is written to free space in the primary data set (even if snapshots are turned off). The obvious benefit is that enabling snapshots doesn't affect write performance. But the size of the snap reserve must be monitored; if the snap reserve is exceeded, free space in the primary data set is consumed. Excessive deletes or overwrites, coupled with maintaining several snapshots, can cause the primary data set to reach 100% capacity even though no "new" data has been written to the volume (see "Network Appliance goes virtual," this page).

				Network Appliance goes virtual
				Network Appliance (NetApp) Inc. has extended its Snapshot technology with the announcement of Data Ontap 7G. The primary new feature of this release is called FlexVol. Prior to Data OnTap 7G, NetApp volumes consisted of a collection of disk spindles. Individual disk spindles were not shared across data volumes. The only way to create a clone of an entire volume was to replicate the entire volume to another set of spindles. This is time-consuming and requires the destination volume to have the same raw capacity as the source volume. FlexVol changes this. A FlexVol has all the features of a traditional NetApp volume with the exception that it's now a logical, instead of a physical, allocation of space. This adds enormous flexibility to capacity allocation, provisioning and volume cloning. A FlexVol can be cloned in a matter of seconds. The clone is read-write and only consumes physical space when data is written to it. Clones can be made of clones. These branches can be split to form independent FlexVols. The technology that permits this is based on the snapshot capabilities inherent in NetApp's operating system.

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