Understanding snapshot disk consumption

Importance of understanding snapshot disk consumption

You need to understand the amount of disk space that snapshots consume and are likely to consume to determine

How often to create a snapshot
How long to keep a snapshot
How much disk space you need for the snapshot reserve

How snapshots consume disk space

Data ONTAP preserves pointers to all the disk blocks currently in use at the time the snapshot is created. When a file is changed, the snapshot still points to the disk blocks where the file existed before it was modified, and changes are written to new disk blocks.

Disk consumption is minimized by preserving individual disk blocks rather than whole files. Snapshots begin to consume extra space only when files in the active file system are changed or deleted. When this happens, the original file blocks are still preserved as part of one or more snapshots, but in the active file system the changed blocks are rewritten to different locations on disk or removed as active file blocks entirely. The result is that, in addition to the disk space used by blocks in the modified active file system, disk space used by the original blocks is still reserved in snapshots to reflect what the active file system was before the change.

Example of how snapshots consume disk space

The following illustration shows how snapshots consume disk space before and after you delete a file named myfile.txt.

Example of how changing file content consumes disk space

Changing the contents of the myfile.txt file creates a situation similar to the one illustrated above. New data written to myfile.txt cannot be stored in the same disk blocks as the current contents because the snapshot is using those disk blocks to store the old version of myfile.txt. Instead, the new data is written to new disk blocks. As the following illustration shows, there are now two separate copies of myfile.txt on disk--a new copy in the active file system and an old one in the snapshot.

Disk consumption by multiple snapshots

Multiple snapshots can reference the same file without consuming extra disk space. Suppose a snapshot contains a 1-MB file that has not changed since Data ONTAP created the snapshot. If you delete that file from the active file system, the snapshot then consumes 1 MB of disk space.

The same version of that 1-MB file might be referenced by several snapshots: hourly.0, hourly.1, and hourly.2. If those snapshots all contain the 1-MB file that has not changed since Data ONTAP created them, only 1 MB of disk space is consumed by the snapshots even though all three snapshots contain the file. This is because they are all pointing to the same disk blocks.

For detailed information

The following sections discuss how to reserve disk space for snapshots and how to monitor snapshot disk space usage: