9-10 Dell PowerVault 720N, 740N, and 760N System Administrator and Command Reference Guide
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Even with the snapshot reserve, the job of administering snapshot disk space con-
sumption is important. There is no way to prevent snapshots from consuming disk
space greater than the amount reserved for them. Consider what would happen in
the example if all files in the active file system were deleted. Before the deletion, the
df output was as follows:
Filesystem kbytes used avail capacity
/vol/vol0/ 3000000 3000000 0 100%
/vol/vol0/.snapshot 1000000 500000 500000 50%
After removing all the data in the file system, the df command generates the follow-
ing output:
Filesystem kbytes used avail capacity
/vol/vol0/ 3000000 2500000 500000 83%
/vol/vol0/.snapshot 1000000 3500000 0 350%
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The entire 3 GB in the active file system moved into snapshots, along with the
500 MB that were in snapshots before, making a total of 3.5 GB of snapshot data.
This is 2.5 GB more than the space reserved for snapshots. Because the active file
system cannot use that space, the space shows up as used by the active file system
even though no files are there.
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Whenever snapshots consume more than 100% of the snapshot reserve, the system
is in danger of becoming full. In this case, you can create files only after you remove
enough snapshots.
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For example, if 500 MB of data were added to the active file system in the preceding
example, a df command generates the following information:
Filesystem kbytes used avail capacity
/vol/vol0 3000000 3000000 0 100%
/vol/vol0/.snapshot 1000000 3500000 0 350%
As soon as the filer creates a new snapshot, every block in the system is referenced
by some snapshot. Therefore, no matter how many files you remove from the active
file system, there is still not room to add any more. The only way to recover from this
situation is to remove enough snapshots to free more disk space. Refer to Display-
ing Snapshot Statistics in Chapter 9 for information about how to use the
snap
list command to determine which snapshot to delete.