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Snapshots 9-9
NOTE: The numbers in this example were rounded off to make the example easier to
understand. Also, to make the output easier to read, the “Mounted on” column is not
included in the sample
df
output in the following sections.
In this example, the
vol0
volume contains 4 GB of disk space. It has 1 GB (or 25%)
reserved for snapshots (the idea of reserving space for snapshots is described in
more detail later). That leaves 3 GB for the active file system, and 2 GB of the file sys-
tem is in use.
It is important to understand that the
/vol/vol0/.snapshot
line counts data that
exists only in a snapshot. Because data that also exists in the active file system needs
to be stored on disk anyway, it is misleading if the filer charged the space to snap-
shots. In the example, half of the 1 GB reserved for the snapshot is used.
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By default, the snapshot reserve is 20% of disk space. For information about how to
adjust the amount of the snapshot reserve, refer to “Changing the Snapshot
Reserve.”
This section uses several examples to explain the advantages of reserving disk space
for snapshots.
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If the filer created a snapshot when the disks were full, removing files from the active
file system wouldn’t create any free space because everything in the active file sys-
tem would also be referenced by the newly created snapshot. The filer would have to
delete the snapshot before it could create any new files.
The following example shows how disk space being freed by deleting files in the
active file system ends up in the snapshot:
If the filer creates a snapshot when the active file system is full and there is still space
remaining in the snapshot reserve, the
df
command output is as follows:
Filesystem kbytes used avail capacity
/vol/vol0/ 3000000 3000000 0 100%
/vol/vol0/.snapshot 1000000 500000 500000 50%
If you remove 100 MB of files, the disk space used by these files is no longer part of
the active file system, so the space is reassigned to the snapshots instead. If you
enter the
df
command, the output is as follows:
Filesystem kbytes used avail capacity
/vol/vol0/ 3000000 2900000 100000 97%
/vol/vol0/.snapshot 1000000 600000 400000 60%
The filer reassigns 100 MB of space from the active file system to the snapshot
reserve. Because there was reserve space for snapshots, removing files from the
active file system freed space for new files.