Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

16 GB is the space for applications to write. Any data or binaries that are required
by each instance of the virtual machine can still be part of the first 4 GB of the
shared extent. Since most of the writes are expected to take place on the 16 GB
portion, you should allocate the master image in such a way that the 16 GB of
space is not shared, as shown in the following commands:
# touch /vdi_images/master_image
# /opt/VRTS/bin/setext -r 4g -f chgsize /vdi_images/master_image
# dd if=/dev/zero of=/vdi_images/master_image seek=16777215 \
bs=1024 count=1
The last command creates a 16 GB hole at the end of the file. Since holes do not
have any extents allocated, the writes to hole do not need to be unshared.
Data mining, reporting, and testing
It is very common to create one or more copies of production data for the purpose
of generating reports, mining, and testing. These cases frequently update the
copies of the data with the most current data and one or more copies of the data
always exists. FileSnaps can be used to create multiple copies instantly. The
application that uses the original data may see a slight performance hit due to
the unsharing of data that may take place during updates. This slight impact on
performance may still be present even when all FileSnaps have been deleted. An
important point to note though is that many of these use cases almost always
have one or more copies all of the time.
Comparison of the logical size output of the fsadm
-S shared, du, and df commands
The fsadm -S shared, du, and df commands report different values for the size
of a FileSnap. The fsadm -S shared command displays this size as the "logical
size," which is the logical space consumed, in kilobytes, and accounts for both
exclusive blocks and shared blocks. This value represents the actual disk space
needed if the file system did not have any shared blocks. The value from the fsadm
-S shared command differs from the output of du -sk command since the du
command does not track the blocks consumed by VxFS structural files. As a result,
the output of the du -sk command is less than the logical size output reported
by the fsadm -S shared command.
The following examples show output from the fsadm -S shared, du, and df
commands:
299Administering FileSnaps
Comparison of the logical size output of the fsadm -S shared, du, and df commands