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

Best practices with FileSnaps
The key to obtaining maximum performance with FileSnaps is to minimize the
copy-on-write overhead. You can achieved this by enabling lazy copy-on-write.
Lazy copy-on-write is easy to enable and usually results in significantly better
performance. If lazy copy-on-write is not a viable option for the use case under
consideration, an efficient allocation of the source file can reduce the need of
copy-on-write.
Virtual desktops
Virtual desktop infrastructure (VDI) operating system boot images are a very good
use case for FileSnaps. The parts of the boot images that may change are user
profile, page files (or swap for UNIX/Linux) and application data. It would be ideal
to separate such data from boot images to minimize unsharing. It is also important
to allocate a single extent to the master boot image file.
Virtual desktop example
The following example uses a 4 GB master boot image that has a single extent
that will be shared by all snapshots.
# touch /vdi_images/master_image
# /opt/VRTS/bin/setext -r 4g -f chgsize /vdi_images/master_image
The master_image file can be presented as a disk device to the virtual machine
for installing operating system. Once the operating system is installed and
configured, the file is ready for snapshots.
Write intensive applications
When virtual machines are spawned to perform certain tasks that are write
intensive, significant amount of unsharing can take place. Symantec recommends
that you optimize performance by enabling lazy copy-on-write. If the use case
does not allow enabling lazy copy-on-write, with careful planning, you can reduce
the occurrence of unsharing. The easiest way to reduce unsharing is to separate
the application data to a file other than the boot image. If this is not possible due
to the nature of your applications, then you can take actions similar to the
following example.
Write intensive applications example
Assume that the disk space required for a boot image and the application data is
20 GB. Out of this, only 4 GB is used by the operating system and the remaining
Administering FileSnaps
Best practices with FileSnaps
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