Storage Foundation 7.4.1 Administrator's Guide - Linux

Using FileSnaps to optimize write intensive applications for virtual machines

When virtual machines are spawned to perform certain tasks that are write intensive, a significant amount of unsharing can take place. Veritas 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 you cannot do this due to the nature of your applications, then you can take actions similar to the following 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 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=20971520 \
bs=1024 count=1

The last command creates a 20 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.