VMware backups with Exclude unused and deleted blocks option turned on may be slow

Problem

VMware backups with Exclude unused and deleted blocks option turned on may be slow, especially if a Virtual Machine being backed up has fragmented free space or a large number of very small files. Restores of these backups may also be very slow.

Error Message

There is no error message and the backup will eventually complete successfully, however backup performance may be lower than expected.

The lines in VxMS provider logs will indicate the time that NetBackup takes to perform the exclusion:

 09:11:34.0169 : rvp_map_exclude_unallocated:pintf.cpp:1404 <INFO> : Excluding blocks on /var/data
 09:28:32.0971 : excludeUnallocated:pExt2fsFileSystem.cpp:1289 <INFO> : Excluded 83022738 excluded sectors

In this example it took 17 minutes to exclude unused blocks on the volume /var/data.

Cause

With the option to Exclude unused and deleted blocks turned on, NetBackup has to examine the filesystem and find which blocks are in use. In addition, with this option enabled, NetBackup will perform reads of contiguous used blocks of data (extents). If free space in the virtual machine is fragmented, extents may be small resulting in a much larger number of read operations required to back up the same amount of data, compared to scenario of the option turned off. During a restore, these extents are written back to the datastore one at a time, therefore restore performance will also be impacted.

As a result, while backups may be smaller with this option enabled, the time it takes to complete these backups may be longer.

Solution

Consolidating free space on all volumes of a VM may improve performance, however it may also increase the used size on the datastore if the virtual disk was thin-provisioned. This command will perform free space consolidation on all volumes on a Windows 2008 R2-based VM:

defrag /C /X /V

Otherwise, consider disabling this option.