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QueryChangedDiskAreas call incorrectly returns the entire thin-provisioned disk (2039052)

Symptoms

When called on large thin-provisioned disks, QueryChangedDiskAreas(*) might return the entire disk as change info, rather than only the in-use areas of the thin-provisioned disk.

Cause

QueryChangedDiskAreas(*) relies on the underlying host file system to provide the initial list of changed areas.

This is not always guaranteed to be the minimal list, and in the worst case could be the entire disk.

The response depends on the host file system, the size of the virtual disk, the pattern of I/O, and the format of the guest file system. The issue is especially noticeable on Linux ext2 and ext3 file systems.

Resolution

This is a known issue, and is currently being reviewed by VMware.

This article will be updated as information becomes available.

To work around this issue:

  1. Activate change tracking before installing the file system. This allows Changed Block Tracking to track which thin-provisioned sectors are written-to, and which are not.
  2. After installing the file system, initiate a full backup by calling QueryChangedDiskAreas() with a numeric change ID instead of "*" so it uses the change tracking mechanism instead of the underlying host file system.

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