Windows Agent

Mr.OneProCloudJuly 1, 2025About 7 min

Windows Agent

Important Notice

Compatibility of Windows Agent with Other Backup Software

The Windows Agent captures I/O changes through Windows VSS (Volume Shadow Copy Service) and disk filtering drivers. However, due to limitations of Windows VSS, if two software applications attempt to invoke VSS services during overlapping time periods, it may cause VSS snapshot failures and prevent the system from meeting the predefined RPO requirements. Therefore, the Windows Agent cannot run concurrently with other backup software that uses the same mechanism, such as Acronis, Veeam, Veritas, Commvault, etc.

If similar backup software is already installed on the system, it is recommended to pause or uninstall that software before installing the Windows Agent to avoid conflicts. Additionally, users may consider consulting their platform provider to confirm whether agentless data backup is supported, which can help prevent conflicts between different backup solutions and ensure proper system backup and recovery.

OS Support

Click Agent Support Matrix to view the compatibility list and get the latest support status.

FileSystem & Partition Types

FileSystem

NTFS
FAT16
FAT32

Partitions

Primary/Extended
Logical Disk Manager(LDM)
Basic disk (Partition type: MBR/GPT)

Support and Limitations

Basic Requirements

CPU: x86-64-bit processor
- CPU Usage Control: You can limit the average CPU usage of DiskSyncAgent by configuring the CPU_USAGE_SETTING option. By default, the CPU usage limit is set to 30%.
- Increase Sync Speed: If your network is sufficient, increasing the CPU usage limit can improve synchronization speed in object storage mode.
- It is important to note that increasing the CPU usage limit may slow down or destabilize the system if the user's computer performance is low or if other CPU-intensive tasks are running simultaneously. Therefore, before increasing the CPU usage limit, it is necessary to evaluate the system's performance and resource utilization.
Memory Requirements:
- To ensure the normal operation of the DiskSyncAgent service, it is recommended to have at least 1 GB of available memory.
- The DiskSyncAgent service typically occupies 220-350 MB of memory. Insufficient system memory may cause the DiskSyncAgent service to run slowly or crash.
Agent Space Usage: To ensure proper installation and operation of the software, it is recommended to reserve at least 200 MB of free space.
Network Connection: To ensure stability and speed of the connection to the target endpoint, it is recommended to use a network connection of at least 10 Mbps.
System Firmware: To ensure compatibility with the software, BIOS or UEFI firmware is required.
Virtualization Support: DiskSyncAgent supports full virtualization, but support for paravirtualization (e.g., XenServer) is limited. Manual repair may be required during final boot.

Disk Space Recommendation

The Windows Agent uses Windows VSS to create consistent snapshots without interrupting system operations, ensuring data integrity and enabling quick recovery, thus improving synchronization efficiency and business continuity. Therefore, the following disk space requirements apply during the synchronization process.

Recommended Free Space for VSS Snapshots: It is recommended that the free space on the volume used for VSS snapshots be at least 10%. If the volume data is frequently updated, the proportion of reserved free space should be increased to avoid synchronization failures due to insufficient space.
Impact of High Disk I/O on VSS Snapshots: Excessive disk I/O during host synchronization can prevent VSS snapshot data from being maintained properly. The Windows system prioritizes business data access and may abandon VSS snapshot differential data, causing synchronization failures. This issue may occur during operations like data backups, large database transactions, table index updates, full disk searches, or heavy temporary table operations.
Recommended Actions to Address Synchronization Failures Due to High Disk I/O:
1. Reduce Disk I/O: Adjust business operations, such as pausing backup tasks during synchronization or optimizing SQL queries to reduce disk I/O.
2. Designate a Non-Synchronization Disk: Assign a disk as a non-synchronization disk, or add a new disk and allocate a volume dedicated to VSS snapshot data. Then, configure the VSS_SPEC_MAX_? setting to direct VSS storage to the dedicated volume, which can help alleviate high I/O impact but may not fully resolve the issue.

Disk Volume Requirements

The number of disks must be fewer than 32.
Offline synchronization is not supported for source disks. All disks must be online before synchronization. You can configure disks that do not need data synchronization through the source configuration file.
Disks that do not need synchronization (except boot disks) can be specified through configuration. Refer to the EXCLUDE_DISKS configuration item in the configuration file. Excluded disks can be offline. Note: Modifying the EXCLUDE_DISKS configuration item must be completed before the first service startup (node registration). Once the host node registration is completed, this configuration item cannot be modified. If you need to modify it, you need to clear the resources and re-register the node.
If there are shared disks mounted to multiple hosts on the source side, only one host's shared disk can be selected for synchronization during migration. The shared disks on other hosts can be excluded through the EXCLUDE_DISK configuration item.
The free space ratio of the volume should not be less than 3%. If the system incremental data is large, additional free space is required to reserve space for storing VSS snapshots. Insufficient space may cause snapshot abnormalities, leading to synchronization failure.
Simple volumes and partition volumes synchronization are supported, but not volumes with areas, mirror volumes, or RAID volumes.
Windows dynamic disks cannot be used as system boot partitions.
Windows operating system UEFI boot is not supported.

File System and Volume Snapshot Requirements

NTFS file system supports VSS synchronization mode, which only synchronizes valid data.
Raw devices or other file systems outside of NTFS do not support VSS mode and are synchronized based on the actual disk capacity.
In VSS mode, the VSS service on the source host must be running normally. You can use the vssadmin command line provided by Windows to create and delete shadows.
In cloud disk mode, the MsiscsiService service must be running normally.
During data synchronization on the source host, do not manually delete Windows VSS shadows. Otherwise, synchronization failure may occur, and you will need to restart full synchronization.

Network Requirements

In cloud disk mode, the source host needs to be able to access port 3260 of the target endpoint and ensure that iSCSI communication is normal.
In cloud disk mode, if the source disk contains iSCSI disks, use caution and do not change the original Initiator Name to avoid affecting the normal operation of the business system.
In cloud disk mode synchronization, the network bandwidth from the source host to the target host should not be less than 5 Mbps to ensure the stability of the iSCSI target disk. Bandwidth less than 5 Mbps may cause instability of the iSCSI disk, resulting in synchronization failure. Ensure network stability, low latency, and low jitter.
Support proxy mode, set the correct proxy server and port through the interface during installation and startup stages.

System Patch Requirements

To ensure the normal operation of the DiskSyncAgent service, you need to install patch KB4474419 on three types of systems: Windows 2008 32-bit, Windows 2008 64-bit, and Windows 2008 R2 64-bit. After installing the patch, restart the system before starting the DiskSyncAgent service. For patch installation methods, please refer to the appendix.

Security Software Requirements

If your host has security software installed, it is recommended to completely disable it before synchronization (for some software, the exit function cannot completely disable the software. If you are unsure whether the software can be completely disabled, it is recommended to uninstall the related software first).
If you cannot completely disable the software, pay attention to the security software's warning messages during the installation and startup of the Agent service. If a warning message window appears, be sure to set trust for all operations of the software to avoid interception or warning from the security software. Do not temporarily disable security software for the installation and startup of Windows Agent, as this may damage the Agent service. If this happens, you need to restart the host to restore the host service. For the list of security software that affects Agent data synchronization, please refer to Appendix 2.

Time Synchronization Requirements

Under conditions where the host can connect to the Internet, it needs to synchronize with the Internet time to maintain consistency. If you cannot connect to the Internet, you need to synchronize with the time server within the intranet.

Language Support

The Windows Agent currently supports the following system languages:

Chinese (中文)
English
Spanish (Español)

HyperBDR configuration requirements

If the boot mode of the source Windows host is UEFI and the target platform does not support native UEFI boot, a public network needs to participate in UEFI repair transition host.

FAQ

Solutions for Windows Agent Synchronization Failures (153315 / 154000 / 154001 / etc.)

When using the Windows Agent, common error codes include 153315, 154000, and 154001, etc. These errors are usually related to Windows VSS (Volume Shadow Copy Service) VolSnap events. Below is a detailed analysis of these errors.

VSS Reserved Storage Space Exhaustion: This issue occurs when the VSS reserved storage space is quickly used up, leading to an automatic snapshot cleanup. It can manifest under various event IDs, including Event ID 23 (VS_DIFF_AREA_CREATE_FAILED_LOW_DISK_SPACE) and Event ID 36 (VS_ABORT_NO_DIFF_AREA_SPACE_USER_IMPOSED), but may also appear with other IDs.
Differential Data Write Failure: This happens when the system is unable to handle differential data writes, triggering a snapshot cleanup to ensure normal I/O operations continue. To avoid unexpected snapshot deletions due to high I/O or insufficient storage, Microsoft recommends moving VSS snapshots to a disk with more available space or using a separate disk not involved in VSS snapshots. This helps ensure snapshot stability and business continuity. This problem is typically associated with Event ID 25 (VS_ABORT_SNAPSHOTS_OUT_OF_DIFF_AREA), but could also occur with other event IDs.

Tips

Starting from version v6.2.0, if a VSS exception occurs (e.g., the VSS snapshot is deleted due to high I/O load), the Windows Agent will be unable to continue reading incremental data. In this case, a full synchronization will be automatically performed upon the next sync trigger to ensure data integrity.

How to Resolve?

References

Event ID	Source	Message
1	VolSnap	The shadow copy of volume %2 could not create a diff area file on volume %3.
2	VolSnap	The shadow copy of volume %2 could not be created because volume %3, which is specified as part of the diff area, is not an NTFS volume or an error was encountered while trying to determine the file system type of this volume.
3	VolSnap	The shadow copy of volume %2 could not lock down the location of the diff area file on volume %3.
14	VolSnap	The shadow copies of volume %2 were aborted because of an IO failure on volume %3.
16	VolSnap	The shadow copies of volume %2 were aborted because volume %3, which contains a diff area file for this shadow copy, was force dismounted.
23	VolSnap	There was insufficient disk space on volume %3 to create the shadow copy of volume %2. Diff area file creation failed.
24	VolSnap	There was insufficient disk space on volume %3 to grow the diff area for shadow copies of %2. As a result of this failure all shadow copies of volume %2 are at risk of being deleted.
25	VolSnap	The shadow copies of volume %2 were aborted because the diff area file could not grow in time. Consider reducing the IO load on this system to avoid this problem in the future.
33	VolSnap	The oldest shadow copy of volume %2 was deleted to keep disk space usage for shadow copies of volume %2 below the user defined limit.
35	VolSnap	The shadow copies of volume %2 were aborted because the diff area file failed to grow.
36	VolSnap	The shadow copies of volume %2 were aborted because the diff area file could not grow due to a user imposed limit.
38	VolSnap	There was a user imposed limit that prevented disk space on volume %3 from being used to grow the diff area for shadow copies of %2. As a result of this failure all shadow copies of volume %2 are at risk of being deleted.
40	VolSnap	The shadow copies of volume %2 were aborted because volume %3 has been dismounted.
41	VolSnap	When preparing a new volume shadow copy for volume %2, the shadow copy storage on volume %3 did not have sufficiently large contiguous blocks. Consider deleting unnecessary files on the shadow copy storage volume or use a different shadow copy storage volume.