Common backup errors and what they mean
Backups, when running smoothly gives you peace of mind. But, from time to time, your backups will throw you an error. The error doesn’t necessarily mean the backup was unsuccessful, but may start driving you crazy not knowing what the error means…
I’ve listed a few common errors and some of their causes below.
1. Permission errors
On the surface this may seem self-explanatory. The backup application must be run under an account that has sufficient permissions to read the selected data for backup, or permissions errors will arise. There are, however, situations where files or folders copied from another server may have missing or improperly set permissions. In these cases, it’s necessary to manually and recursively update the permissions on the folder in question.
2. Read error
Read errors usually arise when you have proper permissions to access the data, but the read operation fails. This is often caused by failing hardware, and is usually accompanied by an OS error message.
3. File size changed during backup
This message will appear when a file is modified during the backup process. This often occurs with log files, where the file is written to exactly when the file is being read by the backup software. This means that the exact contents of the file cannot be relied upon. In these cases, it’s recommended to enable some form of snapshotting to ensure file consistency.
4. Failed to create VSS snapshot
VSS, the Microsoft Volume Shadow copy Service, was not able to successfully snapshot the disk volume. VSS can be quite complex with potentially dozens of application specific VSS Writers that need to be called to perform a snapshot, so there are multiple possible points of failure. Luckily, there will almost always be detailed logs available in the Windows Event viewer to help track down the issue. Often it’s as simple as one of the component services needing a restart. In some cases, certain applications will replace the default Volume Shadow copy Service with their own implementation. This is common with some older backup and archiving applications. In these cases, the replacement VSS implementation may not be fully compliant and will need to be removed.
5. Path no longer available
This error message often occurs when multiple backup or archiving applications exist on the same server. The backup application will be working from a specific volume snapshot path that is presented from a temporary snapshot. The other application may cause the snapshot to be purged, causing the snapshot path to disappear mid-backup. It is not recommended to run multiple applications of this type on the same server, but if absolutely critical, care must be made to schedule them apart.
6. File locked
This error occurs when you have the correct permissions for reading a file for backup, but another application has opened the file in “exclusive lock” mode. This prevents the files from being read at all, even if a Volume Snapshot is performed. This lock mode is often found on temp files, such as the common MS Office temp files (~$ms_office_file). However, you may find that some applications have been designed to erroneously open standard data files in exclusive lock mode. The only recourse in this case would be to schedule a downtime for the offending application so that backups can be performed.
7. Checksum does not match
This error if often caused by failing hardware. When a file is being backed up, a checksum is taken of the original file. The backed up data is then validated to ensure that it was processed correctly. The error usually means that the file read operations are returning inconsistent data, and there is a high likely-hood that the disk may be failing.