Challenge
Backup or restore fails with either of the following errors:
Error: Client error: Zlib decompression error: [-3].
OR
Error: Client error: Failed to decompress LZ4 block: Incorrect decompression result or length
Cause
Cyclic Redundancy Check (CRC) failed while extracting data from a backup file. This indicates that one or more blocks in that file are corrupted.
Typically this error indicates hardware malfunction during storage or transmission of the backup file. Most types of storage are subject to a small degree of unrecoverable failure – this is commonly called Bit Rot.
Solution
If you are attempting restore and have multiple copies of the backup file, try using a different copy. If you have other restore points, try an older full backup or an incremental associated with an older full. If you are unable to recover the complete VM, file level recovery may still be possible.
If the error is encountered during backup, try running an active full backup. This will bypass the need to read data from previous backup files.
The Windows event log on the repository server may contain Disk Event 7: “The device… has a bad block.” If so, run CHKDSK /F or /R. A small number of bad blocks are normal on most storage devices. If additional bad block events occur after running CHKDSK, check the SMART status and consider replacing the disk. If using a Network Attached Storage device, contact your storage vendor to determine what diagnostics are available.
More Information
Synthetic full backups and reverse incremental backups both have the potential to copy data from corrupted backup files. Active full backups do not copy any data from existing backup files. Although it is best to perform periodic full-VM recovery tests, you can schedule monthly or quarterly active full backups to reduce the possibility that your latest backup contains corrupted data.