When managing backups for LXC containers with Proxmox Backup Server (PBS), users often encounter questions about how the system handles interrupted or subsequent backups. Specifically, understanding the concept of “Server Resume” in this context is crucial for efficient data management. This article clarifies the backup process, especially concerning the behavior of backups after interruptions or when run repeatedly.
For a new backup, particularly the very first one, it’s essential to understand that Proxmox Backup Server needs to perform a comprehensive process. This involves several steps on the backup client side before data is even transmitted:
- Data Reading: The backup client starts by reading all the data and files within the LXC container that is being backed up.
- Chunking: This continuous stream of data is then divided into smaller, manageable segments known as chunks.
- Hashing, Compression, and Encryption: Each chunk undergoes a series of transformations. It is hashed to create a unique identifier, compressed to save space, and optionally encrypted for security.
If a previous backup snapshot exists due to a successful prior backup, PBS can optimize the process significantly. In such cases:
- Snapshot Registration: Chunks that are already part of the previous backup snapshot are simply registered with the server, avoiding redundant uploads.
- Selective Upload: Only new or modified chunks, which are not part of the prior snapshot, are uploaded to the server.
However, when there is no prior successful backup snapshot, typically during the very first backup of an LXC container:
- Full Chunk Upload: All generated chunks are uploaded to the server, regardless of whether similar chunks might already exist from other backups.
On the server side, Proxmox Backup Server operates efficiently by:
- Duplicate Chunk Discarding: The server intelligently discards any uploaded chunks that it already has in its storage. This deduplication process saves storage space and bandwidth over time.
Therefore, in the absence of a successful backup snapshot, an LXC backup will always initiate from the beginning, re-reading all data and creating chunks. While this might seem like a restart, the server is still efficient as it avoids rewriting existing data chunks. Once a successful backup is completed and a snapshot is created, subsequent backups become significantly faster due to the incremental nature of PBS. It skips uploading chunks that are already backed up, focusing only on changes since the last snapshot. However, it’s important to note that data reading on the client side is currently always performed in full for each backup, irrespective of whether it’s a full or incremental backup.
To manage backup processes effectively, especially with large LXC containers or in environments where interruptions are possible, consider these options:
- Local PBS with Synchronization: Set up a local Proxmox Backup Server for initial backups. Perform backups to this local server (which should be faster and less prone to interruption). Once the backup is successful, synchronize the data to your actual, possibly remote, PBS server. This strategy aims to establish an initial snapshot locally to facilitate efficient incremental backups later, even when targeting the external PBS server directly. If local storage is sufficient, maintaining the local PBS and sync setup long-term can be a robust solution.
- Aggressive Exclusion and Gradual Backup Expansion: To minimize the initial backup size and time, start with aggressive exclusions of non-critical data within the LXC container. Perform a backup with these exclusions. Gradually relax the exclusions, adding more data in each subsequent backup iteration. Remember to prune the initial “reduced” backups after a full backup is established to avoid confusion between them and regular backups.
Understanding how Proxmox Backup Server handles initial and subsequent backups, especially the “server resume” behavior in terms of efficiency and deduplication, is key to designing an effective backup strategy for your LXC containers. By leveraging local backups and intelligent exclusion strategies, you can optimize backup times and ensure data integrity while minimizing resource usage.
