No Silver Bullets

Snapshots don't come free of disadvantages. The main disadvantage, as previously discussed, is that using snapshots requires careful planning. If you intend to use snapshots as part of your backup strategy for a folder, you need to store the contents of that folder on snapshot-capable storage to begin with.

Another issue is performance. LVM in particular is known for taking a performance hit when it must keep multiple snapshots taken from the same filesystem [3].

The biggest problem with LVM snapshots, however, is that their life is finite. LVM snapshots track the changes done to the original filesystem (see the box "How Do LVM Snapshots Work?"). If enough changes are done to the original filesystem, the LVM snapshot will run out of space to register the changes. Modern LVM supports dynamic expansion of the snapshot volume if it detects it is running out of room [4], but hard drive space is finite and the snapshot may yet be dropped if it needs to grow past the physical storage medium's capabilities.

It is worth noticing that many applications with data you might want to back up don't guarantee the consistency of their files while the application is running. For example, a busy database might work asynchronously, keeping many operations in a RAM cache and committing them to disk periodically. Creating a backup that perfectly mirrors the filesystem the database is stored in will then create a copy with inconsistent data, because many database operations may not have been committed to disk. It is important to check the documentation of the programs you are backing up in order to learn of potential pitfalls.

Conclusions

Classic Unix tools are not advisable for making backups for files and folders that are being modified during the process, because backup corruption can occur as a result. This article has covered LVM as a safer alternative for Linux, but there are other tools for the task, including ZFS and BTRFS snapshots.

Many programs, especially services intended for production, suggest their own means for creating backups (Figure 4). Databases are notorious for including their own backup utilities, and if you are indeed using a real database (such as MariaDB or PostgreSQL) you should consider using their tools instead of backup utilities that operate at filesystem or block device level.

Figure 4: Production-ready programs offer instructions for backing up their data – and even provide their own backup functionality. OpenCart, a popular e-commerce platform, has a utility for copying its database on the fly.

The Author

Rubén Llorente is a mechanical engineer who ensures that the IT security measures for a small clinic are both legally compliant and safe. In addition, he is an OpenBSD enthusiast and a weapons collector.