File links are used to create shortcuts from one file location to another on your computer. If used unwisely, though, these links can cause many types of software--including CrashPlan--to interact with the same files more than once.
This article describes how to use soft and hard links in order to avoid problems for CrashPlan and other software.
For issues with file links relating to Apple's Time Machine software, please read Using CrashPlan And Time Machine.
How file links work
Your computer uses files to physically locate information on the hard drive (or other storage drive, like a solid state drive). For an in-depth examination of exactly how your computer works with files and file links, check our External Resources.
File links refer to files, but they don't contain much information themselves. There are two different types of file links that operate slightly differently: soft links and hard links.
A soft link is a type of file link that references another file. Soft links are also called "symlinks" in some environments. Soft links can point to any file on your system, even supporting links from one hard drive or logical volume to another. In fact, soft links can also point to a file or path that doesn't exist yet. Because soft links are so versatile, they are used throughout many UNIX operating systems, including Mac OS X and many Linux distros.
Soft link backup considerations
- CrashPlan backs up soft links, but not the target of the soft link. If you back up a soft link with CrashPlan, be aware that only the link is backed up—not the target file.
- Soft links can link to files with relative paths or absolute paths. When you restore a soft link, it is restored in the same state it was backed up in (relative or absolute). It is possible to restore a soft link and its target in such a way that the link does not accurately point to the target.
- Windows Server utilizes soft links for folder redirection, which allows administrators to move the location of folders on the computer or to another drive.
A hard link is a type of file link that references a specific physical location on a storage drive. Hard links are more efficient than soft links, because they don't deal with the complexities supported by soft links. Hard links are also limited in some ways, such as being restricted from hard linking directly to a folder, in order to prevent some complex problems. However, hard links represent another instance of a file without actually being a separate copy, and that can cause problems for some software.
Hard link backup considerations
- CrashPlan follows hard links when examining your drive for files to back up. As a result, using lots of hard links can quickly drive up the total file count that CrashPlan backs up, and an abnormally high file count can cause poor performance or other unexpected behavior from CrashPlan.
- CrashPlan follows hard links when examining your drive for files to back up. As a result, using hard links can cause CrashPlan to examine the same information more than once, which can add to backup processing time.
- Some software and utilities, such as Apple's Time Machine utility, use hard links to accomplish their tasks.
De-duplication and hard links
Hard links can cause CrashPlan to examine the same file more than once, but CrashPlan will not waste space by backing up the same information multiple times. CrashPlan uses data de-duplication to back up all your information as efficiently as possible.
For more information on data de-duplication, read How Backup Works.
Always avoid back up of hard links
If your backup selection includes multiple hard links to the same file, CrashPlan processes the links individually, which can cause poor performance.
To prevent this, you can deselect files and folders containing hard links with CrashPlan's file selection feature.
Avoid use of hard links
Many types of software can exhibit strange behavior when working with hard links. If your system can operate well without using hard links, you may want to avoid using them altogether.
Be aware of your soft links
Soft links can trick you into thinking some information is backed up, but in fact only the soft link is backed up. You may want to test your backups by making sure that you can restore files from your backup.
Soft links do not carry the same risks as hard links, but they can cause you potential headaches when restoring unless you plan ahead and carefully consider your use of soft links.