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10.1.3 Race conditions

If some other process is modifying the file system while tar is reading or writing files, the result may well be inconsistent due to race conditions. For example, if another process creates some files in a directory while tar is creating an archive containing the directory’s files, tar may see some of the files but not others, or it may see a file that is in the process of being created. The resulting archive may not be a snapshot of the file system at any point in time. If an application such as a database system depends on an accurate snapshot, restoring from the tar archive of a live file system may therefore break that consistency and may break the application. The simplest way to avoid the consistency issues is to avoid making other changes to the file system while tar is reading it or writing it.

When creating an archive, several options are available to avoid race conditions. Some hosts have a way of snapshotting a file system, or of temporarily suspending all changes to a file system, by (say) suspending the only virtual machine that can modify a file system; if you use these facilities and have tar -c read from a snapshot when creating an archive, you can avoid inconsistency problems. More drastically, before starting tar you could suspend or shut down all processes other than tar that have access to the file system, or you could unmount the file system and then mount it read-only.

When extracting from an archive, one approach to avoid race conditions is to create a directory that no other process can write to, and extract into that.

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