26.8 Files and Secondary Storage

After Emacs changes a file, there are two reasons the changes might not survive later failures of power or media, both having to do with efficiency. First, the operating system might alias written data with data already stored elsewhere on secondary storage until one file or the other is later modified; this will lose both files if the only copy on secondary storage is lost due to media failure. Second, the operating system might not write data to secondary storage immediately, which will lose the data if power is lost or if there is a media failure.

Although both sorts of failures can largely be avoided by a suitably configured system, such systems are typically more expensive or less efficient. In lower-end systems, to survive media failure you can copy the file to a different device, and to survive a power failure (or be immediately notified of a media failure) you can use the write-region function with the write-region-inhibit-fsync variable set to nil. Although this variable is ordinarily t because that can significantly improve performance, it may make sense to temporarily bind it to nil if using Emacs to implement database-like transactions that survive power failure on lower-end systems. See Writing to Files.

On some platforms when Emacs changes a file other processes might not be notified of the change immediately. Setting write-region-inhibit-fsync to nil may improve notification speed in this case, though there are no guarantees.