I have read that I should not mount /var/tmp as a temporary filesystem (e.g., as tmpfs), because files in /var/tmp must not be deleted on reboot.  Is that correct?Suppose I did mount /var/tmp as a tmpfs, so its contents are deleted on every reboot.  Would that be bad?  If yes, what could go wrong?  How bad would it be?
What can go wrong if /var/tmp is on a temporary filesystem?
filesystems;directory structure;tmp
Files in /var/tmp are expected to be persistent across reboots. From theFHS:The /var/tmp directory is made available for programs that require temporary  files or directories that are preserved between system reboots. Therefore,  data stored in /var/tmp is more persistent than data in /tmp. Files in /var/tmp are often cache files or temporary files that should notdisappear in the event of a sudden power failure. They cannot be expected tolive forever though. It is common to clear oldfiles from /var/tmp on aschedule.Here are some examples of /var/tmp's usage:Some implementations of vi (e.g. nvi) put their crash recovery files in /var/tmp. If that's a temporary filesystem, you don't get a chance of recovering anything. Vim puts its crash recovery files in the same directory as the file being edited.I use a Firefox plugin that allows me toedit text fields in Vim. To accomplish this, the plugin creates a temporaryfile in /var/tmp (/tmp is the default though) and passes the file to Vim.If my computer loses power while I am using this feature, my writing will besafe and sound in /var/tmp.Text editing tools such as ex and sudoedit put temporary files in/var/tmp. If /var/tmp was mounted as tmpfs, you would risk losing data tounexpected power failures.The git-archive(1) manpage has the following example.git archive --format=tar --prefix=junk/ HEAD | (cd /var/tmp/ && tar xf -)  Create a tar archive that contains the contents of the latest commit on  the current branch, and extract it in the /var/tmp/junk directory.It is possible that the /var/tmp directory was chosen so that the extractedarchive contents would not be lost to sudden power failure.Since /var/tmp is cleared periodically but never unexpectedly, it is commonto store temporary logs and test databases there. For example, in the arpdmanpage, /var/tmp is used as the location of a test database for the sakeof some examples.arpd -b /var/tmp/arpd.db   Start arpd to collect gratuitous ARP, but not messing with kernel functionality.  In summary, your system is unlikely to incur severe damage if you mount  /var/tmp as a tmpfs. Doing so may be undesirable though as you would risk  losing information to power failures and reboots.