What does the file mod timestamp of temp-21331.rdb say? It sounds like a leftover from a crash.
You can delete it.
The documentation is definitely correct. When rewriting, all info is written to a temp file (compressed), and when complete, the dump.rdb file is replaced by this temp-file. There should however be no leftovers during normal usage. What is important: you always need enough free disk space for this operation to succeed. A safe guideline is: 140% times the redis memory limit (it would be 200% if no compression was applied).
Hope this helps, TW
