Why do SQL databases use a write-ahead log over a command log?

Question

I read about Voltdb\'s command log. The command log records the transaction invocations instead of each row change as in a write-ahead log. By recording only the invocation, the

Answer 1

From the description of Postgres' write ahead http://www.postgresql.org/docs/9.1/static/wal-intro.html and VoltDB's command log (which you referenced), I can't see much difference at all. It appears to be the identical concept with a different name.

Both sync only the log file to the disk but not the data so that the data could be recovered by replaying the log file.

Section 10.4 of VoltDB explains that their community version does not have command log so it would not pass the ACID test. Even in the enterprise edition, I don't see the details of their transaction isolation (e.g. http://www.postgresql.org/docs/9.1/static/transaction-iso.html) needed to make me comfortable that VoltDB is as serious as Postges.