I don't believe this is the case - it's not an in-memory database.
Even if it is (and I can't see anything on their developer website) then if it's ACID compliant, like they say, then it will have to be writing to the disk at some point - at the very least it will need a transaction log. Otherwise, it's not really usable for anything you need to persist to long term storage!
Yes, its not clear on their site, but the gigaom article states: "As its name implies, MemSQL achieves its fast performance in part by keeping data in memory".
Yes, it does write to disk (append only logging I think) but my point was, that if you are keeping ALL your data in memory, you can optimise the storage for fast queries, as opposed to a hybrid / paged memory and disk system.
Oops, sorry - I'm wrong. Actually, according to their documentation:
"By default, MemSQL runs with full durability enabled: transactions are committed to disk as a log and later compressed into full-database snapshots. The snapshot and log files are stored in the datadir directory configured in memsql.cnf. When MemSQL is restarted, it will recover itself into the state it was in before the shutdown, by reading the snapshot and log files in datadir" [1]
I missed that piece of documentation on their website. I think they could do better to explain better the advantages and how their technology works. Their product overview doesn't really tell you much of anything, except that they have an enhanced query parser. [2]
Even if it is (and I can't see anything on their developer website) then if it's ACID compliant, like they say, then it will have to be writing to the disk at some point - at the very least it will need a transaction log. Otherwise, it's not really usable for anything you need to persist to long term storage!