Memory Mapped files and atomic writes of single blocks

Question

If I read and write a single file using normal IO APIs, writes are guaranteed to be atomic on a per-block basis. That is, if my write only modifies a single block, the operating

Answer 1

The technique of keeping a journal seems to be the only way. I don't know how this works with multiple apps writing to the same file. The Cassandra project has a good article on how to get performance with a journal. The key thing is to make sure of, is that the journal only records positive actions (my first approach was to write the pre-image of each write to the journal allowing you to rollback, but it got overly complicated).

So basically your memory-mapped file has a transactionId in the header, if your header fits into one block you know it won't get corrupted, though many people seem to write it twice with a checksum: [header[cksum]] [header[cksum]]. If the first checksum fails, use the second.

The journal looks something like this:

[beginTxn[txnid]] [offset, length, data...] [commitTxn[txnid]]

You just keep appending journal records until it gets too big, then roll it over at some point. When you startup your program you check to see if the transaction id for the file is at the last transaction id of the journal -- if not you play back all the transactions in the journal to sync up.