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 system guarantees that either the whole block is written, or nothing at all.

How do I achieve the same effect on a memory mapped file?

Memory mapped files are simply byte arrays, so if I modify the byte array, the operating system has no way of knowing when I consider a write "done", so it might (even if that is unlikely) swap out the memory just in the middle of my block-writing operation, and in effect I write half a block.

I'd need some sort of a "enter/leave critical section", or some method of "pinning" the page of a file into memory while I'm writing to it. Does something like that exist? If so, is that portable across common POSIX systems & Windows?

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.