This is a quickie in response to a blog comment from my previous post on instant initialization: How to tell if you have instant initialization enabled? The comment was:
I must say, I love instant initialization. It's awesome. But I always wondered why it's not available for the log file. I assume there's a technical reason… but what is it? Does it depend on having the rest of the file be zeroed out? Doesn't it already know where it's start and stop points are anyways, since the log is circular?
I couldn't remember the exact answer so I discussed with Peter Byrne on the Storage Engine dev team and now I have the answer to share. There is a lot of metadata kicking around in the Storage Engine about the transaction log (mostly in the boot page – see my post Search Engine Q&A #20: Boot pages, and boot page corruption), including where to start reading the log during crash recovery. However, there's nothing that can be used after a crash occurs to determine where the active transaction log ends (i.e. where should crash recovery stop processing log records).
The way this is done is to have each log sector have parity bits stamped on it. When the log is first created, it is zero-initialized (with zero being an illegal log sector parity value). As the log is written, each sector has parity bits in it. When the end of the log is reached, and it wraps around to the start of the log file, the parity bits are flipped, so that overwritten log sectors have the opposite parity from when they were last written. When a crash occurs, log sectors are read and processed until a log sector with an out-of-sequence parity is found.
This entire process will not work if there's already random data in the space used by the log file – some of the random data could just look like a valid set of parity bits and cause the recovery system to try to process a log sector full of garbage, leading to a suspect database, at best.
So – it's not just a "there wasn't time" – there really is a good, architectural reason why instant initialization cannot be done with the transaction log.