(This post first appeared on SQLperformance.com four years ago as part of a blog series, before that website was mothballed later in 2022 and the series was curtailed. Reposted here with permission, with a few tweaks.)
In the second part of this series, I described the structural hierarchy of the transaction log. As this post is chiefly concerned with the Virtual Log Files (VLFs) I described, I recommend you read the second part before continuing.
When all is well, the transaction log will endlessly loop, reusing the existing VLFs. This behavior is what I call the circular nature of the log. Sometimes, however,