Time spent by the various sessions waiting for space in the log buffer or otherwise waiting for memory to be made available to write log records. Consistently high values of this metric can indicate the log devices’ inability to keep up with the amount of log generated by the server.
When a user transaction is committed (either explicitly using a COMMIT statement, or implicitly), SQL Server writes all changes from the Log Cache out to the log files on disk. This process is termed a log flush. The user that issued the commit must wait until the log flush is complete before they can continue. If the log flush takes a long time, this will degrade the user's response time.