Is data written to the transaction logs in any particular order?
As new data arrives, the Store writes it to the log files in a strict first in/first out (FIFO) order. This procedure is true for both Exchange 2000 Server and Exchange Server 5.5; however, because Exchange 2000 can have multiple databases in one storage group, one set of Exchange 2000 logs will contain transactions for all the databases in the group interleaved in the order of their arrival (in fact, blocks of data in a single transaction can be interleaved with blocks from other concurrent transactions).
After the Store has saved an incoming transaction to the log file, it applies the data to the in-memory copy of the corresponding database pages; eventually (when its workload allows), the Store writes the data to the database file on disk. During typical Exchange operation, the log files are write only—the Store reads the logs only when it restarts and has to replay the logs. The Microsoft white paper "Best Practices for Exchange Database Management" (http://www.microsoft.com/exchange/techinfo/administration/55/bestdbmanage.asp) explains the logging and recovery process for Exchange 5.5; the process is basically identical for Exchange 2000.