Understanding the Mailbox Store in Exchange Server 2013

          The primary component of the mailbox store is the mailbox database.
          Mailbox databases contain the data, data definitions, indexes, checksums, flags, and other information that constitute mailboxes in Exchange Server 2013. Mailbox databases hold data that is private to an individual user, and contain mailbox folders generated when a mailbox is created for that user. The mailbox database can be hosted on a single server, or can be distributed across multiple
          These files are:
• Mailbox database (.edb file). Is the main repository for mailbox data, it directly accessed
by the Extensible Storage Engine (ESE), has a B-tree structure that helps to provide quick access and
enables users to access data on any page within just one input-output cycle.
• Transaction log (.log file). Record action performed on a database, such as sending or receiving a message, are called transactions.
Operations that are committed to the transaction log are later written to the database itself (in an
.edb file). Until the transaction is committed to the mailbox database, the only existence of this data is
in the RAM memory and in the transaction logs. All transactions, complete or incomplete, are logged
to maintain data integrity in case of a service interruption. Each database has its own set of transaction logs.
• Checkpoint file (.chk). Store data that indicate when a transaction is successfully committed to the database. It help the ESE to replay log files on an inconsistent database in case of database recovery. The ESE will start with the transaction that is present in the log file, but is not yet written to checkpoint file. Each database’s log prefix determines its checkpoint file name.
• Temporary file (Tmp.edb). Used for processing transactions. Tmp.edb contains temporary information that is deleted when all stores in the storage group are dismounted or the Exchange Information Store service is stopped. This file does not exceed 1 MB.
• Reserve log files (E##res0001.jrs - E##res000A.log per database, where ## is the log prefix). Used to reserve space for additional log files if the disk that stores log files becomes full.
Exchange Server 2013 only uses these files as emergency storage when the disk becomes full, and it
cannot write new transactions to disk. When Exchange Server 2013 runs out of disk space, it writes
the current transaction to disk, using up the space reserved by the 10 reserve transaction logs, and
then dismounts the database. The reserved transaction logs ensure minimal loss of data that is in
transit to the database. The reserved transaction logs are always 1 MB each.