Exchange Server 2013 AutoReseed or Automatic Reseed

What is AutoReseed?

Automatic Reseed, or AutoReseed, is a feature that's the replacement for what normally an administrator do for a disk failure, database corruption event, or other issue that necessitates a reseed of a database copy. AutoReseed is designed to automatically restore database redundancy after a disk failure by using spare disks that have been provisioned on the system.

Overview

1. It requires some standard disk/storage structure which involves pre-mapping a set of volumes (including spare) and databases using mountpoints.
2. MS EXchange Replication Service periodically scans the database copies that have a status of FailedAndSuspended.If all database copies on a volume configured for AutoReseed are in a FailedandSuspended state for 15 consecutive minutes, the AutoReseed workflow is initiated.
3. AutoReseed will try to resume the failed and suspended copies up to three times, with a 5-minute sleep in between each attempt.
4. Sometimes, after a FailedandSuspended database copy is resumed, the copy remains in a Failed state.AutoReseed will automatically suspend a database copy that has been Failed for 10 consecutive minutes to keep the workflow running. If the suspend and resume actions don’t result in a healthy database copy, the workflow continues.
5. When it finds a copy with that status, it performs some prerequisite checks. For example, it will verify that a spare disk is available, that the database and its log files are configured on the same volume, and in the appropriate locations that match the required naming conventions.
4. If the prerequisite checks pass successfully, the Disk Reclaimer function within the Microsoft Exchange Replication service allocates, remaps and formats a spare disk according to the timelines in the table below. AutoReseed will attempt to assign a spare volume up to 5 times, with 1-hour sleeps in between each try.
5. Once a spare has been assigned, AutoReseed will perform an InPlaceSeed operation using the SafeDeleteExistingFiles seeding switch. All databases that were on the affected disk are re-seeded using the active copy of the database as the seeding source.
6. Once all retries are exhausted, the workflow stops. If, after 3 days, the database copy is still FailedandSuspended, the workflow state is reset and it starts again from Step 1. This reset/resume behavior is useful (and intentional) since it can take a few days to replace a failed disk, controller, etc.

AutoReseed is configured using three properties of the DAG.

AutoDagVolumesRootFolderPath - refers to the mount point that contains all of the available volumes includes spare volumes
AutoDagDatabasesRootFolderPath - refers to the mount point that contains the databases
AutoDagDatabaseCopiesPerVolume - used to configure the number of database copies per volume.

In this example, there are three volumes, two of which will contain databases (VOL1 and VOL2), and one of which is a blank, formatted spare (VOL3).

1. All three volumes are mounted under a single mount point (C:\ExchVols). This represents the directory used to get storage for Exchange databases.
2. The root directory of the mailbox databases is mounted as another mount point (C:\ExchDBs).
3. A directory structure is created so that a parent directory is created for the database (MDB1 and MDB2), and under the parent directory, two subdirectories are created: one database file and one for the log files (MDB1 DB and MDB1 Logs).

Disk Reclaimer

The Disk Reclaimer component automatically formats spare disks in preparation for automatic reseeding at different intervals, depending on the state of the disk. In order for the Disk Reclaimer to format a disk, certain conditions must met:

• The Disk Reclaimer must be enabled. It is enabled by default, but it can be disabled using Set-DatabaseAvailabilityGroup

• The volume must have a mount point in the root volumes path (by default, C:\ExchangeVolumes).

• The volume must not have any mount points in the database volumes path (by default, C:\ExchangeDatabases).

• If the volume contains any files, none of the files must have been touched for 24 hours.