Identical calls to Public Folders tend to be slower than the same identical calls to mailboxes.
This is often a result of the following factors:
1. Size:
Public folders are generally much larger than individual mailboxes.
- Average range for a mailbox is 2 – 10GB.
- Average range for a Public Folder is 10GB – 100GB.
This means that the migration will run for a longer period of time, and that there is more possibility of errors during migration, for the following reasons:
- Simple connection problems
- Maintenance issues
- Other intermittent, infrequent problems
The intermittent problems cause MigrationWiz to focus only on the first part of the data (e.g., the first 25GB) because every time MigrationWiz starts a (Full/Delta Pass) migration, it has to check if any of the items have changed or have not been previously migrated.
This problem can be overcome by adding two project Advanced Options, which will prevent the Full/Delta Pass code from executing on folders that have completed in a previous migration.
These need to be added in the Support section, under Support Options.
The Advanced Options to add are:
-
MaintainWatermarkCompletionState=1
-
SkipImportWhenPublicFolderExists=1
2. Number of folders retrieved:
There are differences in the structure of Public Folder databases and mailboxes. This results in a lower number of Public Folders being retrieved at a time. There are limitations on Public Folder EWS calls that do not exist for normal mailboxes.
- When attempting to retrieve the folder structure for a mailbox, you can retrieve all folders at once across the hierarchy. EWS calls are designed this way because mailboxes generally only have hundreds of folders, which is manageable for such calls.
- When retrieving the folder structure for Public Folders, you can only retrieve one level at a time. This is especially troublesome when a Public Folder has thousands (or tens of thousands) of folders, which is not uncommon.
3. Permissions:
The number of permissions being migrated with Public Folders are normally much higher than with mailbox migrations. Permissions have to be retrieved for each folder on both creation and update, unless the Advanced Option has been set to only retrieve them for creation.
- For mailbox migration, we only retrieve and migrate permissions for the well-known folders (Inbox, Sent Items, Deleted Items, etc.). Custom folders do not have permissions migrated.
- For Public Folders, each folder has custom permissions that have to be migrated. This ensures that only trusted users have access to the given folder.
- A separate call, in addition to the call for the initial retrieval of the folders, has to be made to retrieve permissions (i.e., it cannot include permission in the initial list of properties to retrieve). This means that not only do we need to make a call to the Exchange server to retrieve all the folders at the current point in the hierarchy, but we have to make a separate call *per folder* to Exchange to retrieve that folder's current permissions. Due to the large number of folders, this can significantly increase the initial folder retrieval and migration. However, this can be skipped by setting the Advanced Option SkipImportFolderWhenPublicFolderExists=1.
Notes:
- If a large number of folders are mail-enabled, this can add time to the migration. The good news here is that this is only ever done on the first pass when a folder is created.
- There have been reports that Microsoft throttles Public Folder EWS calls more than EWS calls that go against mailboxes.
- Connections to Public Folders do not seem to be as stable as connections to mailboxes.