Symptoms
Consider the following scenario. In Microsoft SQL Server, you create a snapshot backup of many databases at the same time. To do this, you use the Volume Shadow Copy Service (VSS), or you use the Virtual Backup Device Interface (VDI). In this scenario, the snapshot backup operation fails. Additionally, you receive the following error message if you use the VSS to create the snapshot backup:

ERROR: Selected writer ‘Microsoft Writer (Service State)’ is in failed state!
– Status: 8 (VSS_WS_FAILED_AT_PREPARE_SNAPSHOT)
- Writer Failure code: 0×800423f4 (<Unknown error code>)
- Writer ID: {WriterID}
- Instance ID: {InstanceID}You receive one of the following error messages if you use the VDI to create the snapshot backup:
Error message 1

[Microsoft][ODBC SQL Server Driver][SQL Server] Insufficient resources to create UMS scheduler.
Msg 3267, SevLevel 16, State 1, SQLState 42000
Error message 2

[Microsoft][ODBC SQL Server Driver][SQL Server] Could not create worker thread.
Msg 3013, SevLevel 16, State 1, SQLState 42000
Error message 3The following message may be logged in the SQL Server error log:

2008-12-06 02:26:24.90 spid420 SubprocessMgr::EnqueueSubprocess: Limit on ‘Max worker threads’ reachedThe number of databases that you try to back up when this problem occurs varies. The number of databases depends on the following conditions:The configuration of SQL ServerOther activities in SQL Server
Resolution
In SQL Server, the snapshot backup of each database uses five threads in the Sqlservr.exe process. Additionally, other activities may also use threads in the Sqlservr.exe process. Depending on the configuration of SQL Server, the available threads may be used up if you create a snapshot backup of many databases at the same time.