how many databases can be backed up by default for a Single SQL server

Bill England 0 Reputation points
2026-10-05T15:25:58.47+00:00

Dealing with an issue where a user has a SQL server hosting 920 databases and is ruuning into errors when attempting to backup. Error being received is as follows:

SQL Server has ~940 user databases. During the Ninja Image Backup, SqlServerWriter.txt shows many VDI/BACKUP DATABASE operations followed by Error message: Cannot create worker thread Then multiple 3013 / 3201/ 3202/ OS 995

Thanks,

Bill E.

SQL Server | Other
SQL Server | Other

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  1. AI answer

    2026-10-05T15:26:12.8133333+00:00

    There is no single default limit that allows backing up all databases on one SQL Server instance at once.

    For the error pattern shown (Cannot create worker thread during many VDI/BACKUP DATABASE operations), the relevant documented limit is for snapshot-style backups of many databases at the same time:

    • SQL Server uses five threads in Sqlservr.exe for the snapshot backup of each database.
    • If many databases are backed up simultaneously, available worker threads can be exhausted.
    • Microsoft recommends creating a snapshot backup of fewer than 35 databases at the same time.
    • For Transact-SQL snapshot backup, the maximum number of databases in that feature is 64. If there are more than 64 databases on the server, SQL Server returns Msg 925 ... The maximum allowed is 64.

    Given a server with about 920–940 databases, the failure is consistent with too many databases being processed concurrently, which can lead to Cannot create worker thread and follow-on backup failures.

    For third-party backup products that use VDI/VSS, check these items:

    1. Verify the SQL login NT SERVICE\SQLWriter exists and is in the sysadmin server role on the instance being backed up.
    2. Verify the SQL Server VSS Writer service is started and its startup account is Local System.
    3. Run VSSADMIN LIST WRITERS from an elevated command prompt and confirm SqlServerWriter is present and in the Stable state.
    4. Reduce the number of databases backed up at the same time.

    If the backup method is Azure VM SQL snapshot backup, the documented concurrent backup limit is 20 databases per VM.

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  2. Erland Sommarskog 137.9K Reputation points MVP Volunteer Moderator
    2026-10-05T21:16:43.26+00:00

    It seems like you are taking backups through a third-party software. You should ask the vendor how many databases at a time they support. If they try to back up all 920/940 at the same time, that may not have a happy ending, if the server is not equipped for the situation.

    • What does "SELECT @@version" report?
    • How much RAM do you have?
    • What is max server memory set to?
    • How many cores do you have in the machine?

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  3. Deepesh Dhake 1,245 Reputation points
    2026-10-07T03:19:10.6466667+00:00

    Cause: A VSS image backup makes SQL Writer start a VDI snapshot backup of every database on the imaged volumes at the same time. With about 940 databases, SQL Server runs out of worker threads, so you get "Cannot create worker thread." This is the documented scenario in Microsoft KB 943471, which recommends snapshotting fewer than 35 databases at a time.

    Fix: Don't let the image backup take application-consistent SQL backups of 940 databases. Back up SQL natively to disk, and have the image capture the .bak files rather than relying on VSS for SQL recovery.

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