SQL Server Error 9001

Error number

9001

Severity

21 (fatal: the connection ends; may indicate a serious problem)

State

Varies: 3, 4, 5 and 16 appear in Microsoft's examples; their meanings are not documented

Product

Microsoft SQL Server Database Engine

Versions

All supported SQL Server versions (2016 to 2025), Azure SQL Managed Instance

Category

Database State & Recovery

Last reviewed

24 October 2026

What it means

Error 9001 means SQL Server lost the ability to write to or read from a database's transaction log, so no transaction can continue in that database until the underlying problem is fixed and the database restarts.

Every change in a database must be written to its transaction log first. When the log becomes unusable, because its disk failed, the file is damaged, a TDE key cannot be reached or the log filled during a rollback, SQL Server takes the log offline and raises 9001 at severity 21. Severity 20 and above ends the current connection, so applications see their sessions dropped. SQL Server then shuts the database down and tries to restart it once all connections are gone (error 3422); for tempdb or a system database it may have to restart the whole service (error 3449).

9001 is the end result, not the cause. Microsoft's guidance is to resolve the errors logged before it first. Look in the SQL Server error log for 9002 (log full), 823 or 17053 (operating system I/O errors), 17204 (file could not be opened), 3313 or 3314 (redo or undo failed) or a decryption error, and in the Windows System event log for disk or controller warnings.

Common causes

• The storage holding the log file failed or became unavailable (Microsoft's examples show OS error 21, "The device is not ready", and 1117, an I/O device error).
• The log file is physically damaged, so SQL Server cannot read or write it.
• Transparent Data Encryption could not decrypt or encrypt the log: the key vault or EKM provider was unreachable or failed.
• The log filled up (9002) during a large transaction or rollback, for example on a full disk or a log with a size limit.

Diagnose it

Read-only These queries only read metadata and dynamic management views. They do not change data, permissions or configuration, so they are safe to run first.

EXEC sp_readerrorlog 0, 1, N'9001';
EXEC sp_readerrorlog 0, 1, N'operating system error';
EXEC sp_readerrorlog 0, 1, N'AppDb';

SELECT name, state_desc, user_access_desc, log_reuse_wait_desc
FROM sys.databases
WHERE name = N'AppDb';

SELECT mf.name AS logical_name, mf.physical_name, mf.size * 8 / 1024 AS size_mb,
       vs.volume_mount_point, vs.available_bytes / 1048576 AS free_mb
FROM sys.master_files AS mf
CROSS APPLY sys.dm_os_volume_stats(mf.database_id, mf.file_id) AS vs
WHERE mf.database_id = DB_ID(N'AppDb') AND mf.type_desc = N'LOG';

How to fix it

Each fix is labelled with what it changes. Try read-only checks and the least invasive fix first, and test changes outside production.

ALTER DATABASE AppDb SET OFFLINE WITH ROLLBACK IMMEDIATE;
ALTER DATABASE AppDb SET ONLINE;
DBCC CHECKDB (N'AppDb') WITH NO_INFOMSGS, ALL_ERRORMSGS;

RESTORE DATABASE AppDb FROM DISK = N'D:\Backups\AppDb_full.bak' WITH NORECOVERY, REPLACE;
RESTORE LOG AppDb FROM DISK = N'D:\Backups\AppDb_log_1.trn' WITH RECOVERY;

Examples

The error log when a log volume fails

Shown as documented by Microsoft; not reproduced here because it needs a log file that becomes unavailable. The sequence shows the operating system error first and 9001 as its result.

-- No statement: logged while the database is in use
EXEC sp_readerrorlog 0, 1, N'9001';

Error: 823, Severity: 24, State: 2.
The operating system returned error 21(The device is not ready.) to SQL Server during a read at offset 0x000009afde6000 in file 'G:\Data\Files\ContosoDb_4.ldf'. Additional messages in the SQL Server error log and system event log may provide more detail. This is a severe system-level error condition that threatens database integrity and must be corrected immediately. Complete a full database consistency check (DBCC CHECKDB). This error can be caused by many factors; for more information, see SQL Server Books Online.
Error: 9001, Severity: 21, State: 3.
The log for database 'ContosoDb' is not available. Check the event log for related error messages. Resolve any errors and restart the database.

How to prevent it

Put transaction logs on reliable, monitored storage, keep drivers and firmware current, and alert on 823, 17053, 9002 and 9001 in the error log so the first sign of trouble is noticed.

Size logs with headroom and keep log backups running so a long transaction cannot fill the log during rollback. With TDE and an external key store, monitor access to the key vault or EKM provider.

Common questions

Learn more on CodeWithSQL

• Working with large databases

Official documentation

• MSSQLSERVER_9001 (Microsoft Learn)
• The transaction log
• Database states

Last technically reviewed 24 October 2026; examples and diagnostics run on SQL Server 2025 (17.0.1145.4). Message text checked against the engine's sys.messages catalogue.