Fix SQL Server LocalDB Startup Failures (SQL Server 2025 LocalDB on NVMe)

Fix SQL Server LocalDB Startup Failures (SQL Server 2025 LocalDB on NVMe)

Scope

Use this on Windows machines where MSSQLLocalDB (or other LocalDB instances) fail to start with errors like:

  • SQL Server process failed to start
  • LocalDB event log error 575
  • SQL error log shows misaligned log IOs
  • SQL error log shows missing system DB files under a fake path like F:\dbs\...\mkmastr.nativeproj\...

This runbook fixes two issues we saw together:

  1. NVMe sector-size reporting issue (PhysicalBytesPerSectorForPerformance = 32768) that crashes LocalDB at startup.
  2. Stale/bad LocalDB instance metadata inside old master.mdf (system DB paths pointing to F:\dbs\...).

Important (Read First)

  • Run the registry + fsutil steps in Administrator PowerShell.
  • Recreating a LocalDB instance deletes that instance’s LocalDB system files and any databases stored inside that instance folder.
  • Back up LocalDB data before deleting instances.
  • Reboot is required after the registry change before retesting LocalDB.

Step 0: Optimistic classroom workflow (fast first pass)

When helping many student laptops, try this first before full diagnostics.

0A) Recreate the default instance first (no registry change, no reboot)

sqllocaldb stop MSSQLLocalDB -k
sqllocaldb delete MSSQLLocalDB
sqllocaldb create MSSQLLocalDB -s

If this succeeds, validate and stop here:

sqllocaldb i MSSQLLocalDB

Expected:

  • State: Running

0B) If 0A fails, classify quickly from error.log

Get-Content "$env:LOCALAPPDATA\Microsoft\Microsoft SQL Server Local DB\Instances\MSSQLLocalDB\error.log" -Tail 120

Interpretation:

  • If the log references F:\dbs\..., this is stale instance metadata. Continue with instance recreate steps (Step 6), including custom instances as needed.
  • If the log shows misaligned log IOs or early fatal startup crash, continue with the storage-path fix (Step 1 -> Step 2 -> Step 3 -> Step 4).
  • If unclear, continue with Step 1 for full diagnosis.

Step 1: Confirm the symptom (Admin PowerShell)

sqllocaldb i
sqllocaldb start MSSQLLocalDB

If it fails, inspect sector info:

fsutil fsinfo sectorinfo C:

Typical problematic output:

  • LogicalBytesPerSector : 512
  • PhysicalBytesPerSectorForAtomicity : 4096
  • PhysicalBytesPerSectorForPerformance : 32768

Also confirm the NVMe driver is Microsoft stornvme (the registry fix below targets that driver):

Get-CimInstance Win32_PnPSignedDriver |
  Where-Object { $_.DeviceName -match 'NVMe|NVM Express' -or $_.FriendlyName -match 'NVMe|NVM Express' } |
  Select-Object DeviceName, DriverProviderName, DriverVersion, InfName

Expected controller driver for this fix: stornvme.inf.

Step 2: Apply the NVMe sector-size override (Admin PowerShell)

Run this exactly:

New-ItemProperty `
  -Path "HKLM:\SYSTEM\CurrentControlSet\Services\stornvme\Parameters\Device" `
  -Name "ForcedPhysicalSectorSizeInBytes" `
  -PropertyType MultiString `
  -Value "* 4095" `
  -Force

Verify the value exists:

Get-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Services\stornvme\Parameters\Device' |
  Select-Object ForcedPhysicalSectorSizeInBytes

Expected:

  • ForcedPhysicalSectorSizeInBytes : {* 4095}

Step 3: Reboot (Required)

Reboot the computer now. Do not skip this.

The driver-level registry override does not take effect until after reboot.

Step 4: Re-test LocalDB after reboot

Open PowerShell (Admin is fine) and test:

sqllocaldb start MSSQLLocalDB

If it still fails, inspect the latest LocalDB SQL error log:

Get-Content "$env:LOCALAPPDATA\Microsoft\Microsoft SQL Server Local DB\Instances\MSSQLLocalDB\error.log" -Tail 120

Interpretation:

  • If you still see misaligned log IOs followed by immediate fatal crash, the sector fix did not take effect. Re-check Step 2 and ensure the machine was rebooted.
  • If LocalDB gets much further (for example, SQL Server is now ready for client connections) but then fails trying to open model/msdb under F:\dbs\..., the instance metadata is stale/bad and must be recreated (next steps).

List all instances:

sqllocaldb i

Optional: find non-system database files inside LocalDB instance folders:

Get-ChildItem "$env:LOCALAPPDATA\Microsoft\Microsoft SQL Server Local DB\Instances" -Recurse -File -Include *.mdf,*.ldf |
  Where-Object {
    $_.Name -notin @(
      'master.mdf','mastlog.ldf',
      'model.mdf','modellog.ldf',
      'msdbdata.mdf','msdblog.ldf',
      'tempdb.mdf','templog.ldf'
    )
  } |
  Select-Object FullName, Length, LastWriteTime

Optional: back up the whole LocalDB instance folder tree:

$src = "$env:LOCALAPPDATA\Microsoft\Microsoft SQL Server Local DB\Instances"
$dst = "$env:USERPROFILE\Desktop\LocalDB-Instances-Backup-$(Get-Date -Format yyyyMMdd-HHmmss)"
Copy-Item $src $dst -Recurse -Force

Step 6: Recreate affected LocalDB instances (No reboot required)

Default instance (MSSQLLocalDB)

sqllocaldb stop MSSQLLocalDB -k
sqllocaldb delete MSSQLLocalDB
sqllocaldb create MSSQLLocalDB -s

Expected result:

  • LocalDB instance "MSSQLLocalDB" created ...
  • LocalDB instance "MSSQLLocalDB" started.

Custom instance example (ProjectModels)

sqllocaldb stop ProjectModels -k
sqllocaldb delete ProjectModels
sqllocaldb create ProjectModels
sqllocaldb start ProjectModels

Notes:

  • Use -s on create if you want it started immediately.
  • Repeat for each custom LocalDB instance on the machine.

Step 7: Validate success

sqllocaldb i MSSQLLocalDB
sqllocaldb i ProjectModels

Expected:

  • State: Running for instances you started

Optional SQL-level test (default instance):

sqlcmd -S "(localdb)\MSSQLLocalDB" -Q "select @@version"

If sqlcmd is not installed, the sqllocaldb state check is sufficient for a basic validation.

Step 8: If it still fails after all steps

Check the error log again and classify the failure:

A) Still crashes early with misaligned log IOs

Do this:

  • Re-check ForcedPhysicalSectorSizeInBytes registry value (* 4095)
  • Confirm NVMe controller uses stornvme.inf
  • Confirm the machine was rebooted after Step 2
  • Re-run fsutil fsinfo sectorinfo C:

B) Fails after SQL Server is now ready for client connections and references F:\dbs\...

Do this:

  • Delete/recreate all affected LocalDB instances (Step 6)
  • Do not reuse the old instance master.mdf

C) A brand-new LocalDB instance still fails on the same F:\dbs\... path

This suggests an install-level LocalDB packaging/problem on that machine.

Try:

  1. Install/update SQL Server 2025 LocalDB from a newer CU package.
  2. Recreate the LocalDB instances again.

Step 9: Rollback (only if needed)

If you need to remove the NVMe override later (Admin PowerShell):

Remove-ItemProperty `
  -Path "HKLM:\SYSTEM\CurrentControlSet\Services\stornvme\Parameters\Device" `
  -Name "ForcedPhysicalSectorSizeInBytes"

Then reboot.

Quick field checklist (per computer)

  1. (Fast pass) Recreate MSSQLLocalDB (stop/delete/create -s). If it runs, stop here.
  2. Open Admin PowerShell.
  3. Run fsutil fsinfo sectorinfo C: and note if PhysicalBytesPerSectorForPerformance is 32768.
  4. Apply ForcedPhysicalSectorSizeInBytes = "* 4095" under stornvme.
  5. Reboot.
  6. Test sqllocaldb start MSSQLLocalDB.
  7. If it fails with F:\dbs\... in error.log, delete/recreate the affected LocalDB instances.
  8. Validate sqllocaldb i <instance> shows State: Running.