Windows Storage Spaces is incorrectly giving "Low capacity" warning after windows update

Question

I have a simple pool of two 1TB drives (HDD) setup in a two way mirror (only one disk created). Yesterday it was showing that 1 TB of the 1.81 TB pool capacity was used. Which sound correct because I had around 500 GB of data on that drive.

This morning (after a nightly windows update (cumulative 2020-05) I got a "low capacity" error. It even showed that 1.81 TB of 1.81 TB was used and all physical drives were at 99.9%

I checked the properties of the drive and it was showing the correct numbers:

Also I selected all the files and double checked the total amount used on disk:

The strangest part is that when I copied over a 50 gb test file to that drive, the numbers in control panel changed again. Strangely it dropped down to 1.7 TB, which is still wrong.

BTW the service/system is working fine. I have a second pool which is giving me the correct information.

I've already tried:

• Cleaned recycle bin but it did not make any difference.
• A Chkdsk gave no errors and did also not resolve anything
• "Optimize drive usage" in control panel had no results

Answer 1

After a long search I found a solution myself by running the following PowerShell command (administrative privileges)

Optimize-Volume -DriveLetter S -Verbose

Replace S with your drive letter.

I've got the following result back:

VERBOSE: Invoking slab consolidation on RISA (S:)...
VERBOSE: Slab Analysis:  0% complete...
VERBOSE: Slab Analysis:  100% complete...
VERBOSE: Slab Analysis:  100% complete.
VERBOSE: Retrim:  0% complete...
VERBOSE: Slab consolidation was skipped because there were few evictable slabs.
VERBOSE: Retrim:  100% complete.
VERBOSE: Performing pass 1:
VERBOSE: Retrim:  3% complete...
VERBOSE: Retrim:  4% complete...
VERBOSE: Retrim:  5% complete...
VERBOSE: Retrim:  100% complete.
VERBOSE: Post Defragmentation Report:
VERBOSE: Volume Information:
VERBOSE:   Volume size                 = 15,99 TB
VERBOSE:   Cluster size                = 4 KB
VERBOSE:   Used space                  = 581,13 GB
VERBOSE:   Free space                  = 15,43 TB
VERBOSE: Allocation Units:
VERBOSE:   Slab count                  = 65534
VERBOSE:   Slab size                   = 256 MB
VERBOSE:   Slab alignment              = 127,00 MB
VERBOSE:   In-use slabs                = 2326
VERBOSE: Slab Consolidation:
VERBOSE:   Space efficiency            = 100%
VERBOSE:   Potential purgable slabs    = 1
VERBOSE:   Slabs pinned unmovable      = 1
VERBOSE:   Successfully purged slabs   = 0
VERBOSE:   Recovered space             = 0 bytes
VERBOSE: Retrim:
VERBOSE:   Backed allocations          = 3324
VERBOSE:   Allocations trimmed         = 998
VERBOSE:   Total space trimmed         = 249,50 GB

It's clear that the retrim action reclaimed all of my lost pool space. I always thought that "Trim" was something specific for SSD's but in this case it has probably other meanings too. According to microsoft Docs

-ReTrim
Generates TRIM and Unmap hints for all currently unused sectors of the volume, 
notifying the underlying storage that the sectors are no longer needed and can be purged.
This can recover unused capacity on thinly provisioned drives.

Because I'm using only two 1 TB HDD's for a 16 TB volume the last part is problay significant here: "This can recover unused capacity on thinly provisioned drives."

I'm going to dive deeper into what "retrim" really does, because it's still somewhat of a mystery why this problem occurred after a Windows update

Answer 2

Can you do the following:

1. Check if the same error occurs when logged in through a different user account.
2. Can you give us a chkdsk log since we don't know what chkdsk did. Were there errors?

The cause of this error is likely to be a System Corruption. Press Windows Key + X and select Command Prompt (Admin) (it might be replaced by powershell and if so, type cmd in the search and run it as an administrator) Then, type the following command:

sfc /scannow

If that command didn't work, then try these:

DISM.exe /online /cleanup-image /scanhealth
DISM.exe /online /cleanup-image /restorehealth

If the error is still occurring, do the following:

NOTE: This step is the one you should be careful around cause editing the registry can cause more problems so follow this step carefully. Creating a backup is advised.

Press Windows Key + R and type regedit Navigate to HKEY_CURRENT_USER / Software / Microsoft / Windows / CurrentVersion / Policies / Explorer. If the key NoLowDiskSpaceChecks doesn't exist, then right-click to create a new DWORD value with that name. Then double click on the created value and type 1. Press OK and exit REGEDIT.

Good luck!