Datastore usage suddenly spikes to 100% and the production VMs go inaccessible on VMFS datastore.

Products

• VMware vCenter Server

Issue/Introduction

Symptoms:

• Virtual Machines go inaccessible state due to storage capacity issue.
• Events will be received on vCenter as shown below:
  Message from ##.###.###.##: There is no more space for virtual disk VM_Name.vmdk. You might be able to continue this session by freeing disk space on the relevant volume, and clicking Retry. Click Cancel to terminate this session.

Environment

VMware vSphere ESXi 6.x
VMware vSphere ESXi 7.x
VMware vSphere ESXi 8.x

Cause

VMFS uses Small File Blocks (SFBs) and Large File Blocks (LFBs) internally for space allocation. Both of these resource pools were fully consumed, leaving no available blocks to handle new write requests. This indicates that the space consumption on the datastore was genuine (No issue reported).

Cause validation:

• The /var/run/log/vmkernel.log file reports “No space left on device” errors when Virtual Machine attempted to issue I/O requests confirming that the datastore ran completely out of space:
  2025-09-17T16:30:08.141Z cpu24:8875503)Fil6: 4094: 'Datastore_Name': Fil6 file IO (<FD c41 r104>) : No space left on device
2025-09-17T16:30:08.141Z cpu24:8875503)Fil6: 4060: ioCtx: 0x45c971180f80, world: 2112600, overallStatus: No space left on device, token: 0x45c9403a9900, tokenStatus: Success, txnToken: 0x0, txnTokenStatus: N, totalIOSize: 65536, maxIOLength: 26677370880
2025-09-17T16:30:08.142Z cpu24:8875503)Fil6: 4066: numAddrs: 0, numTbz: 0, numPBsToAlloc: 0, numBlocksToAlloc: 1 numTBZBlocks: 0, numPBsToUnmap: 0, numBlocksToUnmap: 0
2025-09-17T16:30:08.142Z cpu24:8875503)Fil6: 4073: opType: 2, ioFlags: 272, allocFlags: 0, ioStateTxn: 0, ioStateNoTxn: 0, txnState: 3, numResolvedAddrMaps: 0, numTxnPendingAddrMaps: 16
2025-09-17T16:30:08.142Z cpu24:8875503)Fil6: 4078: wakeupOnIOComplete: 0, isIOIssued: 0, isIODone: 0, isRead: 0 isSync: 0, canBlock: 1, lastPartialBlock: 0
2025-09-17T16:30:08.142Z cpu24:8875503)Res3: 1874: 'Datastore_Name': [rt 1] Converted clusters updated (on-disk 30688, in-memory 30496)
2025-09-17T16:30:12.886Z cpu43:8875503)Fil3: 10596: SFB totalNumResources: 15726592 numFreeResources: 19521 numNotAvailable 14336
2025-09-17T16:30:13.048Z cpu43:8875503)Fil3: 10604: LFB totalNumResources: 30716 numFreeResources: 28 numNotAvailable 30688

  Both SFBs and LFBs reported numNotAvailable at high values, proving that allocation failure was due to exhausted datastore capacity.

Resolution

• Expand the datastore.
  or
• Migrate few VMs to other datastore to release the space. 

Additional Information

Storage vMotion and VM power on fails with error "No space left on device" on VMFS6 datastore