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Friday, 7 September 2012

vSphere 5 FAQ: VMFS-5


This article provides information regarding the new VMFS-5 filesystem in vSphere 5.0, as well as details regarding troubleshooting the upgrade between VMFS-3 and VMFS-5.


VMFS-5 Overview

Why switch to VMFS-5?
  • VMFS-5 has improved scalability and performance.
  • VMFS-5 does not use SCSI-2 Reservations, but uses the ATS VAAI primitives.
  • VMFS-5 uses GPT (GUID Partition Table) rather than MBR, which allows for pass-through RDM files greater than 2TB.
  • Newly created VMFS-5 datastores use a single block size of 1MB.
  • VMFS-5 has support for very small files (<1KB) by storing them in the metadata rather than in the file blocks.
  • VMFS-5 uses sub-blocks of 8K rather than 64K, which reduces the space used by small files.
  • VMFS-5 uses SCSI_READ16 and SCSI_WRITE16 cmds for I/O (VMFS-3 used SCSI_READ10 and SCSI_WRITE10 cmds for I/O).
What are the limitations for VMFS-5?
  • VMFS-5 still limits the number of extents to 32 and the total datastore size to 64TB, but the individual extents are no longer limited to 2TB each. For example, a datastore can be created with a LUN size of 64TB, or a datastore can be created with up to 32 extents up to maximum size of 64TB.
  • Only pass-through RDMs (Raw Device Mapping) can be created with a size >2TB. Non-pass-through RDMs and virtual disk files are still limited to 512B ~ 2TB.
  • Passthrough RDMs are supported up to ~60TB in size.
  • Both upgraded and newly-created VMFS-5 volumes supported the larger Passthrough RDM size.
Non-passthrough RDMs are still limited to 2TB minus 512B.

The actual maximum size of a LUN will depend on the capabilities  of  RAID controller/adapter driver that is used on the vSphere host. 
For vSphere 5.0 storage configuration maximums pertaining to 2TB LUN support please review Configuration Maximums for VMware vSphere 5.0

VMFS-5 Partitioning

How to check my VMFS-5 partition information?

To allow files greater than 2TB (pass through RDMs only - up to ~60TB in size), the partition table type has been changed from MBR to GPT. To view GPT partitioning, use the partedUtil command. For more information on this command for VMFS-5, seeUsing the partedUtil command line utility on ESX and ESXi (1036609).
My upgraded VMFS-5 partition information indicates MBR not GPT. Why?

Only extending the LUN/datastore beyond the 2TB limit automatically changes the partition type from MBR to GPT. If your LUN/datastore has not gone beyond this size, then the partition table stays at MBR.

Upgrading from VMFS-3 to VMFS-5

Can I upgrade while my virtual machines are running?
Yes. Upgrading from VMFS-3 to VMFS-5 can be done on-the-fly (virtual machines do not need to be powered-off, suspended, or migrated).
Do I have to use the command-line to upgrade to VMFS-5? 
The upgrade to from VMFS-3 to VMFS-5 can be done either via the ESXi 5.0 command-line or via vSphere Client.
Note: Ensure that all ESX hosts accessing the LUN are already on ESXi 5.0.
  • To upgrade to VMFS-5 using the vSphere Client, Under Configuration > Storage, highlight the desired VMFS-3 datastore, and click on Upgrade to VMFS-5...
  • To upgrade to VMFS-5 using the ESXi host command-line, use the command:

    # vmkfstools -T /vmfs/volumes/<VMFS3datastore>
My upgraded VMFS-5 does not have a 1MB block size. Why?

Upgraded VMFS-5 partitions will retain the partition characteristics of the original VMFS-3 datastore, including file block-size, sub-block size of 64K, etc. To take full advantage of all the benefits of VMFS-5, migrate the virtual machines to another datastore(s), delete the existing datastore, and re-create it using VMFS-5.

Troubleshooting VMFS-5 Upgrade Issues

The upgrade to VMFS-5 fails with these errors: 
  • There are hosts accessing this datastore which don't support VMFS-5
    To resolve this issue, ensure all ESXi hosts accessing the datastore are already running ESXi 5.0.
  • An error occurred during host configuration. Operation failed, diagnostics report: Unable to Upgrade Filesystem: File system on device /vmfs/devices/disks/<device> cannot be online upgraded now because it is being used by some legacy host.
    To resolve this issue, check if any non-ESXi 5.0 hosts (either standalone, or connected to another vCenter Server) is accessing this datastore, and has virtual machines running. If so, then run one of the:
Note: Once the datastore has been upgraded to VMFS-5, it is no longer visible on legacy ESXi hosts.