Understanding VMFS Datastores

I have gone through vSphere 5.5 Storage Documentation and find some useful information, so i thought to share with you some key points.

Datastores are logical containers and datastores can also be used for storing ISO images, virtual machine templates, and floppy images.

Different VMFS versions and their compatibilities with ESX/ESXi hosts :

VMFS ESX/ESXi 3.x host ESX/ESXi 4.x host ESXi 5.x host

  • RW- Read and Write Support
  • RO- Read Only Support
  • N- No access, ESXi 5.x does not support VMFS2. If your datastore was formatted with             VMFS2 , First upgrade the datastore to VMFS3

How VMFS5 Differs from VMFS3:

  • Greater than 2TB storage devices for each VMFS extent.
  • Support of virtual machines with large capacity virtual disks, or disks greater than 2TB. Because VMFS3 datastores do not support large capacity virtual disks, you cannot move virtual disks greater than 2TB from a VMFS5 datastore to a VMFS3 datastore.
  • Increased resource limits such as file descriptors.
  • Standard 1MB file system blocks size with support of 2TB virtual disks.
  • Greater than 2TB disk size for RDMs.
  • Support of small files of 1KB.
  • Any file located on a VMFS5 datastore, new or upgraded from VMFS3, can be opened in a shared mode by a maximum of 32 hosts. VMFS3 continues to support 8 hosts or fewer for file sharing. This affects VMware products that use linked clones, such as View Manager.
  • If you upgrade vmfs3 to vmfs5 Block size remain same of VMFS3.

VMFS Datastores and Storage Disk Formats

VMFS datastore use either the master boot record (MBR) format or the GUID partition table (GPT) format.

With ESXi 5.x , If you create a new datastore , the device is formatted with GPT. This is the reason you can create datastore larger than 2TB and upto 64TB for a single extent

VMFS3 uses MBR format for their storage devices, consider following items when working with VMFS3:

  • When you upgrade VMFS3 datastore to VMFS5 datastore, datastore uses MBR format Conversion to GPT happen only after you expands the datastore to a size larger than 2TB.
  • only after you expand the datastore to a size larger than 2TB.

VMFS Datastores as Repositories

ESXi can format SCSI-based storage devices as VMFS datastores. VMFS datastores primarily serve as

Repositories for virtual machines. In ESXi 5.5 with VMFS5, you can have up to 256 VMFS datastores per host, with the maximum size of 64TB. The required minimum size for a VMFS datastore is 1.3GB, however, the recommended minimum size is 2GB.

  • When you run multiple virtual machines, VMFS provides specific locking mechanisms for virtual machine files, so that virtual machines can operate safely in a SAN environment where multiple ESXi hosts share the same VMFS datastore.
  • As a cluster file system, VMFS lets multiple ESXi hosts access the same VMFS datastore concurrently. You can connect up to 128 hosts to a single VMFS datastore.

VMFS Metadata Updates

A VMFS datastore store virtual machine files, directories, symbolic links, RDM descriptor files, and so on.

The datastore also maintains a consistent view of all the mapping information for these objects. This

Mapping information is called metadata. Metadata is updated each time you perform datastore or virtual machine management operations.

For Example :

  • Creating, growing, or locking a virtual machine file
  • Changing a file’s attributes
  • Powering a virtual machine on or off
  • Creating or deleting a VMFS datastore
  • Expanding a VMFS datastore
  • Creating a template
  • Deploying a virtual machine from a template
  • Migrating a virtual machine with vMotion
  • When metadata changes are made in a shared storage environment, VMFS uses special locking mechanisms to protect its data and prevent multiple hosts from concurrently writing to the metadata.

VMFS Locking Mechanisms

In shared storage environment, when multiple hosts access the same VMFS datastore, specific locking mechanisms are used. These locking mechanism prevent multiple hosts from concurrently writing to the metadata and ensure that no data corruption occurs.

VMFS supports SCSI reservations and atomic test and set (ATS) locking.

 SCSI Reservations: VMFS uses SCSI Reservation on storage devices that do not support hardware acceleration SCSI reservations lock an entire storage device while an operation that requires metadata protection is performed. After the operation completes, VMFS releases the reservation and other operations can continue. Because this lock is exclusive, excessive SCSI reservations by a host can cause performance degradation on other hosts that are accessing the same VMFS.

Atomic Test and Set: For storage devices that support hardware acceleration, VMFS uses the ATS algorithm, also called hardware assisted locking. In contrast with SCSI reservations, ATS supports discrete locking per disk sector. For information about hardware acceleration, see Chapter 24, “Storage Hardware Acceleration,” on page 231 in VMware Storage Documentation 5.5.

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