How to Manage VMFS Datastores in VMware

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Virtual machine file system (VMFS), exclusive to VMware, functions as both a volume manager and a filesystem; it controls block devices associated with a host or hosts and resides on the same pool of storage resources that it manages. VMFS is used to store disc images and the files that make up a virtual machine or template; unlike most other filesystems, VMFS is optimized for use with large files.

Multiple servers can write data to, and read data from, a VMFS volume. ESX and ESXi Server use a file-locking mechanism, called SCSI reservation, to preserve the integrity of data contained on a volume. SCSI reservation occurs when a VMFS operation modifies the metadata of logical unit number (LUN) -- an individual, unique, block-based storage device (the term "LUN" is often used interchangeably with "disk" or "drive"). Examples of VMFS operations include creating a virtual machine or template, turning a VM on, or creating or deleting a file. These reservations are dynamic, rather than static; once an operation is complete, the host will release the lock.

VMFS deposits files in a storage container called a datastore, a separate filesystem that runs on top of a volume. Datastores reside on a block-based device such as iSCSI. Virtual machine disks (VMDKs) are housed in the datastore.

VMFS-5, released with vSphere and ESXi 5.0, can handle volume sizes of up to 64TB and datastores of 2TB - 512B in size. VMFS is limited to 256 volumes per host and a maximum of 64 hosts can be associated with a single volume.

To ensure the best performance, administrators should determine what kind of datastore is most suitable for a particular virtual machine. Datastores running on low-end disks or lacking redundancy, for example, should be associated with low-priority VMs. Continue reading to learn how to configure and manage your datastores in VMware vSphere.

Create a New Datastore

VMFS volumes should be created in vSphere, rather than via the fdisk tool or from the ESX/ESXi installer. The start sectors on volumes formatted in the ESX/ESXi installer or via command line will not be aligned at 128K, negatively impacting disk performance as a result.

  1. In vSphere, select the ESX or ESXi host. Click the Configuration tab and then select "Storage" from the Hardware pane.Select Storage
  2. Click "Add Storage." Select "Disk/LUN" as the storage type and then click "Next."Add Storage
  3. Select the LUN to format. Click "Next." Choose from the following options if the selected disk is not blank:

    • Use All Available Partitions - Erase all data from the LUN and dedicate the entire disk for use with the datastore.
    • Use Free Space- Create a VMFS volume on the remaining space available on the disk.

    VMFS Volume

  4. Create a unique name for the datastore and then click "Next."
  5. Select a block size from the drop-down menu. The block size determines the maximum file size a volume can manage. 1MB corresponds to 256GB, 2MB to 512GB, 4MB to 1024GB, and 8MB to 2048GB. The block size limits the size of the VMDK.VMDK
  6. Check "Maximize Capacity" to use the entire LUN, if applicable. For the best performance possible, it's recommended to have a 1:1 ratio of VMFS volumes to LUNs, although LUNs can hold more than one VMFS volume at a time. Click "Next."
  7. Review your selections and then click "Finish" to create the VMFS datastore.

Mount a Datastore

Datastores in VMware aren't locked to their original host and can be mounted to, or unmounted from, a server. To associate an existing datastore with a new server:

  1. Follow the instructions in steps 1 and 2 in the above section. When prompted to select a disk, check the Label column to find the LUN that holds the existing datastore.
  2. Select the LUN and then choose "Keep Existing Signature" or "Assign a New Signature." Select the former if the server is a copy of a now-defunct host; select the latter if the server is a snapshot of an existing host that is still active.

  3. Follow the remaining prompts to attach the datastore to a new host.

Unmount a Datastore

  1. Go to the Configuration tab after selecting the ESX or ESXi host and then click "Datastores."
  2. Right-click the appropriate datastore and then select "Unmount" from the context menu.
  3. If the datastore is shared, deselect the host to unmount the datastore; select a host to keep the volume mounted. Click "Next."
  4. Click "Finish" to unmount the datastore from the target host or hosts.

Increase the Size of the Datastore

Administrators can increase the size of a VMFS datastore using an extent, which represents a partition on a LUN. VMFS volumes can accommodate multiple extents -- up to 32 at a time if each extent is the maximum 2TB in size. You can also grow an existing extent so long as it has available free space after it.

  1. Select the target host and then click the Configuration tab. Choose "Storage" from the Hardware pane.
  2. Right-click the appropriate datastore and then choose "Properties" from the context menu.
  3. Click "Increase." Select a device from the list. To add a new extent to the datastore, select an unexpandable device; to increase the size of an existing extent, select an expandable device. Click "Next."

  4. Select one of the following options:

    • Use Free Space to Add New Extent - Allocate available space on the disk to the new extent.
    • Use Free Space to Expand Existing Extent - Add additional storage space to an existing extent.
    • Use Free Space- Use all remaining, available space on the disk to deploy a new extent.
    • Use All Available Partitions - Reformat the disk, removing any existing datastores, and dedicate all available space on the disk to create a new extent. The disk must not be blank.

  5. Check "Maximize Capacity" to use all available space; otherwise, enter the desired size, in GB, into the available field. Click "Next."
  6. Click "Finish" to increase the size of the datastore.

As stated above, it's important to have a firm understanding of the requirements of your environment before associating a virtual machine with a datastore. You should have a good idea, for example, of the block size to choose when first setting up your VMFS volume; selecting too small a block size affects how large your VMDKs can be, which could have deleterious effects in the long run. Make your choices wisely when configuring your datastore.

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Petra Jorgenson

Petra Jorgenson is a professional writer with over six years of IT experience. She specializes in computer architecture, operating systems, networking, virtualization and web design. She has written support documentation for a leading BSS/OSS system and has over 100 published articles addressing a multitude of technology-related topics. Jorgenson is working on obtaining her MCITP certification.