Configure storage

This section describes how physical storage hardware maps to virtual machines (VMs), and the software objects used by the management API to perform storage-related tasks. Detailed sections on each of the supported storage types include the following information:

  • Procedures for creating storage for VMs using the CLI, with type-specific device configuration options
  • Generating snapshots for backup purposes
  • Best practices for managing storage
  • Virtual disk QoS (Quality of Service) settings

Storage repositories (SRs)

A Storage Repository (SR) is a particular storage target, in which Virtual Machine (VM) Virtual Disk Images (VDIs) are stored. A VDI is a storage abstraction that represents a virtual hard disk drive (HDD).

SRs are flexible, with built-in support for the following drives:

Locally connected:

  • IDE
  • SATA
  • SCSI
  • SAS

Remotely connected:

  • iSCSI
  • NFS
  • SAS
  • Fibre Channel

The SR and VDI abstractions allow for advanced storage features to be exposed on storage targets that support them. For example, advanced features such as thin provisioning, VDI snapshots, and fast cloning. For storage subsystems that don’t support advanced operations directly, a software stack that implements these features is provided. This software stack is based on Microsoft’s Virtual Hard Disk (VHD) specification.

SR commands provide operations for creating, destroying, resizing, cloning, connecting and discovering the individual VDIs that they contain.

A storage repository is a persistent, on-disk data structure. For SR types that use an underlying block device, the process of creating an SR involves erasing any existing data on the specified storage target. Other storage types such as NFS, create a container on the storage array in parallel to existing SRs.

Each XenServer host can use multiple SRs and different SR types simultaneously. These SRs can be shared between hosts or dedicated to particular hosts. Shared storage is pooled between multiple hosts within a defined resource pool. A shared SR must be network accessible to each host. All servers in a single resource pool must have at least one shared SR in common.

CLI operations to manage storage repositories are described in SR commands.

Virtual disk image (VDI)

A virtual disk image (VDI) is a storage abstraction that represents a virtual hard disk drive (HDD). VDIs are the fundamental unit of virtualized storage in XenServer. VDIs are persistent, on-disk objects that exist independently of XenServer hosts. CLI operations to manage VDIs are described in VDI commands. The on-disk representation of the data differs by SR type. A separate storage plug-in interface for each SR, called the SM API, manages the data.

Physical block devices (PBDs)

Physical block devices represent the interface between a physical server and an attached SR. PBDs are connector objects that allow a given SR to be mapped to a host. PBDs store the device configuration fields that are used to connect to and interact with a given storage target. For example, NFS device configuration includes the IP address of the NFS server and the associated path that the XenServer host mounts. PBD objects manage the run-time attachment of a given SR to a given XenServer host. CLI operations relating to PBDs are described in PBD commands.

Virtual block devices (VBDs)

Virtual Block Devices are connector objects (similar to the PBD described above) that allows mappings between VDIs and VMs. In addition to providing a mechanism for attaching a VDI into a VM, VBDs allow for the fine-tuning of parameters regarding QoS (Quality of Service) and statistics of a given VDI, and whether that VDI can be booted. CLI operations relating to VBDs are described in VBD commands.

Summary of storage objects

The following image is a summary of how the storage objects presented so far are related:

Graphical overview of storage repositories and related objects

Virtual disk data formats

In general, there are the following types of mapping of physical storage to a VDI:

  1. Logical volume-based VHD on a LUN: The default XenServer block-based storage inserts a logical volume manager on a disk. This disk is either a locally attached device (LVM) or a SAN attached LUN over either Fibre Channel, iSCSI, or SAS. VDIs are represented as volumes within the volume manager and stored in VHD format to allow thin provisioning of reference nodes on snapshot and clone.

  2. File-based QCOW2 on a LUN: VM images are stored as thin-provisioned QCOW2 format files on a GFS2 shared-disk filesystem on a LUN attached over either iSCSI software initiator or Hardware HBA.

  3. File-based VHD on a filesystem: VM images are stored as thin-provisioned VHD format files on either a local non-shared filesystem (EXT type SR) or a shared NFS target (NFS type SR).

VDI types

For most SR types, VHD format VDIs are created. You can opt to use raw at the time you create the VDI. This option can only be specified by using the xe CLI. For GFS2 SRs, QCOW2 VDIs are created.

To check if a VDI was created with type=raw, check its sm-config map. The sr-param-list and vdi-param-list xe commands can be used respectively for this purpose.

Create a raw virtual disk by using the xe CLI

  1. Run the following command to create a VDI given the UUID of the SR you want to place the virtual disk in:

    xe vdi-create sr-uuid=sr-uuid type=user virtual-size=virtual-size \
        name-label=VDI name sm-config:type=raw
    
  2. Attach the new virtual disk to a VM. Use the disk tools within the VM to partition and format, or otherwise use the new disk. You can use the vbd-create command to create a VBD to map the virtual disk into your VM.

Convert between VDI formats

It is not possible to do a direct conversion between the raw and VHD formats. Instead, you can create a VDI (either raw, as described above, or VHD) and then copy data into it from an existing volume. Use the xe CLI to ensure that the new VDI has a virtual size at least as large as the VDI you are copying from. You can do this by checking its virtual-size field, for example by using the vdi-param-list command. You can then attach this new VDI to a VM and use your preferred tool within the VM to do a direct block-copy of the data. For example, standard disk management tools in Windows or the dd command in Linux. If the new volume is a VHD volume, use a tool that can avoid writing empty sectors to the disk. This action can ensure that space is used optimally in the underlying storage repository. A file-based copy approach may be more suitable.

VHD-based and QCOW2-based VDIs

VHD and QCOW2 images can be chained, allowing two VDIs to share common data. In cases where a VHD-backed or QCOW2-backed VM is cloned, the resulting VMs share the common on-disk data at the time of cloning. Each VM proceeds to make its own changes in an isolated copy-on-write version of the VDI. This feature allows such VMs to be quickly cloned from templates, facilitating very fast provisioning and deployment of new VMs.

As VMs and their associated VDIs get cloned over time this creates trees of chained VDIs. When one of the VDIs in a chain is deleted, XenServer rationalizes the other VDIs in the chain to remove unnecessary VDIs. This coalescing process runs asynchronously. The amount of disk space reclaimed and time taken to perform the process depends on the size of the VDI and amount of shared data.

Both the VHD and QCOW2 formats support thin provisioning. The image file is automatically extended in fine granular chunks as the VM writes data into the disk. For file-based VHD and GFS2-based QCOW2, this approach has the considerable benefit that VM image files take up only as much space on the physical storage as required. With LVM-based VHD, the underlying logical volume container must be sized to the virtual size of the VDI. However unused space on the underlying copy-on-write instance disk is reclaimed when a snapshot or clone occurs. The difference between the two behaviors can be described in the following way:

  • For LVM-based VHD images, the difference disk nodes within the chain consume only as much data as has been written to disk. However, the leaf nodes (VDI clones) remain fully inflated to the virtual size of the disk. Snapshot leaf nodes (VDI snapshots) remain deflated when not in use and can be attached Read-only to preserve the deflated allocation. Snapshot nodes that are attached Read-Write are fully inflated on attach, and deflated on detach.

  • For file-based VHDs and GFS2-based QCOW2 images, all nodes consume only as much data as has been written. The leaf node files grow to accommodate data as it is actively written. If a 100 GB VDI is allocated for a VM and an OS is installed, the VDI file is physically only the size of the OS data on the disk, plus some minor metadata overhead.

When cloning VMs based on a single VHD or QCOW2 template, each child VM forms a chain where new changes are written to the new VM. Old blocks are directly read from the parent template. If the new VM was converted into a further template and more VMs cloned, then the resulting chain results in degraded performance. XenServer supports a maximum chain length of MAXIMUM_SUPPORTED_VHD_CHAIN_LENGTH. Do not approach this limit without good reason. If in doubt, “copy” the VM using XenCenter or use the vm-copy command, which resets the chain length back to 0.

VHD-specific notes on coalesce

Only one coalescing process is ever active for an SR. This process thread runs on the SR master host.

If you have critical VMs running on the master server of the pool, you can take the following steps to mitigate against occasional slow I/O:

  • Migrate the VM to a host other than the SR master

  • Set the disk I/O priority to a higher level, and adjust the scheduler. For more information, see Virtual disk QoS settings.