6.1 Guidelines for Using NSS in a Xen Virtualization Environment

Consider the following guidelines when planning to use NSS in a virtualization environment:

6.1.1 Host Server Issues

Running NSS on the Host Server Is Not Supported

NSS pools and volumes are not supported on the Xen host server in a Xen virtualization environment. You can install NSS on the guest servers from inside the guest server environment, just as you would if the guest servers were physical servers.

When you create a virtual machine, you must assign devices to it. If you plan to use the virtualization guest server as a node in a cluster and you need to be able to fail over cluster resources to different physical servers, you must assign SAN-based physical devices to the virtual machine. You create the NSS pools and volumes from within the guest server.

If you install Novell Cluster Services™ in the host server environment, the cluster resources use shared Linux POSIX volumes, and do not use shared NSS pools.

If you install Novell Cluster Services in the guest server environment, the guest server is a node in the cluster. The disk sharing is managed by Novell Cluster Services from within the guest server environment. You can use shared NSS pools as cluster resources that run on the guest server and on other nodes in that cluster.

For information about deployment scenarios using shared NSS pools in clusters in a virtualization environment, see Configuring Novell Cluster Services in a Xen Virtualization Environment in the OES 2 SP2: Novell Cluster Services 1.8.7 for Linux Administration Guide.

Using RAIDs

In a Xen virtualization environment, if you need to use RAIDs for device fault tolerance in a high-availability solution, we recommend that you use standard hardware RAID controllers. Hardware RAIDs provide better performance over using software RAIDs on the virtualization host server or guest server.

To get the best performance from a software RAID, create a RAID device on the Xen host and present that device to the guest VM. Each of the RAID’s segments must be on different physical devices. It is best to present the entire physical RAID device or a physical partition of the RAID device to the guest VM, and to not present just a file-backed virtual device.

NSS is not supported to run in the virtualization host server environment, so NSS software RAIDs cannot be used there. Xen supports using Linux mdadm for software RAIDs on the host server.

If you attempt to create and manage a software RAID on the guest server in a production environment, make sure to present different physical devices to the guest VM that you want to use for the software RAID. Using segments from virtual devices that actually reside on the same physical device on the host server slows performance and provides no protection against failed hardware devices. The maximum number of disks that can be presented to the VM is 16 (xvda to xvdp). Xen provides a mechanism to dynamically add and remove drives from a VM, but that capability is currently not supported in paravirtualized NetWare.

Using NSS software RAIDs in a virtualization guest server environment has not been tested.

Using Multipath Devices

If it is available, use your storage vendor’s multipath I/O management solution for the storage subsystem. In this case, the multiple paths are resolved as a single device that you can assign to a virtual machine.

Do not use multipath management tools in the guest environment.

If a storage device has multiple connection paths between the device and the host server that are not otherwise managed by third-party software, use Linux multipathing to resolve the paths into a single multipath device. When assigning the device to a VM, select the device by its multipath device node name (/dev/mapper/mpathN). The guest server operating system is not aware of the underlying multipath management being done on the host. The device appears to the guest server as any other physical block storage device. For information, see Managing Multipath I/O for Devices in the SLES 10 SP3: Storage Administration Guide.

6.1.2 Virtual Machine Issues

Assigning Physical Disks or Disk Partitions to the Virtual Machine

For the best performance on a Xen guest server, NSS pools and volumes on NetWare should be created on block storage devices that are local SCSI devices, Fibre Channel devices, iSCSI devices, or partitions on those types of devices.

SATA or IDE disks have slower performance because special handling is required when working through the Xen driver to ensure that data writes are committed to the disk in the order intended before it reports back.

Assigning File-Backed Disk Images for Virtual Devices

Novell supports file-backed disk images on virtual machines, but does not recommend using them for important data because the volume can become corrupt after a power failure or other catastrophic failure. For example, file-backed volumes might be useful for training and sales demonstrations.

WARNING:Data corruption can occur if you use Xen file-backed disk images for NSS volumes on the guest server in the event of a power failure or other catastrophic failure.

6.1.3 Guest Server Issues

Unless otherwise indicated, the issues in this section apply to both OES 2 Linux and NetWare, and to NetWare 6.5 SP7.

Initializing Virtual Disks

The primary virtual disk (the first disk you assign to the virtual machine) is automatically recognized when you install the guest operating system. The other virtual devices must be initialized before any space is shown as available for creating a pool. Without initializing the devices, no space is shown as available for pool creation. For information, see Section 6.3, Initializing New Virtual Disks on the Guest Server.

Configuring Write Barrier Behavior for NetWare in a Guest Environment

Write barriers are needed for controlling I/O behavior when writing to SATA and ATA/IDE devices and disk images via the Xen I/O drivers from a guest NetWare server. This is not an issue when NetWare is handling the I/O directly on a physical server.

The XenBlk Barriers parameter for the SET command controls the behavior of XenBlk Disk I/O when NetWare is running in a virtualization environment. The setting appears in the Disk category when you issue the SET command in the NetWare server console.

Valid settings for the XenBlk Barriers parameter are integer values from 0 to 255, with a default value of 16. A non-zero value specifies the depth of the driver queue, and also controls how often a write barrier is inserted into the I/O stream. A value of 0 turns off XenBlk Barriers.

A value of 0 (no barriers) is the best setting to use when the virtual disks assigned to the guest server’s virtual machine are based on physical SCSI, Fibre Channel, or iSCSI disks (or partitions on those physical disk types) on the host server. In this configuration, disk I/O is handled so that data is not exposed to corruption in the event of power failure or host crash, so the XenBlk Barriers are not needed. If the write barriers are set to zero, disk I/O performance is noticeably improved.

Other disk types such as SATA and ATA/IDE can leave disk I/O exposed to corruption in the event of power failure or a host crash, and should use a non-zero setting for the XenBlk Barriers parameter. Non-zero settings should also be used for XenBlk Barriers when writing to Xen LVM-backed disk images and Xen file-backed disk images, regardless of the physical disk type used to store the disk images.

To configure XenBlkBarriers:

  1. In the server console on the guest NetWare server, enter

    SET XenBlkBarriers=value

    For example, to turn off XenBlk Barriers for virtual disks based on physical SCSI, Fibre Channel, and iSCSI disks, enter

    SET XenBlkBarriers=0

NSS Features that Are Not Supported in a Virtualization Environment

Some NSS features are not supported in a Xen guest server environment.

Table 6-1 NSS Feature Support in a Guest Server Environment

NSS Feature

NSS on Linux

NSS on NetWare

Data shredding

Not supported

Not supported

Multipath I/O

Not applicable; not supported on Linux

Not supported

Software RAIDs

Not tested

Not tested