Storage Domains are a key feature in Red Hat Virtualization (RHV), but they’re a little bit tricky. In a Storage Domain, you group disks and add them to a storage pool. You can then create virtual machines on the storage pool, and those virtual machines can access the disks in the storage pool as if they were physical disks. However, once you create a storage domain, you can never create another one. That’s because once a storage domain is created you can no longer add new disks to it.
Red Hat Virtualization (RHV) is an open source solution for computer virtualization that enables users to create guest operating systems (VMs) while retaining the benefits of virtualization like virtual hardware resources, virtual storage, and virtual networks. RHV allows users to create virtual storage domains within a RHV server. These domains are based on dedicated storage and share all access to data such as files, directories, and virtual disk images with other domains.
Storage domains and storage pool management
Storage Pool Manager (SPM) is the data center node responsible for making changes to storage domains at the request of Red Hat Virtualization Manager (RHVM). All nodes can make changes to the data in the storage domains, but only SPM can make changes to the configuration of the storage domains. Auto-dial selects the SPM host. If the current SPM node fails, it is replaced by another node that is operational as SPM.
SPM must be run to add storage domains. Therefore, administrators should register a host (hypervisor) before setting up a data center from scratch. Once the host is part of the data center, you can start configuring the data center’s storage domains.
In the NFS data domain, SPM creates the virtual machine’s disk as a file in the file system, either as a QCOW2 file for thin provisioning (sparse) or as a regular file for pre-allocation (RAW).
In an iSCSI or Fibre Channel data domain, SPM creates a volume group (VG) on the logical unit numbers (LUNs) provided for the storage domain and creates the virtual machine’s disk as a logical volume (LV) in that volume group. For a virtual disk with a pre-allocated size, SPM creates a logical volume with the specified size (in GB). For a virtual disk with a thin provisioning format, SPM first creates a 512 MB logical volume. The host on which the virtual machine is running continuously monitors the logical volume. When the host determines that it needs more space, it notifies SPM and SPM adds an additional 512 MB to the logical volume.
In terms of performance, a virtual disk with a pre-provisioned format (RAW) is significantly faster than a virtual disk with a thin-provisioned format (QCOW2). The best practice is to use a thin-provisioning format for virtual desktops that do not require intensive I/O and a pre-provisioning (RAW) format for virtual servers.
Configuring an iSCSI-based storage domain
Red Hat Virtualization supports the use of iSCSI storage to create a storage domain. The iSCSI storage domain is configured to provide storage through a specific iSCSI target. Hosts are iSCSI initiators that are part of the iSCSI target specified by the data domain. Each iSCSI LUN can only be used by one storage domain at a time.
The following procedure describes how to configure the iSCSI LUN as a data domain in the data center through the administration portal when you are logged on as the superuser admin.
1. In the System section, click the Storage tab and then click New.
In the New Domain window, use the Data Center menu to select the data center for the iSCSI storage domain. On the Area Function menu, choose the area type Data as Storage. Choose iSCSI from the Storage Type menu to create a storage domain based on iSCSI. Select an SPM host from the Host to Use menu. If SPM is not available, select one of the available hypervisor hosts. Enter a name for the iSCSI-based storage domain in the Name field.
2. In the Discover Targets section of the window, enter the address and port of the iSCSI target. Then click on Discover to discover the iSCSI target.
3. Enter the iSCSI target using the arrow key for that target.
4. Click the plus button next to the iSCSI target name to display the unused LUNs available for that iSCSI target.
5. Select the LUN by checking the corresponding box. Click OK to create a new iSCSI-based data domain with this LUN.
6. On the Storage tab, the new iSCSI-based storage domain is active.
Connecting an ISO public domain to a data centre
Unlike data domains, ISO domains can be used by multiple data centers simultaneously. This can be useful to limit the storage requirements of the installation media used to create virtual machines.
The following procedure describes how to connect an existing ISO storage domain to the new data center using the administrator portal.
1. In the System section, click the Memory tab. Click the name of the ISO storage domain and click the Data Center tab to list the data centers where the ISO storage domain is available.
2. On the Data Center tab, click Attach. The Connect to Data Center window appears. Select the data center to which you want to assign the ISO domain.
Click OK to connect the ISO domain to this data center.
3. On the Data Center tab, verify that the new data center is listed.
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