About XenServer SRs

A XenServer storage repository (SR) is a storage container on which virtual disks are stored. Both storage repositories and virtual disks are persistent, on-disk objects that exist independently of XenServer. SRs can be shared between servers in a resource pool and can exist on different types of physical storage device, both internal and external, including local disk devices and shared network storage. A number of different types of storage are available when you create a new storage repository using the New Storage Repository wizard and, depending on the type of storage selected, a number of advanced storage features can be configured in XenCenter, including:

• Dynamic multipathing. On Fibre Channel and iSCSI SRs, you can configure storage multipathing using round robin mode load balancing. See Storage Multipathing for more information.
• Thin provisioning. On NetApp and Dell EqualLogic SRs, you can choose the type of space management used. By default, allocated space is thickly provisioned and all virtual allocation guarantees are fully enforced on the filer, guaranteeing that virtual disks will never run out of space and consequently experience failed writes to disk. Thin provisioning allows the disks to be better utilized, as physical capacity is allocated only as a VM needs it - when it writes - allowing for over provisioning of the available storage and maximum utilization of your storage assets.
• Reclaiming Freed Space. On a thinly provisioned block-based SRs, you can free up unused space (for example, deleted VDIs in a LUN). The reclaimed space can then be reused by the storage repository. For more information, see Reclaiming Freed Space.
• Live LUN Expansion. Live LUN Expansion enables you to increase the size of the LUN without any VM downtime. For more information, see Live LUN Expansion.

When you configure a server or pool, you nominate a default SR which is used to store crash dump data and images of suspended VMs, and which will be the default SR used for new virtual disks. At pool level, the default SR must be a shared SR. Any new virtual disks, crash dump files or suspended VM images created within the resource pool are stored in the pool’s default SR, providing a mechanism to recover from physical server failure. For standalone servers, the default SR can be local or shared. When you add shared storage to a standalone server, the shared storage will automatically become the default SR for that server.

Note that it is possible to use different SRs for VMs, crash dump data and suspended VM using the XenServer xe command line interface (CLI). See the XenServer Administrator’s Guide for more information.