Citrix Systems 5.6 manual FlexVols, Aggregates

The easiest way to create NetApp SRs is to use XenCenter. See the XenCenter help for details. See the section called “Creating a shared NetApp SR over iSCSI” for an example of how to create them using the xe CLI.

FlexVols

NetApp uses FlexVols as the basic unit of manageable data. There are limitations that constrain the design of NetApp-based SRs. These are:

•maximum number of FlexVols per filer

•maximum number of LUNs per network port

•maximum number of snapshots per FlexVol

Precise system limits vary per filer type, however as a general guide, a FlexVol may contain up to 200 LUNs, and provides up to 255 snapshots. Because there is a one-to-one mapping of LUNs to VDIs, and because often a VM will have more than one VDI, the resource limitations of a single FlexVol can easily be reached. Also, the act of taking a snapshot includes snapshotting all the LUNs within a FlexVol and the VM clone operation indirectly relies on snapshots in the background as well as the VDI snapshot operation for backup purposes.

There are two constraints to consider when mapping the virtual storage objects of the XenServer host to the physical storage. To maintain space efficiency it makes sense to limit the number of LUNs per FlexVol, yet at the other extreme, to avoid resource limitations a single LUN per FlexVol provides the most flexibility. However, because there is a vendor-imposed limit of 200 or 500 FlexVols, per filer (depending on the NetApp model), this creates a limit of 200 or 500 VDIs per filer and it is therefore important to select a suitable number of FlexVols taking these parameters into account.

Given these resource constraints, the mapping of virtual storage objects to the Ontap storage system has been designed in the following manner. LUNs are distributed evenly across FlexVols, with the expectation of using VM UUIDs to opportunistically group LUNs attached to the same VM into the same FlexVol. This is a reasonable usage model that allows a snapshot of all the VDIs in a VM at one time, maximizing the efficiency of the snapshot operation.

An optional parameter you can set is the number of FlexVols assigned to the SR. You can use between 1 and 32 FlexVols; the default is 8. The trade-off in the number of FlexVols to the SR is that, for a greater number of FlexVols, the snapshot and clone operations become more efficient, because there are fewer VMs backed off the same FlexVol. The disadvantage is that more FlexVol resources are used for a single SR, where there is a typical system-wide limitation of 200 for some smaller filers.

Aggregates

When creating a NetApp driver-based SR, you select an appropriate aggregate. The driver can be probed for non-traditional type aggregates, that is, newer-style aggregates that support FlexVols, and lists all aggregates available and the unused disk space on each.

Note:

Aggregate probing is only possible at sr-createtime so that the aggregate can be specified at the point that the SR is created, but is not probed by the sr-probecommand.

Citrix strongly recommends that you configure an aggregate exclusively for use by XenServer storage, because space guarantees and allocation cannot be correctly managed if other applications are sharing the resource.

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