Citrix Systems 5.6 manual Thick or thin provisioning, FAS Deduplication, Access Control, Licenses

Thick or thin provisioning

When creating NetApp storage, you can also choose the type of space management used. By default, allocated space is thickly provisioned to ensure that VMs never run out of disk space and that all virtual allocation guarantees are fully enforced on the filer. Selecting thick provisioning ensures that whenever a VDI (LUN) is allocated on the filer, sufficient space is reserved to guarantee that it will never run out of space and consequently experience failed writes to disk. Due to the nature of the Ontap FlexVol space provisioning algorithms the best practice guidelines for the filer require that at least twice the LUN space is reserved to account for background snapshot data collection and to ensure that writes to disk are never blocked. In addition to the double disk space guarantee, Ontap also requires some additional space reservation for management of unique blocks across snapshots. The guideline on this amount is 20% above the reserved space. The space guarantees afforded by thick provisioning will reserve up to 2.4 times the requested virtual disk space.

The alternative allocation strategy is thin provisioning, which allows the administrator to present more storage space to the VMs connecting to the SR than is actually available on the SR. There are no space guarantees, and allocation of a LUN does not claim any data blocks in the FlexVol until the VM writes data. This might be appropriate for development and test environments where you might find it convenient to over-provision virtual disk space on the SR in the anticipation that VMs might be created and destroyed frequently without ever utilizing the full virtual allocated disk.

Warning:

If you are using thin provisioning in production environments, take appropriate measures to ensure that you never run out of storage space. VMs attached to storage that is full will fail to write to disk, and in some cases may fail to read from disk, possibly rendering the VM unusable.

FAS Deduplication

FAS Deduplication is a NetApp technology for reclaiming redundant disk space. Newly-stored data objects are divided into small blocks, each block containing a digital signature, which is compared to all other signatures in the data volume. If an exact block match exists, the duplicate block is discarded and the disk space reclaimed. FAS Deduplication can be enabled on thin provisioned NetApp-based SRs and operates according to the default filer FAS Deduplication parameters, typically every 24 hours. It must be enabled at the point the SR is created and any custom FAS Deduplication configuration must be managed directly on the filer.

Access Control

Because FlexVol operations such as volume creation and volume snapshotting require administrator privileges on the filer itself, Citrix recommends that the XenServer host is provided with suitable administrator username and password credentials at configuration time. In situations where the XenServer host does not have full administrator rights to the filer, the filer administrator could perform an out-of-band preparation and provisioning of the filer and then introduce the SR to the XenServer host using XenCenter or the sr- introduce xe CLI command. Note, however, that operations such as VM cloning or snapshot generation will fail in this situation due to insufficient access privileges.

Licenses

You need to have an iSCSI license on the NetApp filer to use this storage repository type; for the generic plugins you need either an iSCSI or NFS license depending on the SR type being used.

Further information

For more information about NetApp technology, see the following links:

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