Using SVOS RF Data Reduction (ADR)

Only the HNAS Unified VSP F/G/Nx00 support FMD-based compression statistics. When using HNAS gateway models with any system that supports FMD compression, the statistics are not provided to the server.

The Hitachi NAS Platform currently offers hardware accelerated, fixed block de-duplication as a base feature of the NAS Operating System. This is a commonly deployed feature of the NAS OS that has been adopted by many customers. HNAS does not currently offer data compression. As such some customers, have expressed an interest in leveraging the native compression capability offered in SVOS RF ADR in conjunction with HNAS de-duplication to extend capacity savings and lower storage costs.

Customers commonly deploy HNAS with Thin Provisioned HDP Pools. This has been a “best practice” since the introduction of HNAS HDP Integration Support in HNAS OS Server release 12.1. HNAS customers typically deploy NAS spans or pools with HDP Pools that are overprovisioned at a factor of 200-300%. This mechanism allows customers to easily scale NAS capacity incrementally with zero disruption and automatic load-levelling. HNAS is able to obtain available physical free space from the storage system using SCSI based commands, such that the NAS will never allow capacity to be provisioned beyond the currently apportioned capacity to the associated HDP Pool, thereby safeguarding the file systems from unexpectedly running out of capacity.

Contrary to Hitachi Accelerated Flash module drives (FMDs) physical capacity statistics reporting with Hitachi VSP Unified or VSP N, SVOS RF-based data reduction does not currently report in-band the data reduction levels or physical pool utilization level of the host HDP Pool. As such a HNAS system attached to an HDP Pool that is leveraging SVOS RF ADR cannot accurately measure the physical free space available to the HNAS file systems.

Since the HNAS in unable to measure actual physical free space when ADR is deployed, it cannot prevent a catastrophic capacity exhaustion event – an attempt made by the system to write to a mounted file system that fails due to lack of available free space (that was previously provided by HDP Integration).

In such an event, the file system and its host span will fail and remain offline until additional capacity can be acquired and provisioned to the underlying pool. If this is the only HDP pool used by the NAS system, all NAS resource would be rendered offline, until the situation is rectified (by acquisition of additional capacity). While the use of SVOS RF ADR with HDP Pools attached to HNAS is a supported configuration, for these reasons it is not recommended.

Customers who choose to leverage SVOS RF ADR with HDP Pools used by HNAS, must acknowledge and understand that free space management must be performed by consulting the storage system itself for free space information. The storage system is able to report data reduction statistics and available free space, but it will not be able to account for HNAS capacity associated with vacated span chunks or file system free space. Vacated chunks are NAS storage pool chunks that were allocated from the HDP Pool to the NAS span, but whose host file system was deleted – the NAS considers the capacity as free space that is re-usable by other file systems in that span, but the system considers this as allocated.

Customers should also understand that since Data Reduction occurs at the block level (below the HNAS file system), that the amount of free space available to the HNAS will vary according to the changes in the data type and content and the system’s ability to compress the information. Consider that a file system into which was previously written GBs of highly compressible text will consume a much lower amount of physical capacity than an equivalent amount of uncompressible data (e.g. video, rich media), while the file system itself may not have changed its logical size. Therefore, changes in data type written to the system may not lower or raise the utilization level of NAS file systems, but they can alter the utilization level of underlying physical capacity.