DNA. It’s the building block of life—the macromolecule that encodes genes and governs the production of proteins, from which all cellular metabolism derives. It is truly the "molecular blueprint" that determines the properties of all organisms.

I often talk about the Network Appliance™ storage system architecture as having its own "DNA"—a core blueprint from which numerous key features derive and which continues to spawn new evolutionary "variations" that allow the architecture to adapt to environmental changes.

In NetApp storage architectures, this blueprint is based on the WAFL® file system, RAID 4, NetApp use of NVRAM, and a unique approach to Snapshot™ copies. These are the core building blocks that continue to define NetApp storage systems. And they continue to support the evolution of features that carry forth the core DNA—whether that be data protection and retention features (SnapMirror® and SnapVault®), compliance features (LockVault™), or efficient means of replicating working data sets for dev/test/QA environments (FlexClone™).

The Genes: WAFL, RAID 4, NVRAM, and Snapshot
At the core of the NetApp genetic blueprint are four key, interrelated technologies: WAFL, RAID 4, NVRAM, and Snapshot.

WAFL is the Write Anywhere File Layout, an approach to writing data to disk locations that minimizes the conventional parity RAID write penalty. By storing system metadata (inodes, block maps, and inode maps) in the same way application data is stored, WAFL is able to write file system metadata blocks anywhere on the disk. This approach in turn allows multiple writes to be "gathered" and scheduled to the same RAID stripe—eliminating the traditional read-modify-write penalty prevalent in parity-based RAID schemes.

In the case of WAFL, this stripe-at-a-time write approach makes RAID 4 a viable (and even preferred) parity scheme. At the time of its design, the common wisdom was that RAID 4 (which uses a dedicated parity drive) presented a bottleneck for write operations because writes that would otherwise be spread across the data drives would all have to update the single parity drive in the RAID group. WAFL and full-stripe writes, however, eliminate the potential bottleneck and, in fact, provide a highly optimized write path.

This stripe-at-a-time approach to writes also required that the system provide a means of reliably buffering write requests before they are written (en masse) to disk. Nonvolatile RAM allows the system to reliably log writes and quickly acknowledge those writes back to clients.

The final core contribution to the NetApp DNA is the implementation of Snapshot technology, which provides an efficient, point-in-time, consistent view of the file system. Figure 1a presents a simplified view of the WAFL file system (leaving out internal inode and indirect block structures). Figure 1b shows how WAFL creates a new Snapshot copy by simply duplicating the root inode. Both the original root inode and the Snapshot copy then point to the same blocks on disk (same "view" of the file system). Figure 1c shows what happens when one of the baseline file system blocks (block D) is modified by a user process. Only the new data (single write) need be written to disk. This write and any required modifications to intermediate nodes (inode blocks, indirect block maps) are logged into NVRAM, where they can be gathered and coalesced to optimize updates of those intermediate nodes.

The underlying layout, coupled with the episodic, multistripe write approach, ensures that NetApp Snapshot technology is extremely space- and resource-efficient. Effectively, only changed blocks (changes to the baseline file system) are written to disk. The result is that many Snapshot copies can be maintained as efficiently as one; more efficient Snapshot copies allow organizations to create Snapshot copies more frequently, which ensures faster and more up-to-date file or file system recovery.

In addition to providing read-only, point-in-time "versions" of the user file system, Snapshot copies are also used to create periodic "consistency points" within the file system that minimize recovery time in the event of power loss or system failure. These consistency points are taken every few seconds and together with NVRAM-journaled writes ensure rapid recovery of a consistent file system without the need for extensive consistency checks.

Evolution
These are the core building blocks of NetApp storage systems. Over the years, these core technology "genes" have recombined in a number of ways to deliver more and more capable storage systems—the analog of genetic evolution.

Block Storage Protocols
Initial NetApp storage systems were NFS appliances. Over the years, the same core architecture has been extended to support multiple protocols—CIFS initially, and then block-based protocols (Fibre Channel and iSCSI). Block protocols expose LUNs, which are special WAFL containers that exhibit block device characteristics. They inherit the rich lineage of WAFL—including space- and resource-efficient Snapshot copies and clones. Block I/O bypasses all WAFL file semantics and is passed straight through to the WAFL volume management and virtualization layers, which also incorporates the integrated RAID layer

Cluster Failover
The core NVRAM-based write journaling mechanism has been extended in conjunction with controller pairing to provide HA failover capabilities. In these clustered configurations, two controllers are "cross-connected" to each others’ disks, and NVRAM writes are mirrored over an InfiniBand® cable to the partner controller’s NVRAM, ensuring redundant journaling in case of controller failure.

SnapVault, SnapMirror
Snapshot copies are "on-box" point-in-time versions of a file system or LUN. This core technology is the foundation for "off-box" data protection schemes, including SnapVault and SnapMirror. SnapVault effectively propagates Snapshot copies to other NetApp storage devices—typically NearStore® systems—as a disk-to-disk backup solution for high-frequency "incrementals" that are accessed as point-in-time full backups. These, in turn, can be used for user-driven drag-and-drop file recovery and for periodic tape-based full backups without the pressures of production system "backup windows." Open Systems SnapVault (OSSV) extends this functionality to third-party host-based file systems.

Asynchronous SnapMirror also draws from core Snapshot roots to provide data protection capabilities as part of a comprehensive disaster recovery/business continuance architecture.

RAID-DP
The core RAID technology has been augmented with a diagonal dual parity scheme, RAID-DP, that uses a second parity drive in a RAID group and "diagonal" parity computation to survive double-disk failures in a RAID group. The enhanced security of this RAID method allows for the creation of larger RAID groups (14+2 is common), effectively providing better than mirrored protection for user data with no additional parity overhead (still 7:1) and negligible performance impact due to the multistripe write approach of WAFL.

FlexClone
FlexClone uses the same mechanism employed in Snapshot copies to create writable clones of volumes or LUNs. As with Snapshot copies, clones can be created nearly instantaneously with effectively no storage overhead; they share the same underlying storage blocks as their baseline volume/LUN. As the baseline and the clone diverge (for example, due to updates in the clone’s data blocks), those new blocks and their blockmap pointers are written to disk, and the volume/LUN accumulates only the changed blocks.

This scheme affords space- and time-efficient, writable copies of file systems or LUNs that can be used for a range of purposes, including dev/test/QA; database reporting and analytics; and data warehouse extract, transform, and load.

Coupled with SnapMirror to create an incrementally propagated copy of data on a second storage system, FlexClone technology can be used to support these activities entirely "out-of-band" of the production storage system. This concept is depicted in figure 2.

Figure 2) SnapMirror and FlexClone deployed in an out-of-band dev/test/QA scenario.

A-SIS
One of the newer features to evolve out of the NetApp genetic pool is Advanced Single Instance Storage, or A-SIS. Known also as block de-duplication, this feature uses the pointer and block management of WAFL to "squeeze out" duplicate blocks from the file system, replacing pointers to duplicate blocks with pointers to a common block. If files or volumes diverge after block de-duplication, the new (modified) blocks are stitched into the affected block map without impacting any other maps that may share the common block.

Conclusion
Many Network Appliance storage system features owe their existence to the core NetApp DNA: the unique combination of WAFL, RAID 4, NVRAM, and Snapshot that continues to fuel the evolution of the NetApp product line. And there are already additional extensions in the works that promise to deliver on the continued evolution this genetic blueprint affords.

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