NetApp filer

Internal architecture

Most NetApp filers consist of customized computers with Intel or AMD processors using PCI. Each Filer has a proprietary NVRAM adapter to log all writes for performance and to play the data log forward in the event of an unplanned shutdown. One can link two filers together as a cluster, which NetApp (as of 2009) refers to using the less ambiguous term "Active/Active".

Hardware

Each filer model comes with a set configuration of processor, RAM and NVRAM, which users cannot expand after purchase. With the exception of some of the entry point storage controllers, the NetApp filers have at least one PCIe-based slot available for additional network, tape and/or disk connections. In June 2008 NetApp announced the Performance Acceleration Module (or PAM) to optimize the performance of workloads which carry out intensive random reads. This optional card goes into a PCIe slot and provides additional memory (or cache) between the disk and the filer RAM/NVRAM, thus improving performance.

Storage

NetApp supports either SATA, Fibre Channel, or SAS disk drives, which it groups into RAID (Redundant Array of Inexpensive Disks or Redundant Array of Independent Disks) groups of up to 28 (26 data disks plus 2 parity disks). Multiple RAID groups form an "aggregate"; and within aggregates Data ONTAP operating system sets up "flexible volumes" to actually store data that users can access. An alternative is "Traditional volumes" where one or more RAID groups form a single static volume. Flexible volumes offer the advantage that many of them can be created on a single aggregate and resized at any time. Smaller volumes can then share all of the spindles available to the underlying aggregate. Traditional volumes and aggregates can only be expanded, never contracted. However, Traditional volumes can (theoretically) handle slightly higher I/O throughput than flexible volumes (with the same number of spindles), as they do not have to go through an additional virtualisation layer to talk to the underlying disk.

WAFL File System

WAFL, as a robust versioning filesystem, provides snapshots, which allow end-users to see earlier versions of files in the file system. Snapshots appear in a hidden directory: ~snapshot for Windows (SMB) or .snapshot for Unix (NFS). Up to 255 snapshots can be made of any traditional or flexible volume. Snapshots are read-only, although Data ONTAP 7 provides additional ability to make writable "virtual clones", based at "WAFL snapshots" technique, as "FlexClones".

Data ONTAP implements snapshots by tracking changes to disk-blocks between snapshot operations. It can set up snapshots in seconds because it only needs to take a copy of the root inode in the filesystem. This differs from the snapshots provided by some other storage vendors in which every block of storage has to be copied, which can take many hours.

SnapMirror

Snapshots form the basis for NetApp disk replication technology SnapMirror, which effectively replicates snapshots between two NetApp filers. Later versions of Data ONTAP introduced cascading replication, where one volume could replicate to another and then another etc. NetApp also offers a backup product based around replicating and storing snapshots, called SnapVault. Open Systems SnapVault allows Windows and UNIX hosts to back up data to a NetApp filer and store any filesystem changes in snapshots.

SyncMirror

Data ONTAP also implements an option called "SyncMirror" where all the RAID groups within an aggregate or traditional volume can be duplicated to another set of hard disks, typically at another site via a Fibre Channel link. NetApp provides a "MetroCluster" option, that uses "SyncMirror" to provide a geo-cluster or active/active cluster between two sites up to 100 km apart.

SnapLock

Other product options include "SnapLock" which implements a "Write Once Read Many" functionality on magnetic disks instead of to optical media, so that data cannot be deleted until its retention period has been reached. SnapLock exists in two modes: compliance and enterprise. The compliance mode was designed to assist organizations in implementing a comprehensive archival solution that meets strict regulatory retention requirements such as dictated by the SEC and several healthcare governing bodies. Records and files committed to WORM storage on a SnapLock Compliance volume cannot be altered or deleted before the expiration of their retention period. Moreover, a SnapLock Compliance volume cannot be destroyed until all data have reached the end of their retention period.

SnapLock Enterprise is geared toward assisting organizations that are more self-regulated and want to have greater flexibility in protecting digital assets with WORM-type data storage. Data stored as WORM on a SnapLock Enterprise volume are protected from alteration or modification with one main difference from SnapLock Compliance: as the files being stored are not for strict regulatory compliance, a SnapLock Enterprise volume can be destroyed by an administrator with root privileges on the FAS system containing the SnapLock Enterprise volume, even if the designed retention period has not yet passed. In both modes, the retention period can be extended, but not shortened, as this is incongruous with the concept of immutability. In addition, NetApp SnapLock data volumes are equipped with a tamper-proof compliance clock that is used as a time reference to block forbidden operations on files, even if the system time is tampered with.

PAM / Flash Cache

NetApp Filer can have PAM ( Performance Accelerate Module ) or Flash Cache (PAM II) which can reduce read latencies and allows the filer to support more read intensive work without adding any further disk to the underlying RAID.

SnapManager Suite

NetApp also offers products for taking application-consistent snapshots by coordinating the application and the NetApp Storage Array. These products support Microsoft Exchange, Microsoft SQL Server, Microsoft Sharepoint, Oracle, SAP and VMware ESX Server data. These products form part of the SnapManager suite.

Previous limitations

Prior to the release of ONTAP 8, individual aggregate sizes were limited to a maximum of 2TB for FAS250 models and 16TB for all other models.

The limitation on aggregate size, coupled with increasing density of disk drives, served to limit the performance of the overall system. NetApp, like most storage vendors, increases overall system performance by parallelizing disk writes to many different spindles (disk drives). Large capacity drives, therefore limit the number of spindles that can be added to a single aggregate, and therefore limit the aggregate performance.

Each aggregate also incurs a storage capacity overhead of approximately 7-11%, depending on the disk type. On systems with many aggregates this can result in lost storage capacity.

However, the overhead comes about due to additional block-checksumming on the disk level as well as usual file system overhead, similar to the overhead in file systems like NTFS or EXT3. Block checksumming helps to insure that data errors at the disk drive level do not result in data loss.

Data ONTAP 8.0 supports a new 64bit aggregate format, which increases the size limit of FlexVolume to approximately 100TB (depending on storage platform) and also increases the size limit of aggregates to more than 100 TB on newer models (depending on storage platform) thus restoring the ability to configure large spindle counts to increase performance and storage efficiency. ([1])

Model history

This list may omit some models. Information taken from spec.org, netapp.com and storageperformance.org

EOA = End of Availability

SPECsfs with "*" is clustered result. SPECsfs performed include SPECsfs93, SPECsfs97, SPECsfs97_R1 and SPECsfs2008. Results of different benchmark versions are not comparable.