How does the ONTAP cluster work? (part 1)

In my previous series of articles I explained how ONTAP system memory works and talked about:
NVRAM/NVMEM, NVLOGs, Memory Buffer, HA & HA interconnects, Consistency Points, WAFL iNodes, MetroCluster data availability, Mailbox disks, Takeover, Active/Active and Active/Passive configurations, Write-Through, Write Allocation, Volume Affinities (Wafinity), FlexGroup, RAID & WAFL interaction, Tetris and IO redirection, Read Cache, NVRAM size, role of the system battery and boot flash drive, Flash media and WAFL compatibility. Those articles going be a good addition & help to understand this one, so go & check them too.

First I need to tell that clusterization is a very broad term, and in many vendors and technologies, it has a different meaning. ONTAP uses three different types of clusterization and one of the primary purpose of this article to explain each and how they are a different one from another and how they can complement each other and what additional benefits ONTAP can get out of them. Before we go to clusterization, we need to go deeper and explore other ONTAP components to understand how it works.

When someone is speaking about ONTAP cluster, they most probably mean horizontal scaling clusterization used to scale-out storage (the third type of clusterization).


There are a few platforms ONTAP support: FAS appliance, AFF appliance, ASA, and SDS virtual appliance. NetApp FAS storage systems which contain only SSD drives with installed SSD-optimized ONTAP OS called All-Flash FAS (AFF). ASA based on the AFF platform and provides access to the storage only with SAN data protocols, therefore the rest of ONTAP-based systems might be called ”Unified ONTAP” meaning unification of SAN and NAS protocols. There is also a Lenovo DM line of products using Unified ONTAP. NetApp HW appliances are using either SATA, Fibre Channel, SAS, SSD disk drives or LUNs from 3rd party storage arrays, which it groups into RAID groups and then into Aggregates to combine disk performance and capacity into a single pool of storage resources. SDS appliances can use space from the hypervisor as virtual disks and join space into aggregates or can use physical disk drives passed through to ONTAP and build RAID out of them and then aggregates.


FlexVol volume is a logical space that placed on top of an aggregate, each volume can expand or shrink in size plus we can apply and change performance limits to each volume. A FlexVol helps to separate performance & capacity from an aggregate pool of resources and flexibly distribute them as needed. Some volumes need to be big but slow, and some very fast but small; volumes can be re-sized and performance re-balanced, – FlexVol is the technology which achieve this goal in ONTAP architecture. Clients are accessing storage from FlexVol volumes over SAN & NAS protocols. Each volume exists on a single aggregate and served by a single storage node (controller).

If two FlexVol volumes created, each on two aggregates and those aggregates owned by two different controllers, and system admin needs to use space from these volumes through a NAS protocol, then admin will create two file shares, one on each volume. In this case, admin most probably even will create different IP addresses; each will be used to access a dedicated file share. Each volume will have single write waffinity and there will be two buckets of space. Though even if two volumes reside on a single controller, and for example on a single aggregate (thus if the second aggregate exists, it will not be used in this case) and both volumes will be accessed through a single IP address, still there will be two write affinities, one on each volume and there still will be two separate buckets of space. So the more volumes you have, the more write waffinities you’ll have (better parallelization and thus more even CPU utilization, which is good), but then you’ll have multiple volumes (and multiple buckets for space thus multiple file shares).


FlexGroup is a free feature introduced in version 9, which uses the clustered architecture of the ONTAP operating system. FlexGroup provides cluster-wide scalable NAS access with NFS and CIFS protocols. A FlexGroup creates multiple write affinities but on another hand, unlike FlexVol, combines space and performance from all the volumes underneath (thus from multiple aggregates and nodes). A FlexGroup Volume is a collection of constituent FlexVol volumes distributed across nodes in the cluster called “Constituents,” which are transparently joined in a single space. FlexGroup volume combines performance and capacity from all the constituent volumes and thus from all nodes of the cluster where they located. For the end user, each FlexGroup Volume is represented by a single, ordinary file-share with single space equal to the summary of space from all the constituent volumes (not visible to clients) and multiple reads & write waffinities.

NetApp will reveal the full potential of FlexGroup with technologies like NFS multipathing, SMB multichannel, pNFS, SMB CA, and VIP.

Technology NameFlexVolFlexGroup
1. NFS multipathing (session trunking)NoNo
2. SMB multichannelNoNo
3. pNFSYesNo
4. VIP (BGP)YesYes
5. SMB Continuous Availability (SMB CA)YesYes*

*Added in ONTAP 9.6

The FlexGroup feature in ONTAP 9 allows to massively scale in a single namespace to over 20 PB with over 400 billion files, while evenly spreading the performance across the cluster. Starting with ONTAP 9.5 FabricPool supported with FlexGroup, in this case, it is recommended to have all the constituent volumes to back up to a single S3 object storage bucket. FlexGroup supports SMB features for native file auditing, FPolicy, Storage Level Access Guard (SLA), copy offload (ODX) and inherited watches of changes notifications; Quotas and Qtree. SMB Contiguous Availability (CA) supported with FlexGroup in ONTAP 9.6 allows running MS SQL & Hyper-V. FlexGroup also supported on MetroCluster.

Clustered ONTAP

Today’s OS for NetApp AFF, FAS, Lenovo DM line and cloud appliances are known just as ONTAP 9, but before version 9 there was Clustered ONTAP (or Cluster-Mode, Clustered Data ONTAP or cDOT) and 7-mode ONTAP. 7-mode is the old firmware which had capabilities of the first and the second type of clusterization (High Availability and MetroCluster), while Clustered ONTAP 9 has all three HA, MCC, plus horizontal scaling clusterization). The reason why two existed for in parallel was Clustered ONTAP 8 didn’t have all reach functionality from 7-mode, so for a while, it was possible to run both modes (one at a time) on the same NetApp hardware. NetApp spent some time to bring all the functionality to Cluster-Mode, once finished the transition, 7-Mode was deprecated and with that milestone Clustered ONTAP updated to the next version and become ”just” ONTAP 9. ONTAP 8.2.4 was the last version of 7-Mode. Both 7-Mode & Cluster-Mode share a lot of similarities, for example, WAFL file system was used on both, but most were not compatible one with another, in previous example WAFL versions and functionality were different and thus incompatible, only limited compatibility was introduced mostly for migration purposes from 7-mode to Cluster-Mode. The last version of 7-Mide ONTAP 8.2.4 contains WAFL compatible with Cluster-Mode, to introduce fast but offline in-place upgrade to the newest versions of ONTAP.

In version 9, nearly all the features from 7-mode were successfully implemented in ONTAP (Clustered) including SnapLock, FlexCache, MetroCuster, SnapMirror Synchronous, while many new features that not available in 7-Mode were introduced, including features such as FlexGroup, FabricPool, and new capabilities such as fast-provisioning workloads and Flash optimization, NDAS, data compaction, AFF or many others. The uniqueness of NetApp’s Clustered ONTAP is in the ability to add heterogeneous systems (where all systems in a single cluster do not have to be of the same model or generation) to a single (third type) cluster. This provides a single pane of glass for managing all the nodes in a cluster, and non-disruptive operations such as adding new models to a cluster, removing old nodes, online migration of volumes, and LUNs while data is continuously available to its clients.

A node and controller (head or physical appliance) are very similar terms and often used interchangeably. The difference between them is that controller is a physical server with CPU, main board, Memory and NVRAM, while node is an instance of ONTAP OS running on top of controller. A node can migrate, for example when a controller replaced with a new one. ONTAP (the third type) Cluster consist of nodes.

FAS appliances

Are NetApp custom build & OEM hardware. Controllers in FAS systems are computers which running ONTAP OS. FAS systems are used with HDD and SSD drives. SSD often used for caching, but can be used in all-SSD aggregates as well. FAS systems can use NetApp disk shelves to add capacity to the storage or a 3rd party arrays. Each disk shelf connected to one storage system which consists out of one or two controllers (HA pair).

AFF appliances

All-Flash FAS appliance also known as AFF. Usually, NetApp All-Flash systems based on the same hardware as FAS but first one’s OS ONTAP optimized and works only with SSD media on the back end while FAS appliance can use HDD and SSD and SSD as cache. Hare are pairs of appliances which using the same hardware: AFF A700 & FAS9000, A300 & FAS8200, A200 & FAS2600, A220 & FAS2700, but AFF systems do not include FlashCache cards since there is no sense in caching operation from flash media on the flash media. Also, AFF systems do not support FlexArray third-party storage array virtualization functionality. Both AFF & FAS using the same firmware image and nearly all noticeable functionality for the end user are the same for both. However internally data processed and handled differently in ONTAP on AFF systems, for example, used different Write Allocation algorithms than on FAS systems. Because AFF systems have faster underlying SSD drives Inline data deduplication in ONTAP systems nearly not noticeable (no more than 2% performance impact on low-end systems).

ASA appliances

ASA systems based on the AFF platform and provide access over SAN protocols only, therefore to differentiate from ASA the rest of the ONTAP-based systems are called Unified systems meaning unification of SAN & NAS data protocols. ASA systems provide symmetric access to the storage nodes over the network, thus each block device (i.e. LUN or NVMe namespace) accessed over paths from both controllers of the HA pair while rest of the ONTAP-based Unified (non-ASA) systems with SAN protocols normally are using optimized paths only through the controller which owns the LUN and switches to the non-optimized paths only when the optimized paths are not available. See the announcement of ASA in Oct 2019.

Unified ONTAP

Unified ONTAP called all the systems capable of both SAN & NAS protocols. ASA has burned All SAN identity and can serve only SAN data protocols. Thus FAS, AFF, ONTAP Select, Cloud Volumes ONTAP, Cloud Volumes Services are Unified and continue to use ALUA & ANA with SAN protocols.

ONTAP Select

Is software-only solution available in a form of a virtual machine. ONTAP Select is using third-party disk drives and can form its own RAID or use server-based Hardware RAID. If server-based Hardware RAID used, typically aggregates build of a single block device. ONTAP Select can run as single node, HA pair with two nodes or as multiple HA-pair cluster. Previously ONTAP Select was known as Data ONTAP Edge. ONTAP Select used as the platform for Cloud Volumes ONTAP (CVO) offering in public cloud providers.


FAS and AFF systems are using enterprise level HDD, and SSD (i.e., NVMe SSD) physical drives with two ports, each port connected to each controller in an HA pair. HDD and SSD drives can only be bought from NetApp and installed in NetApp’s Disk Shelves for FAS/AFF platform. Physical HDD and SSD drives, partitions on disk drives, and even LUNs imported from third-party arrays with FlexArray functionality are considered in ONTAP as a Disk. In SDS systems like ONTAP Select & ONTAP Cloud, logical block storage like virtual disk or RDM inside ONTAP also considered as a Disk. Do not confuse the general term “disk drive” and “disk drive term used in ONTAP system” because with ONTAP it could be entire physical HDD or SSD drive, a LUN or a partition on a physical HDD or SSD drive. A LUN imported from third-party arrays with FlexArray functionality in HA pair configuration must be accessible from both nodes of the HA pair as HDD or SSD drive. Each ONTAP disk has ownership on it to show which controller owns and serve the disk. An Aggregates can include only disks owned by a single node, therefore each aggregate owned by a node and any objects on top of it, as FlexVol volumes, LUNs, File Shares are served within a single controller. Each node have its own disks and aggregates and serve them. Where both nodes can be utilized simultaneously even though they not serving the same data.


Advanced Drive Partitioning (ADP) can be used in AFF & FAS systems depending on the platform and use-case. FlexArray technology does not support ADP. This technique mainly used to overcome some architectural requirements and reduce the number of disk drives in a NetApp FAS & AFF storage systems. There are three types of ADP:

  • Root-Data partitioning
  • Root-Data-Data partitioning (RD2 also known as ADPv2)
  • and Storage Pool.

Root-Data partitioning used in FAS & AFF systems to create small root partitions on drives to use them to create system root aggregates and therefore not to spend entire two physical disk drives for that purpose while the bigger portion of the disk drive will be used for data aggregate. Root-Data-Data partitioning is used in All-Flash systems only, it used for the same reason as Root-Data partitioning with the only difference that bigger portion of the drive left after root partitioning divided equally by two partitions, each partition assigned to one of the two nodes, therefore reducing the minimum number of drives required for an All-Flash system and reducing waste for expensive SSD space. Storage Pool partitioning technology used in FAS systems to equally divide each SSD drive by four pieces which later can be used for (only) FlashPool cache acceleration, with Storage Pool only a few SSD drives can be divided by up to 4 data aggregates which will benefit from FlashPool caching technology reducing minimally required SSD drives for FlashPool.


In NetApp ONTAP systems, RAID and WAFL are tightly integrated. There are several RAID types available within NetApp FAS and AFF systems:

  • RAID-4 with 1 dedicated parity disk allowing any 1 drive to fail in a RAID group.
  • RAID-DP with 2 dedicated parity disks allowing any 2 drives to fail simultaneously in a RAID group.
  • RAID-TEC US patent 7640484 with 3 dedicated parity drives, allows any 3 drives to fail simultaneously in a RAID group.

RAID-DP’s double parity leads to a disk loss resiliency similar to that of RAID-6. NetApp overcomes the write performance penalty of traditional RAID-4 style dedicated parity disks via WAFL and innovative use of its nonvolatile memory (NVRAM) within each storage system. Each aggregate consists of one or two plexes, and a plex consists of one or more RAID groups. Typical NetApp FAS or AFF storage system have only one plex in each aggregate, two plexes used in local SyncMirror or MetroCluster configurations. Therefore in systems without MetroCluster or local SyncMirror engineers might say “aggregates consist of RAID groups” to simplify things a bit because plex does not play a vital role in such configurations, while in reality an aggregate always have one or two plex and a plex consists of one or more RAID groups (see the picture with aggregate diagram). Each RAID group usually consists of disk drives of the same type, speed, geometry, and capacity. Though NetApp Support could allow a user to install a drive to a RAID group with same or bigger size and different type, speed and geometry for a temporary basis. RAID can be used with partitions too. Any data aggregates if containing more than one RAID group must have same RAID groups across the aggregate, same RAID group size is recommended, but NetApp allows to have an exception in the last RAID group and configure it as small as half of the RAID group size across aggregate. For example, such an aggregate might consist of 3 RAID groups: RG0:16+2, RG1:16+2, RG2:7+2. Within aggregates, ONTAP sets up flexible volumes (FlexVol) to store data that users can access. The reason ONTAP has ”default” RAID group size and that number is smaller than max RAID group size is to allow admin in the future to add only a few disk drives to existing RAID groups instead of adding a new RAID group with the full set of drives

Aggregates enabled as FlashPool consists of both HDD and SSD drives called hybrid aggregates and used in FAS systems. In FlashPool aggregates the same rules applied to the hybrid aggregate as to ordinary aggregates but separately to HDD and SSD drives, thus it is allowed to have two different RAID types: one RAID type for all HDD drives and one RAID type for all SSD drives in a single hybrid aggregate. For example SAS HDD with RAID-TEC (RG0:18+3, RG1:18+3) and SSD with RAID-DP (RG3:6+2). NetApp storage systems running ONTAP combine underlying RAID groups similarly to RAID-0 in plexes and aggregates, while in Hybrid aggregates SSD portion used for cache and therefore capacity from flash media not contributing to overall aggregate space. Also in NetApp FAS systems with FlexArray feature third party LUNs could be combined in a plex/aggregate similarly as in RAID-0. NetApp storage systems running ONTAP can be deployed in MetroCluster and local SyncMirror configurations which are using technique comparably to RAID-1 with mirroring data between two plexes in an aggregate.

Note that ADPv2 does not support RAID-4. RAID-TEC is recommended if the size of the disks used in an aggregate is greater than 4 TiB. RAID type in storage pool cannot be changed. RAID minimums for root aggregate (with force-small-aggregate true) are:

  • RAID-4 is 2 drives (1d + 1p)
  • RAID-DP is 3 drives (1d + 2p)
  • RAID-TEC is 5 drives (2d + 3p)


One or multiple RAID groups form an “aggregate,” and within aggregates ONTAP operating system sets up “flexible volumes” (FlexVol) to store data that hosts can access.

Similarly, to RAID-0, each aggregate merges space from underlying protected RAID groups to provide one logical piece of storage for flexible volumes therefore Aggregate does not provide data protection mechanisms but rather another layer of abstraction. Alongside with aggregates consisted out of disks and RAID groups, other aggregates could consist of LUNs already protected with third-party storage systems and connected to ONTAP with FlexArray, and in similar way it works in ONTAP Select or Cloud Volumes ONTAP. Each aggregate could consist of either LUNs or NetApp RAID groups. 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 the space & disk performance available to the underlying aggregate, and QoS allows to change the performance of flexible volumes on the fly. Aggregates can only be expanded, never downsized. Current maximum physical useful space size in an aggregate is 800 TiB for All-Flash FAS Systems. the limit applies on space in the aggregate rather then number of disk drives and may be different on AFF & FAS systems.


NetApp FlashPool is a feature on hybrid NetApp FAS systems which allows creating a hybrid aggregate with HDD drives and SSD drives in a single data aggregate. Both HDD and SSD drives form separate RAID groups. Since SSD also used for write operations, it requires RAID redundancy contrary to FlashCache which accelerate only read operations. In hybrid aggregate the system allows to use different RAID types for HDD and SSD, for example, it is possible to have 20 HDD 8TB in RAID-TEC while 4 SSD in RAID-DP or even RAID-4 with 960GB in a single aggregate. SSD RAID used as cache and improved performance for read-write operations for FlexVol volumes on the aggregate where SSD added as the cache. FlashPool cache similarly to FlashCache have policies for reading operations but also include write operations, and system administrator could apply those policies for each FlexVol volume located on the hybrid aggregate, therefore could be disabled on some volumes while others could benefit from SSD cache. Both FlashCache & FlashPool can be used simultaneously to cache data from a single FlexVol. To enable an aggregate with FlashPool technology minimum 4 SSD disks required (2 data, 1 parity, and 1 hot spare), it is also possible to use ADP technology to partition SSD into 4 pieces (Storage Pool) and distribute those pieces between two controllers so each controller’s aggregates could benefit from SSD cache when there is a small amount of SSD. FlashPool is not available with FlexArray and is available only with NetApp FAS native disk drives in NetApp’s disk shelves.


FabricPool technology available for all-SSD aggregates in FAS/AFF systems or in Cloud Volumes ONTAP on SSD media. Starting with ONTAP 9.4 FabricPool supported on ONTAP Select platform. Cloud Volumes ONTAP also supports HDD + S3 FabricPool configuration. FabricPool provides automatic storage tiering capability for cold data blocks from fast media (hot tier) on ONTAP storage to cold media to an S3 object storage (cold tier) and back. Each FlexVol volumes on a FabricPool-enabled all-SSD aggregates can have one out of four policies:

  • None – Does not tier data from a volume
  • Snapshot – Migrate cold data blocks captured in snapshots
  • Auto – Migrates cold data blocks from an active file system and snapshots to cold tier
  • All – this policy tiers all the data writing through directly to S3 object storage, metadata though always stays on SSD hot tier.

FabricPool preserves offline deduplication & offline compression savings. FabricPool tier-off blocks from active file system (by default 31-day data not been accessed) & support data compaction savings. Trigger for tiering from hot tier can be adjusted. The recommended ratio is 1:10 for inodes to data files. For clients connected to the ONTAP storage system, all the FabricPool data-tiering operations are completely transparent, and in case data blocks become hot again, they are copied back to fast media to the ONTAP. FabricPool is compatible with the

  • NetApp StorageGRID
  • Amazon S3 and Amazon Commercial Cloud Services (C2S)
  • Google Cloud
  • Alibaba object storage services
  • Azure Blob supported
  • IBM Cloud Object Storage (ICOS) in the cloud
  • IBM Cleversafe (on-prem object storage)

Other object-based SW & services could be used if requested by the customer and that service will be validated by NetApp. The FabricPool feature in FAS/AFF systems is free for use with NetApp StorageGRID external object storage. For other object storage systems such as Amazon S3 & Azure Blob, FabricPool must be licensed per TB to function (alongside costs for FabricPool licensing, the customer needs also to pay for consumed object space). While with the Cloud Volumes ONTAP storage system, FabricPool does not require licensing, costs will apply only for consumed space on the object storage. FlexGroup volumes and SVM-DR supported with FabricPool, also SVM-DR supported with FlexGroups.


NetApp storage systems running ONTAP can have FlashCache cards which can reduce read operations latency and allows the storage systems to process more read intensive work without adding any additional disk drives to the underlying RAID. Usually, one FlashCache module installed per controller, no mirroring performed between nodes and entire space from FlashCache used by a single node only, since read operations do not require redundancy in case of FlashCache failure, but chip-level data protection is available in FlashCache. If the system unexpectedly rebooted, read chance will be lost, but will restore over the time during regular node operation. FlashCache works on node level, by default accelerates any volumes on that node and only read operations. FlashCache caching policies applied on FlexVol level: system administrator can set cache policy on each individual volume on the controller or disable read cache at all. FlashCache technology is compatible with the FlexArray feature. Starting with 9.1 a single FlexVol volume can benefit from both FlashPool & FlashCache cache simultaneously.


FlexArray is NetApp FAS functionality allows to virtualize third-party storage systems, and other NetApp storage systems over SAN protocols and use them instead of NetApp’s disk shelves. With FlexArray functionality RAID protection must be provided with third-party storage array thus NetApp’s RAID-4, RAID-DP and RAID-TEC not used in such configurations. One or many LUNs from third-party arrays could be added to a single aggregate similarly to RAID-0. FlexArray is licensed feature.

NetApp Storage Encryption

NetApp Storage Encryption (NSE) is using specialized purpose-build disks with low level Hardware-based full disk encryption (FDE/SED) chip, some disks are FIPS-certified self-encrypted drives. NSE & FIPS drives compatible nearly with all NetApp ONTAP features and protocols but except for MetroCluster. NSE feature does overall nearly zero performance impact on the storage system. NSE feature similarly to NetApp Volume Encryption (NVE) in storage systems running ONTAP can store encryption key locally in Onboard Key Manager which stores keys in onboard TPM module or through KMIP protocol on dedicated key manager systems like IBM Security Key Lifecycle Manager and SafeNet KeySecure. NSE is data at rest encryption which means it protects only from physical disks theft and does not give an additional level of data security protection. In a standard operational and running ONTAP system, this feature does not encrypt data over the wire. When OS shuts disks down, they lose encryption key and becomes locked and if KeyManager not available or locked, ONTAP couldn’t boot. NetApp has passed NIST Cryptographic Module Validation Program for its NetApp CryptoMod (TPM) with ONTAP 9.2.

Continue to read

How ONTAP Memory work

Zoning for ONTAP Cluster


Please note in this article I described my own understanding of the internal organization of ONTAP systems. Therefore, this information might be either outdated, or I simply might be wrong in some aspects and details. I will greatly appreciate any of your contribution to make this article better, please leave any of your ideas and suggestions about this topic in the comments below.

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6 thoughts on “How does the ONTAP cluster work? (part 1)”

  1. […] There are storage systems where each node serves the same data with symmetrical storage access and have monolithic storage architecture; ONTAP is not one of them. In ONTAP only one node serves each disk/aggregate/FlexVol/LUN at a time, and if that node fails, another takes over (also see the definition what is a disk drive in ONTAP and about exception with ADP drive partitioning). […]

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