Posts Tagged ‘EMC’

Dell Compellent is not an ALUA Storage Array

May 16, 2016

dell_compellentDell Compellent is Dell’s flagship storage array which competes in the market with such rivals as EMC VNX and NetApp FAS. All these products have slightly different storage architectures. In this blog post I want to discuss what distinguishes Dell Compellent from the aforementioned arrays when it comes to multipathing and failover. This may help you make right decisions when designing and installing a solution based on Dell Compellent in your production environment.

Compellent Array Architecture

In one of my previous posts I showed how Compellent LUNs on vSphere ESXi hosts are claimed by VMW_SATP_DEFAULT_AA instead of VMW_SATP_ALUA SATP, which is the default for all ALUA arrays. This happens because Compellent is not actually an ALUA array and doesn’t have the tpgs_on option enabled. Let’s digress for a minute and talk about what the tpgs_on option actually is.

For a storage array to be claimed by VMW_SATP_ALUA it has to have the tpgs_on option enabled, as indicated by the corresponding SATP claim rule:

# esxcli storage nmp satp rule list

Name                 Transport  Claim Options Description
-------------------  ---------  ------------- -----------------------------------
VMW_SATP_ALUA                   tpgs_on       Any array with ALUA support

This is how Target Port Groups (TPG) are defined in section Introduction to asymmetric logical unit access of SCSI Primary Commands – 3 (SPC-3) standard:

A target port group is defined as a set of target ports that are in the same target port asymmetric access state at all times. A target port group asymmetric access state is defined as the target port asymmetric access state common to the set of target ports in a target port group. The grouping of target ports is vendor specific.

This has to do with how ports on storage controllers are grouped. On an ALUA array even though a LUN can be accessed through either of the controllers, paths only to one of them (controller which owns the LUN) are Active Optimized (AO) and paths to the other controller (non-owner) are Active Non-Optimized (ANO).

Compellent does not present LUNs through the non-owning controller. You can easily see that if you go to the LUN properties. In this example we have four iSCSI ports connected (two per controller) on the Compellent side, but we can see only two paths, which are the paths from the owning controller.


If Compellent presents each particular LUN only through one controller, then how does it implement failover? Compellent uses a concept of fault domains and control ports to handle LUN failover between controllers.

Compellent Fault Domains

This is Dell’s definition of a Fault Domain:

Fault domains group front-end ports that are connected to the same Fibre Channel fabric or Ethernet network. Ports that belong to the same fault domain can fail over to each other because they have the same connectivity.

So depending on how you decided to go about your iSCSI network configuration you can have one iSCSI subnet / one fault domain / one control port or two iSCSI subnets / two fault domains / two control ports. Either of the designs work fine, this is really is just a matter of preference.

You can think of a Control Port as a Virtual IP (VIP) for the particular iSCSI subnet. When you’re setting up iSCSI connectivity to a Compellent, you specify Control Ports IPs in Dynamic Discovery section of the iSCSI adapter properties. Which then redirects the traffic to the actual controller IPs.

If you go to the Storage Center GUI you will see that Compellent also creates one virtual port for every iSCSI physical port. This is what’s called a Virtual Port Mode and is recommended instead of a Physical Port Mode, which is the default setting during the array initialization.

Failover scenarios

Now that we now what fault domains are, let’s talk about the different failover scenarios. Failover can happen on either a port level when you have a transceiver / cable failure or a controller level, when the whole controller goes down or is rebooted. Let’s discuss all of these scenarios and their variations one by one.

1. One Port Failed / One Fault Domain

If you use one iSCSI subnet and hence one fault domain, when you have a port failure, Compellent will move the failed port to the other port on the same controller within the same fault domain.


In this example, 5000D31000B48B0E and 5000D31000B48B0D are physical ports and 5000D31000B48B1D and 5000D31000B48B1C are the corresponding virtual ports on the first controller. Physical port 5000D31000B48B0E fails. Since both ports on the controller are in the same fault domain, controller moves virtual port 5000D31000B48B1D from its original physical port 5000D31000B48B0E to the physical port 5000D31000B48B0D, which still has connection to network. In the background Compellent uses iSCSI redirect command on the Control Port to move the traffic to the new virtual port location.

2. One Port Failed / Two Fault Domains

Two fault domains scenario is slightly different as now on each controller there’s only one port in each fault domain. If any of the ports were to fail, controller would not fail over the port. Port is failed over only within the same controller/domain. Since there’s no second port in the same fault domain, the virtual port stays down.


A distinction needs to be made between the physical and virtual ports here. Because from the physical perspective you lose one physical link in both One Fault Domain and Two Fault Domains scenarios. The only difference is, since in the latter case the virtual port is not moved, you’ll see one path down when you go to LUN properties on an ESXi host.

3. Two Ports Failed

This is the scenario which you have to be careful with. Compellent does not initiate a controller failover when all front-end ports on a controller fail. The end result – all LUNs owned by this controller become unavailable.



This is the price Compellent pays for not supporting ALUA. However, such scenario is very unlikely to happen in a properly designed solution. If you have two redundant network switches and controllers are cross-connected to both of them, if one switch fails you lose only one link per controller and all LUNs stay accessible through the remaining links/switch.

4. Controller Failed / Rebooted

If the whole controller fails the ports are failed over in a similar fashion. But now, instead of moving ports within the controller, ports are moved across controllers and LUNs come across with them. You can see how all virtual ports have been failed over from the second (failed) to the first (survived) controller:


Once the second controller gets back online, you will need to rebalance the ports or in other words move them back to the original controller. This doesn’t happen automatically. Compellent will either show you a pop up window or you can do that by going to System > Setup > Multi-Controller > Rebalance Local Ports.


Dell Compellent is not an ALUA storage array and falls into the category of Active/Passive arrays from the LUN access perspective. Under such architecture both controller can service I/O, but each particular LUN can be accessed only through one controller. This is different from the ALUA arrays, where LUN can be accessed from both controllers, but paths are active optimized on the owning controller and active non-optimized on the non-owning controller.

From the end user perspective it does not make much of a difference. As we’ve seen, Compellent can handle failover on both port and controller levels. The only exception is, Compellent doesn’t failover a controller if it loses all front-end connectivity, but this issue can be easily avoided by properly designing iSCSI network and making sure that both controllers are connected to two upstream switches in a redundant fashion.


First Look at UCS Performance Manager

May 12, 2016


perf_gaugeCisco UCS has been in the market for seven years now. It was quite expensive blade chassis when it was first introduced by Cisco in March 2009, but has reached the price parity with most of the server vendors these days.

Over the course of the last seven years Cisco has built a great set of products, which helps UCS customers in various areas:

  • UCS Central for configuration management across multiple Cisco UCS domains
  • UCS Director for infrastructure automation not only of UCS, but also network, storage and virtualization layers (don’t expect it to support any other vendors than Cisco for IP networks, though)
  • UCS Performance Manager for performance monitoring and capacity planning, which can also tap into your network, storage, virtualization and even individual virtual machines

UCS Performance Manager

UCS Performance Manager was first released in October 2014. The product comes in two versions – full and express. PM Express covers only servers, hypervisors and operating systems. The full version on top of that supports storage and network devices. Product is licensed on a per UCS server basis. So you don’t pay for additional network/storage devices or hypervisors.

PM supports vSphere hypervisor (plus Hyper-V), Cisco networking and EMC VNX / EMC VMAX / NetApp FAS storage arrays. By the list of the supported products you may quickly guess that the full version of Performance Manager is targeted mainly at NetApp FlexPod, VCE Vblock and EMC VSPEX customers.

Product architecture

UCS Performance Manager can be downloaded and quickly deployed as a virtual appliance. You might be shocked when you start it up first time, as the appliance by default comes configured with 8 vCPUs and 40GB of RAM. If you’re using it for demo purposes you can safely reduce it to something like 2-4 vCPUs and 8-12GB of RAM. You will experience some slowdowns during the startup, but performance will be acceptable overall.

UCS PM is built on Zenoss monitoring software and is essentially a customized version of Zenoss Service Dynamics with Cisco UCS ZenPacks. You may notice references to Zenoss throughout the management GUI.


Two main components of the solution are the Control Center and the Performance Manager itself. Control Center is a container orchestration product, which runs Performance Manager as an application in Docker containers (many containers).


When deploying Performance Manager you start with one VM and then you can scale to up to four VMs total. Each of the VMs can run in two modes – master or agent. When you deploy the first VM you will have to select it’s role at first login. You have to have one master host, which also runs an agent. And if you need to scale you can deploy three additional agent VMs and build a ZooKeeper cluster. One master host can support up to 500 UCS servers, when configured with 8 vCPUs and 64GB of RAM. Depending on your deployment size you may not ever need to scale to more than one Performance Manager VM.


After you’ve deployed the OVA you will need to log in to the VM’s CLI and change the password, configure the host as a master, set up a static IP, DNS, time zone, hostname and reboot.

Then you connect to Control Center and click “+ Application” button in the Applications section and deploy UCS PM on port 4979. For the hostname use Control Center’s hostname.


Once the UCS PM application is deployed, click on the Start button next to UCS PM line in the Applications section


Performance manager is accessible from a separate link which is Control Center’s hostname prefixed with “ucspm”. So if your CC hostname is ucspm01.domain.local, UCS PM link will be https://ucspm.ucspm01.domain.local:443. You can see it in Virtual Host Names column. You will have to add an alias in DNS which would point from ucspm.ucspm01.domain.local to ucspm01.domain.local, otherwise you won’t be able to connect to it.

When you finally open UCS PM you will see a wizard which will ask you to add the licences, set an admin account and add your UCS chassis, VMware vCenters and UCS Central if you happen to have one. In the full version you will have a chance to add storage and network devices as well.


UCS performance monitoring

Probably the easiest way to start working with Performance Manager is to jump from the dashboard to the Topology view. Topology view shows your UCS domain topology and provides an easy way to look at various components from one screen.


Click on the fabric interconnect and you can quickly see the uplink utilization. Click on the chassis and you will get summarized FEX port statistics. How about drilling down to a particular port-channel or service profile or vNIC? UCS Performance Manager can give you the most comprehensive information about every UCS component with historical data up to 1 year based on the default storage configuration.


Another great feature you may want to straight away drill down into is Bandwidth Usage, which gives you an overview of bandwidth utilization across all UCS components, which you can look at from a server or network perspective. This can let you quickly identify such things as uneven workload distribution between the blades or maybe uneven traffic distribution between fabric interconnect A and B side or SAN/LAN uplinks going to the upstream switches.


You can of course also generate various reports to determine your total capacity utilization or if you’re for example planning to add memory to your blades, you can quickly find out the number of DIMM slots available in the corresponding report.


VMware performance monitoring

UCS Performance Manager is not limited to monitoring only Cisco UCS blade chassis even in the Express version. You can add your hypervisors and also individual virtual machines. Once you add your vCenter to the list of the monitored devices you get a comprehensive list of VMware components, such as hosts, VMs, datastores, pNICs, vNICs and associated performance monitoring graphs, configuration information, events, etc.

Performance Manager can correlate VMware to UCS components and for example for a given VM provide you FC uplink utilization on the corresponding fabric interconnects of the chassis where this VM is running:


If you want to go further, you can add individual VMs to Performance Manager, connected via WinRM/SSH or SNMP. Some cool additional functionality you get, which is not available in VMware section is the Dynamic View. Dynamic View lets you see VM connectivity from the ESXi host it’s running on all the way through to blade, chassis, vNIC, VIC, backplane port, I/O module and fabric interconnect. Which is very helpful for troubleshooting connectivity issues:



UCS Performance Manager is not the only product for performance monitoring in virtualized environments. There are many others, VMware vRealize Operations Manager is one of the most popular of its kind. But if you’re a Cisco UCS customer you can definitely benefit from the rich functionality this product offers for monitoring UCS blade chassis. And if you are a lucky owner of NetApp FlexPod, VCE Vblock or EMC VSPEX, UCS Performance Manager for you is a must.


RecoverPoint VE: iSCSI Network Design

March 29, 2016

recoverpointRecoverPoint is a great storage replication product, which supports Continuous Data Protection (CDP) and gives you RPO figures measured in second compared to a standard asynchronous storage-based replication solutions, where RPO is measured in minutes or even hours.

RecoverPoint comes in three flavours:

  • RecoverPoint SE/EX/CL – physical appliance for replication between VNX (RecoverPoint/SE), VNX/VMAX/VPLEX (RecoverPoint/EX) or EMC and non-EMC (RecoverPoint CL) storage arrays.
  • RecoverPoint VE – virtual edition of RecoverPoint which is installed as a VM and supports the same SE/EX/CL versions.
  • RecoverPoint for Virtual Machines – also a virtual appliance but is array-agnostic and works at a hypervisor level by replicating VMs instead of LUNs.

In this blog post we will be discussing connectivity options for RecoverPoint VE (SE edition). Make sure to not confuse RecoverPoint VE and RecoverPoint for Virtual Machines as it’s two completely different products.

VNX MirrorView ports

MirrorView is an another EMC replication solution integrated into VNX arrays. If there’s a MirrorView enabler installed, it will claim itself the first FC port and the first iSCSI port. When patching VNX iSCSI ports make sure to NOT use the ports claimed by MirrorView.


If you use 1GbE (4-port) I/O modules you can use three ports per SP (all except port 0) and if you have 10GbE (2-port) I/O modules you can use one port per SP. I will talk about workarounds for this in the next blog post.

RPA appliance iSCSI vNICs

Each RecoverPoint appliance has two iSCSI NICs, which can be configured on either one or two subnets. If you use one 10Gb port on each SP as in the example above, then you’re forced to use one subnet. Because you obviously need at least two ports on each SP to have two networks.

If you have 1Gb modules in your VNX array, then you will most likely have two 1Gb iSCSI ports connected on each SP. In that case you can use two iSCSI subnets to reduce the number of iSCSI sessions between RPAs and a VNX.

On the vSphere side you will need to create one or two iSCSI port groups, depending on how many subnets you’ve decided to allocate and connect RPA vNICs accordingly.


VNX iSCSI Connections

RecoverPoint clusters are deployed and connected using a special tool called Deployment Manager. It assigns all IP addresses, connects RecoverPoint clusters to VNX arrays and joins sites together.

Once deployment is finished you will have iSCSI connections created on the VNX array. Depending on how many iSCSI subnets you’re using, iSCSI connections will be configured accordingly.

1. One Subnet Example

Lets look at the one subnet topology first. In this example you have one 10Gb port per VNX SP and two ports on each of the two RPAs all on one subnet. When you right click on the storage array in Unisphere and select iSCSI > Connections Between Storage Systems you should see something similar to this.


As you can see ports iSCSI1 and iSCSI2 on RPA0 and RPA1 are mapped to two ports on the storage array A-5 and B-5. Four RPA ports are connected to two VNX ports which gives you eight iSCSI initiator records on the VNX.


2. Two Subnets Example

If you connect two 1Gb ports per VNX SP and decide to use two subnets, then each SP will have one port on each of the two subnets. Same goes for the RPAs. Each RPA will have one vNIC connected to each subnet.

iSCSI connections will be set up a little bit differently now. Because only the VNX and RPA ports which are on the same subnet should be able to talk to each other.


Every RPA in this example has one IP on the subnet (iSCSI A) and one IP on the subnet (iSCSI B). Similarly, ports A-10 and B-10 on the VNX are configured on iSCSI A subnet. And ports A-11 and B-11 are configured on iSCSI B subnet. Because of that, iSCSI1 ports are mapped to ports A-10/B-10 and iSCSI2 ports are mapped to ports A-11/B-11.

As we are using two subnets in this example instead of 4 RPA ports by 4 VNX ports = 16 iSCSI connections, we will have 2 RPA ports by 2 VNX ports (subnet iSCSI A) + 2 RPA ports by 2 VNX ports (subnet iSCSI B) = 8 iSCSI connections.



The goal of this post was to discuss the points which are not very well explained in RecoverPoint documentation. It’s not a comprehensive guide by any means. You can find the full deployment procedure with prerequisites, installation and configuration steps in EMC RecoverPoint Installation and Deployment Guide.

Issue Joining VNX1 and VNX2 Unisphere Domains

March 21, 2016

no_SSLThe main benefit of using Unisphere Domains is that they give you ability to manage all of your VNXs by connecting to just one array. If you have an old Clariion you’ll have to use a so called Multi-Domain. VNX1 and VNX2 arrays can join a single domain.

Recently I’ve encountered an issue where this didn’t work quite well. When joining VNX1 to a VNX2 I got the following error:

CIMOM Can’t get the VNX hardware class from – ip – Error Connecting SSL. Error details: A system call error (errno=10057).


As it turned out EMC disabled SSL 3.0 support in recent Block OE versions. As a result it’s broken Unisphere Domain connectivity with arrays running Flare 32 Patch 209 or older, which still use SSL 3.0.

Solution is to upgrade Block OE to a version higher than Flare 32 Patch 209 where SSL 3.0 is disabled. Or as a workaround you can connect arrays in a Multi-Domain. To find out how, read one of my earlier blog posts: How to Configure VNX Unisphere Domains

How to Configure VNX Unisphere Domains

March 7, 2016

unisphere_domainVNX storage arrays have a concept of Unisphere Domains which let you manage multiple arrays from one Unisphere GUI. To manage two or more arrays from a single pain of glass you need to join their storage domains. There are typically two scenarios:

  1. Joining arrays of the same generation, such as VNX to VNX or Clariion to Clariion (VNX1 and VNX2 are considered as one generation)
  2. Joining arrays of different generations, such as Clariion to VNX

Same generation arrays

When joining same generation arrays to a single domain you get the benefit of having consistent domain-wide settings across all arrays in the domain, such as: DNS, NTP, LDAP and Global Users. If you go to Unisphere Home screen and click on the Domain button you will find where all domain-wide settings are configured. Once they are set up these settings propagate to all systems within the domain.


There’s also a concept of Domain Master, which keeps and distributes domain-wide settings. Domain Master can be changed manually if you wish to do so by using the Select Domain Master wizard.

To add a new system to the same domain simply click on Add/Remove Systems and follow the wizard.

Different generation arrays

It’s very uncommon to see a Clariion these days, but if you still have one and want to have a single management interface across both your Clariion and VNX arrays you have to use Multi-Domains. You won’t get the benefit of having the same domain-wide settings, but if you have just 2 or 3 arrays it’s not really that hard to set them up manually.

To add a new domain to Multi-Domain configuration click on Manage Multi-Domain Configurations, specify VNX Service Processor IP and assign a name. System will be added to the list of Selected Domains.


Always add another domain to Multi-Domain configuration on a system which is running the highest release of Unisphere within the Multi-Domain, otherwise you’ll get the following error:

This version of user interface software does not support the management server software versions on the provided system.


Once the system is added you will see both arrays in the systems list and will be able to manage both from one Unisphere interface. For the sake of demonstration I used two VNX arrays in the screenshot below. But the same process applies to Clarrion arrays.


Local and global users

Unisphere has two types of user accounts – local and global. Local account can manage the system you have connected to and global account can manage all systems within the same domain.

By default, when array is being installed, global security is initialized and one global user is created. There are no local user accounts on the system by default, which is fine, because each array is created as a member of its own local domain.

In a Multi-Domain configuration you need to make sure you’re logging in to Unisphere using an account, which exists in every domain being managed. Otherwise, each time you log in to Unisphere you will have to manually login to the remote domain on the domain tab, which is quite annoying.

If you have different accounts on each of the arrays, make sure to make them consistent across all systems.



In this post in a few simple steps we went through the Unisphere single domain and multi-domain configurations. If you want to know more details about Unisphere Domain management refer to EMC white paper “Domain Management with EMC Unisphere for VNX“.

VNX LDAP Integration: AD Nested Groups

February 11, 2015

Have you ever stumbled upon AD authentication issues on VNX, even though it all looked configured properly? LDAP integration has always been a PITA on storage arrays and blade chassis as usually there is no way to troubleshoot what the actual error is.


If VNX cannot lookup the user or group that you’re trying to authenticate against in AD, you’ll see just this. Now go figure why it’s getting upset about it. Even though you can clearly see the group configured in “Role Mapping” and there doesn’t seem to be any typos.

Common problem is Nested Groups. By default VNX only checks if your account is under the specified AD group and doesn’t traverse the hierarchy. So for example, if your account is under the group called IT_Admins in AD, IT_Admins is added to Domain Admins and Domain Admins is in “Role Mapping” – it’s not gonna work.


To make it work change “Nested Group Level” to something appropriate for you and this’d resolve the issue and make your life happier.

EMC Isilon Overview

February 20, 2014

isilon_logo_188x110OneFS Overview

EMC Isilon OneFS is a storage OS which was built from the ground up as a clustered system.

NetApp’s Clustered ONTAP for example has evolved from being an OS for HA-pair of storage controllers to a clustered system as a result of integration with Spinnaker intellectual property. It’s not necessarily bad, because cDOT shows better performance on SPECsfs2008 than Isilon, but these systems still have two core architectural differences:

1. Isilon doesn’t have RAIDs and complexities associated with them. You don’t choose RAID protection level. You don’t need to think about RAID groups and even load distribution between them. You don’t even have spare drives per se.

2. All data on Isilon system is kept on one volume, which is a one big distributed file system. cDOT use concept of infinite volumes, but bear in mind that each NetApp filer has it’s own file system beneath. If you have 24 NetApp nodes in a cluster, then you have 24 underlying file systems, even though they are viewed as a whole from the client standpoint.

This makes Isilon very easy to configure and operate. But its simplicity comes at a price of flexibility. Isilon web interface has few options to configure and not very feature rich.

Isilon Nodes and Networking

In a nutshell Isilon is a collection of a certain number of nodes connected via 20Gb/s DDR InfiniBand back-end network and either 1GB/s or 10GB/s front-end network for client connections. There are three types of Isilon nodes S-Series (SAS + SSD drives) for transactional random access I/O, X-Series (SATA + SSD drives) for high throughput applications and NL-series (SATA drives) for archival or not frequently used data.

If you choose to have two IB switches at the back-end, then you’ll have three subnets configured for internal network: int-a, int-b and failover. You can think of a failover network as a virtual network in front of int-a and int-b. So when the packet comes to failover network IP address, the actual IB interface that receives the packet is chosen dynamically. That helps to load-balance the traffic between two IB switches and makes this set up an active/active network.


On the front-end you can have as many subnets as you like. Subnets are split between pools of IP addresses. And you can add particular node interfaces to the pool. Each pool can have its own SmartConnect zone configured. SmartConnect is a way to load-balance connections between the nodes. Basically SmartConnect is a DNS server which runs on the Isilon side. You can have one SmartConnect service on a subnet level. And one SmartConnect zone (which is simply a domain) on each of the subnet pools. To set up SmartConnect you’ll need to assign an IP address to the SmartConnect service and set a SmartConnect zone name on a pool level. Then you create an “A” record on DNS for the SmartConnect service IP address and delegate SmartConnect DNS zone to this IP. That way each time you refer to the SmartConnect zone to get access to a file share you’ll be redirected to dynamically picked up node from the pool.


Each type of node is automatically assigned to what is called a “Node Pool”. Nodes are grouped to the same pool if they are of the same series, have the same amount of memory and disks of the same type and size. Node Pool level is one of the spots where you can configure protection level. We’ll talk about that later. Node Pools are grouped within Tiers. So you can group NL node pool with 1TB drives and NL node pool with 3TB drives into an archive tier if you wish. And then you have File Pool Policies which you can use to manage placement of files within the cluster. For example, you can redirect files with specific extension or file size or last access time to be saved on a specific node pool or tier. File pool policies also allow you to configure data protection and override the default node pool protection setting.

SmartPools is a concept that Isilon use to name Tier/Node Pool/File Pool Policy approach. File placement is not applied automatically, otherwise it would cause high I/O overhead. It’s implemented as a job on the cluster instead which runs at 22:00 every day by default.

Data Layout and Protection

Instead of using RAIDs, Isilon uses FEC (Forward Error Correction) and more specifically a Reed-Solomon algorithm to protect data on a cluster. It’s similar to RAID5 in how it generates a protection block (or blocks) for each stripe. But it happens on a software level, instead of hardware as in storage arrays. So when a file comes in to a node, Isilon splits the file in stripe units of 128KB each, generates one FEC protection unit and distributes all of them between the nodes using back-end network. This is what is called “+1” protection level, where Isilon can sustain one disk or one node failure. Then you have “+2”, “+3” and “+4”. In “+4” you have four FECs per stripe and can sustain four disk or node failures. Note however that there is a rule that the number of data stripe units in a stripe has to be greater than number of FEC units. So the minimum requirement for “+4” protection level is 9 nodes in a cluster.


The second option is to use mirroring. You can have from 2x to 8x mirrors of your data. And the third option is “+2:1” and “+3:1” protection levels. These protection levels let you balance between the data protection and amount of the FEC overhead. For example “+2:1” setting compared to “+2” can sustain two drive failures or one node failure, instead of two node failure protection that “+2” offers. And it makes sense, since simultaneous two node failure is unlikely to happen. There is also a difference in how the data is laid out. In “+2” for each stripe Isilon uses one disk on each node and in “+2:1” it uses two disks on each node. And first FEC in this case goes to first subset of disks and second goes to second.

One benefit of not having RAID is that you can set protection level with folder or even file granularity. Which is impossible with conventional RAIDs. And what’s quite handy, you can change protection levels without recreation of storage volumes, as you might have to do while transitioning between some of the RAID levels. When you change protection level for any of the targets, Isilon creates a low priority job which redistributes data within the cluster.