Tag Archives: Write Through

Dare2Compare Part 4 : HPE provides superior resiliency than Nutanix?

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As discussed in Part 1, we have proven HPE have made false claims about Nutanix snapshot capabilities as part of the #HPEDare2Compare twitter campaign.

In part 2, I explained how HPE/Simplivity’s 10:1 data reduction HyperGuarantee is nothing more than smoke and mirrors and that most vendors can provide the same if not greater efficiencies, even without hardware acceleration.

In part 3, I corrected HPE on their false claim that Nutanix cannot support dedupe without 8vCPUs and in part 4, I will respond to the claim (below) that Nutanix has less resiliency than HPE Simplivity 380.

To start with, the biggest causes of data loss, downtime, outages etc in my experience are caused by human error. From poor design, improper use of a product, poor implementation/validation and a lack of operations procedures or discipline to follow procedures, the number of times I’ve seen properly designed solutions have issues I can count on one hand.

Those rare situations have came down to multiple concurrent failures at different levels of the solution (e.g.: Infrastructure, Application, OS etc), not just things like one or more drive or server failures.

None the less, HPE Simplivity are commonly targeting Resiliency Factor 2 (RF2) and claiming it not to be resilient because they lack a basic understanding of the Acropolis Distributed Storage Fabric and how it distributes data, rebuilds from failures and therefore how resilient it is.

RF2 is often mistakenly compared to RAID 5, where a single drive failure takes a long time to rebuild and subsequent failures during rebuilds are not uncommon which would lead to a data loss scenario (for RAID 5).

Lets talk about some failure scenarios comparing HPE Simplivity to Nutanix.

Note: The below information is accurate to the best of my knowledge and testing, experience with both products.

When is a write acknowledged to the Virtual machine

HPE Simplivity – They use what they refer to as an Omnistack Accelerator card (OAC) which uses “Super capacitors to provide power to the NVRAM upon a power loss”. When a write hits the OAC it is then acknowledged to the VM. It is assumed or even likely that the capacitors will provide sufficient power to commit the writes persistently to flash but the fact is that writes are acknowledged BEFORE it is committed to persistent media. HPE will surely argue the OAC is persistent, but until the data is on something such as a SATA-SSD drive I do not consider it persistent and invite you to ask your trusted advisor/s their option because this is a grey area at best.

This can be confirmed on Page 29 of the SimpliVity Hyperconverged Infrastructure Technology Overview:

OACPowerLossLol

Nutanix – Writes are only acknowledged to the Virtual Machine when the write IO has been checksummed and confirmed written to persistent media (e.g.: SATA-SSD) on the number of nodes/drives based on the configured Resiliency Factor (RF).

Writes are never written to RAM or any other non persistent media and at any stage you can pull the power from a Nutanix node/block/cluster and 100% of the data will be in a consistent state. i.e.: It was written and acknowledged, or it was not written and therefore not acknowledged.

The fact Nutanix only acknowledges writes when data is written to persistent media on two or more hosts makes the platform compliant with FUA and Write Through which for HPE SVT, in the best case is dependant on power protection (UPS and/or OAC Capacitors) means Nutanix is more resilient (less risk) and has a higher level of data integrity than the HPE SVT product.

Checkout “Ensuring Data Integrity with Nutanix – Part 2 – Forced Unit Access (FUA) & Write Through” for more information and this will explain how Nutanix is compliant to critical data integrity protocols such as FUA and Write through and you can make your mind up if the HPE product is or not. Hint: A product is not compliant to FUA unless data is written to persistent media before acknowledgement.

Single Drive (NVMe/SSD/HDD) failure

HPE Simplivity – Protects data with RAID 6 (or RAID 5 on small nodes) + Replication (2 copies). A single drive failure causes a RAID rebuild which is a medium/high impact activity for the RAID group. RAID rebuilds are well known to be slow, this is one reason why HPE chooses (and wisely so) to use low capacity spindles to minimise the impact of RAID rebuilds. But this choice to use RAID and smaller drives has implications around cost/capacity/rack unit/power/cooling and so on.

Nutanix – Protects data with configurable Replication Factor (2 or 3 copies, or N+1 and N+2) along with rack unit (block) awareness. A single drive failure causes a distributed rebuild of the data contained on the failed drive across all nodes within the cluster. This distributed rebuild is evenly balanced throughout the cluster for low impact and faster time to recover. This allows Nutanix to support large capacity spindles, such as 8TB SATA.

Two concurrent drive (NVMe/SSD/HDD) failures *Same Node

HPE Simplivity – RAID 6 + Replication (2 copies) supports the loss of two drive failures and as with a single drive failure causes a RAID rebuild which is a medium/high impact activity for the RAID group.

Nutanix – Two drive failure causes a distributed rebuild of the data contained on the failed drives across all nodes within the cluster. This distributed rebuild is evenly balanced throughout the cluster for low impact and faster time to recover. This allows Nutanix to support large capacity spindles, such as 8TB SATA. No data is lost even when using Resiliency Factor 2 (which is N+1), despite what HPE claims. This is an example of the major advantage Nutanix Acropolis Distributed File System has over the RAID and mirroring type architecture of HPE SVT.

Three concurrent drive (NVMe/SSD/HDD) failures *Same Node

HPE Simplivity – RAID 6 + Replication (2 copies) supports the loss of only two drives per RAID group, at this stage the RAID group has failed and all data must be rebuilt.

Nutanix – Three drive failures again just causes a distributed rebuild of the data contained on the failed drives (in this case, 3) across all nodes within the cluster. This distributed rebuild is evenly balanced throughout the cluster for low impact and faster time to recover. This allows Nutanix to support large capacity spindles, such as 8TB SATA. No data is lost even when using Resiliency Factor 2 (which is N+1). Again, despite what HPE claims. This is an example of the major advantage Nutanix Acropolis Distributed File System has over the RAID and mirroring type architecture of HPE SVT.

Four or more concurrent drive (NVMe/SSD/HDD) failures *Same Node

HPE Simplivity – The RAID 6 + Replication (2 copies) supports the loss of only two drives per RAID group, any failures 3 or more result in a failure RAID group and a total rebuild of the data is required.

Nutanix – Nutanix can support N-1 drive failures per node, meaning in a 24 drive system, such as the NX-8150, 23 drives can be lost concurrently without the node going offline and without any data loss. The only caveat is the lone surviving drive for a hybrid platform must be an SSD. This is an example of the major advantage Nutanix Acropolis Distributed File System has over the RAID and mirroring type architecture of HPE SVT.

Next let’s cover off failure scenarios across multiple nodes.

Two concurrent drive (NVMe/SSD/HDD) failures in the same cluster.

HPE Simplivity – RAID 6 protects from 2 drive failures locally perRAID group whereas Replication (2 copies) supports the loss of one copy (N-1). Assuming the RAID groups are intact, data would not be lost.

Nutanix – Nutanix has configurable resiliency (Resiliency Factor) of either 2 copies (RF2) or three copies (RF3). Using RF3, under any two drive failure scenario there is no data loss and it causes a distributed rebuild of the data contained on the failed drives across all nodes within the cluster.

When using RF2 and block (rack unit) awareness, in the event two or more drives fail within a block (which is up to 4 nodes of 24 SSDs/HDDs), there is no data loss. In fact, in this configuration Nutanix can support the loss of up to 24 drives concurrently e.g.: 4 entire nodes and 24 drives without data loss/unavailability.

When using RF3 and block awareness, Nutanix can support the loss of up to 48 drives concurrently e.g.: 8 entire nodes and 48 drives without data loss/unavailability.

Under no circumstances can HPE Simplivity support the loss of ANY 48 drives (e.g.: 2 HPE SVT nodes w/ 24 drives each) and maintain data availability.

This is another example of the major advantage Nutanix Acropolis Distributed File System has over the RAID and mirroring type architecture of HPE SVT. Nutanix distributes all data throughout the ADSF cluster, which is something HPE SVT cannot do which impacts both performance and resiliency.

Two concurrent node (NVMe/SSD/HDD) failures in the same cluster.

HPE Simplivity – If the two HPE SVT nodes mirroring the data both go offline, you have data unavailability at best, with data loss at worst. As HPE SVT is not a cluster, (note the careful use of the term “Federation”) it scales essentially in pairs and each pair cannot fail concurrently.

Nutanix – With RF3 even without the use of block awareness, any two nodes and all drives within those nodes can be lost, with no data unavailability.

Three or more concurrent node (NVMe/SSD/HDD) failures in the same cluster.

HPE Simplivity – As previously discussed, HPE SVT cannot support the loss of any two nodes, so three or more makes matters worse.

Nutanix – With RF3 and block awareness, up to eight (yes 8!!) can be lost along with all drives within those nodes, with no data unavailability. That’s up to 48 SSD/HDDs concurrently failing without data loss.

So we can clearly see Nutanix provides a highly resilient platform and there are numerous configurations which ensure two drive failures do not cause data loss despite what the HPE campaign suggests.

https://twitter.com/HPE_SimpliVity/status/872073088569139200

The above tweet would be like me configuring a HPE Proliant server with RAID 5 and complaining HPE lost my data when two drive fails, it’s just ridiculous.

The key point here is when deploying any technology to understand your requirements and configure the underlying platform to meet/exceed your resiliency requirements.

Installation/Configuration

HPE Simplivity – Dependant on vCenter.

Nutanix – Uses PRISM which is a fully distributed HTML 5 GUI with no external dependancies regardless of Hypervisor choice (ESXi, AHV, Hyper-V and XenServer). In the event any hypervisor management tool (e.g.: vCenter) is down, PRISM is fully functional.

Management (GUI)

HPE Simplivity – Uses a vCenter backed GUI. If vCenter is down, Simplivity cannot be fully managed. In the event a vCenter goes down, best case scenario vCenter HA is used, then management will have a short interruption.

Nutanix – Uses PRISM which is a fully distributed HTML 5 GUI with no external dependancies regardless of Hypervisor choice (ESXi, AHV, Hyper-V and XenServer). In the event any hypervisor management tool (e.g.: vCenter) is down, PRISM is fully functional.

In the event of a node/s failing, PRISM being a distributed management layer continues to operate.

Data Availability:

HPE Simplivity – RAID 6 (or RAID 60) + Replication (2 copies), Deduplication and Compression for all data. Not configurable.

Nutanix – Configurable resiliency and data reduction with:

  1. Resiliency Factor 2 (RF2)
  2. Resiliency Factor 3 (RF3)
  3. Resiliency Factor 2 with Block Awareness
  4. Resiliency Factor 3 with Block Awareness
  5. Erasure Coding / Deduplication / Compression in any combination across all resiliency types.

Key point:

Nutanix can scale out with compute+storage OR storage only nodes, in either case, resiliency of the cluster is increased as all nodes (or better said, Controllers) in our distributed storage fabric (ADSF) help with the distributed rebuild in the event of drive/s or node/s failures. Therefore restoring the cluster to a fully resilient state faster, to therefore be able to support subsequent failures.

HPE Simplivity – Due to HPE SVTs platform not being a distributed file system, and working in a mirror style configuration, adding additional nodes to the “per datacenter” limit of eight (8) does not increase resiliency. As such the platform does not improve as it grows which is a strength of the Nutanix platform.

Summary:

Thanks to our Acropolis Distributed Storage Fabric (ADSF) and without the use of legacy RAID technology, Nutanix can support:

  1. Equal or more concurrent drive failures per node than HPE Simplivity
  2. Equal or more concurrent drive failures per cluster than HPE Simplivity
  3. Equal or more concurrent node failures than HPE Simplivity
  4. Failure of hypervisor management layer e.g.: vCenter with full GUI functionality

Nutanix also has the follow capabilities over and above the HPE SVT offering:

  1. Configurable resiliency and data reduction on a per vDisk level
  2. Nutanix resiliency/recoverability improves as the cluster grows
  3. Nutanix does not require any UPS or power protection to be compliant with FUA & Write Through

HPE SVT is less resilient during the write path because:

  1. HPE SVT acknowledge writes before committing data to persistent media (by their own admission)

Return to the Dare2Compare Index:

Why Nutanix Acropolis hypervisor (AHV) is the next generation hypervisor – Part 5 – Resiliency

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When discussing resiliency, it is common to make the mistake of only looking at data resiliency and not considering resiliency of the storage controllers and the management components required to service the business applications.

Legacy technologies such as RAID and Hot Spare drives may in some cases provide high resiliency for data, however if they are backed by a dual controller type setups which cannot scale out and self heal, the data may be unavailable or performance/functionality severely degraded following even a single component failure. Infrastructure that is dependant on HW replacement to restore resiliency following a failure is fundamentally flawed as I have discussed in: Hardware support contracts & why 24×7 4 hour onsite should no longer be required.

In addition if the management application layer is not resilient, then data layer high-availability/resiliency may be irrelevant as the business applications may not be functioning properly (i.e.: At normal speeds) or at all.

The Acropolis platform provides high resiliency for both the data and management layers at a configurable N+1 or N+2 level (Resiliency Factor 2 or 3) which can tolerate up to two concurrent node failures without losing access to Management or data. In saying that, with “Block Awareness”, an entire block (up to four nodes) can fail and the cluster still maintains full functionality. This puts the resiliency of data and management components on XCP up to N+4.

In addition, the larger the XCP cluster, the lower the impact of a node/controller/component failure. For a four node environment, N-1 is 25% impact whereas for an 8 node cluster N-1 is just a 12.5% impact. The larger the cluster the lower the impact of a controller/node failure. In contrast a dual controller SAN has a single controller failure, and in many cases the impact is 50% degradation and a subsequent failure would result in an outage. Nutanix XCP environments self heal so that even for an environment only configured for N-1, it is possible following a self heal than subsequent failures can be tolerated without causing high impact or outages.

In the event the Acropolis Master instance fails, full functionality will return to the environment after an election which completes within <30 seconds. This equates to management availability greater than “six nines” (99.9999%). Importantly, AHV has this management resiliency built-in; it requires zero configuration!

For more information see: Acropolis: Scalability

As for data availability, regardless of hypervisor the Nutanix Distributed Storage Fabric (DSF) maintains two or three copies of data/parity and in the event of a SSD/HDD or node failure, the configured RF is restored by all nodes within the cluster.

Data Resiliency

While we have just covered why resiliency of data is not the only important factor, it is still key. After all, if a solution which provides shared storage looses data, its not fit for purpose in any datacenter.

As data resiliency is such a foundation to the Nutanix Distributed Storage Fabric, the Data resiliency status is displayed on the Prism Home Screen. In the below screenshot we can see is that the ability to provide resiliency in both steady state and in the event of a failure (Rebuild Capacity) are both tracked.

In this example, all data in the cluster is compliant with the configured Resiliency Factor (RF2 or 3) and the cluster has at least N+1 available capacity to rebuild after the loss of a node.

dataresiliency1

To dive deeper into the resiliency status, simply click on the above box and it will expand to show more granular detail of the failures which can be tolerated.

The below screen shot shows things like Metadata, OpLog (Persistent Write Cache) and back end functions such as Zookeeper are also monitored and alerted when required.

resiliency2

In the event either of these is not in a normal or “Green” state, PRISM will alert the administrator. In the event the alert is the cause of a node failure, Prism automatically notifies Nutanix support (via Pulse) and dispatches the required part/s, although typically an XCP cluster will self-heal long before delivery of hardware even in the case of an aggressive Hardware Maintenance SLA such as 4hr Onsite.

This is yet another example of Nutanix not being dependent on Hardware (replacement) for resiliency.

Data Integrity

Acknowledging a Write I/O to a guest operating system should only occur once the data is written to persistent media because until this point, it is possible for data loss to occur even when storage is protected by battery backed cache and uninterruptible power supplies (UPS).

The only advantage to acknowledging writes before this has occurred is performance, but what good is performance when your data lacks integrity or is lost?

Another commonly overlooked requirement of any enterprise grade storage solution is the ability to detect and recover from Silent Data Corruption. Acropolis performs checksums in software for every write AND on every read. Importantly Nutanix is in no way dependent on the underlying hardware or any 3rd party software to maintain data integrity, all check summing and remediation (where required) is handled natively.

Pro tip: If a storage solution does not perform checksums on Write AND Read, DO NOT use it for production data.

In the event of Silent Data Corruption (which can impact any storage device from any vendor), the checksum will fail and the I/O will be serviced from another replica which is stored on a different node (and therefore physical SSD/HDD). If a checksum fails in an environment with Erasure Coding, EC-X recalculates the data the same way as if a HDD/SSD failed and services the I/O.

In the background, the Nutanix Distributed Storage Fabric will discard the corrupted data and restore the configured Resiliency Factor from the good replica or stripe where EC-X is used.

This process is completely transparent to the virtual machine and end user, but is a critical component of the XCP’s resiliency. The underlying Distributed Storage Fabric (DFS) also automatically protects all Acropolis management components, this is an example of one of the many advantages of the Acropolis architecture where all components are built together, not bolted on afterwards.

An Acropolis environment with a container configured with RF3 (Replication Factor 3) provides N+2 management availability. As a result, it would take an extraordinarily unlikely failure of three concurrent node failures before a management outage could potentially occur. Luckily XCP has an answer for this albeit unlikely scenario as well, Block Awareness is a capability where with 3 or more blocks the cluster can tolerate the failure of an entire block (up to 4 nodes) without causing data or management to go offline.

Part of the Acropolis story around resiliency goes back to the lack of complexity. Acropolis enables rolling 1-click upgrades and includes all functionality. There is no single point of failure; in the worst-case scenario if the node with Acropolis master fails, within 30 seconds the Master role will restart on a surviving node and initiate VMs to power on. Again this is in-built functionality, not additional or 3rd party solutions which need to be designed/installed & maintained.

The above points are largely functions of the XCP rather than AHV itself, so I thought I would highlight a AHV’s Load Balancing and failover capabilities.

Unlike traditional 3-tier infrastructure (i.e.: SAN/NAS) Nutanix solutions do not require multi-pathing as all I/O is serviced by the local controller. As a result, there is no multi-pathing policy to choose which removes another layer of complexity and potential point of failure.

However in the event of the local CVM being unavailable for any reason we need to service I/O for all the VMs on the node in the most efficient manner. AHV does this by redirecting I/O on a per vDisk level to a random remote stargate instance as shown below.

pervmpathfailover

AHV can do this because every vdisk is presented via iSCSI and is its own target/LUN which means it has its own TCP connection. What this means is a business critical application such as MS SQL / Exchange or Oracle with multiple vDisks will be serviced by multiple controllers concurrently.

As a result all VM I/O is load balanced across the entire Acropolis cluster which ensures no single CVM becomes a bottleneck and VMs enjoy excellent performance even in a failure or maintenance scenario.

For more information see: Acropolis Hypervisor (AHV) I/O Failover & Load Balancing

Summary:

  1. Out of the box self healing capabilities for:
    1. SSD/HDD/Node failure/s
    2. Acropolis and PRISM (Management layer)
  2. In-Built Data Integrity with software based checksums
  3. Ability to tolerate up to 4 concurrent node failures
  4. Management availability of >99.9999 (Six “Nines”)
  5. No dependency on Hardware for data or management resiliency

For more information see: Ensuring Data Integrity with Nutanix – Part 2 – Forced Unit Access (FUA) & Write Through

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Integrity of I/O for VMs on NFS Datastores – Part 1 – Emulation of the SCSI Protocol

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This is the first of a series of posts covering how the Integrity of I/O is ensured for Virtual Machines when writing to VMDK/s (Virtual SCSI Hard Drives) running on NFS datastores presented via VMware’s ESXi hypervisor as a “Datastore”.

Note: To be crystal clear, this post is not talking about presenting NFS direct to Windows or any other guest operating system.

This process is patented (US7865663) by VMware and its inventors and on the patent the process is called “SCSI Protocol Emulation”.

This series will first cover the topics in a vendor agnostic manner, meaning I am talking in general about VMware + any NFS storage on the VMware HCL with NFS support.

Following the vendor agnostic posts, I will follow with a series of posts focusing specifically on Nutanix, as the motivation for the series was to cover off this topic for existing or potential Nutanix customers, some of whom are less familiar with NFS and have asked for clarification, especially around virtualizing Business Critical Applications (vBCA) such as Microsoft SQL and Exchange.

The below diagram visualizes shows how storage can be presented to an ESXi host and what this series will focus on.

A VM accesses its .vmx and .vmdk file/s via a datastore the same way, regardless of the underlying storage protocol (DAS SCSI, iSCSI , NFS , FCP).

GUID-AD71704F-67E4-4AC2-9C22-10B531755566-high

In the case of NFS datastores, SCSI protocol emulation is used to allow the Guest Operating System (OS) and application/s to read and write via SCSI even when the underlying storage (which is abstracted by the hypervisor) is served via NFS which does not natively support the same commands.

Image Source: https://pubs.vmware.com/vsphere-50/index.jsp?topic=%2Fcom.vmware.vsphere.introduction.doc_50%2FGUID-2E7DB290-2A07-4F54-9199-B68FCB210BBA.html

In the following section, and throughout this series, many images shown are from the patent (US7865663) and are the property of the patent owners, not the author of this article.

The areas which I will be focusing on are the ones where there has been the most concern in the industry, especially for business critical applications, such as Microsoft SQL and Microsoft Exchange, being how are the VM operating system and application/s (or data integrity) are impacted when issuing commands when the storage is abstracted by the hypervisor and served to via NFS which does not have equivalent I/O commands as SCSI.

Some examples areas of concern around the industry for VMs running on datastores backed by NFS are:

1. SCSI Aborts / Resets
2. Forced Unit Access (FUA) & Write Through
3. Write Ordering
4. Torn I/O (Writes + Reads)

In this first part, we will look at the SCSI Protocol Emulation process and discuss SCSI Aborts and Resets and how the SCSI protocol emulation process deals with these.

Below is a diagram showing the flow of an I/O request for a VM writing SCSI commands to a VMDK (formatted as NTFS) through the SCSI emulation process and through to the NFS storage.

US07865663-20110104-D00005

The first few steps in my opinion are fairly self explanatory, where it gets interesting for me, and one of the points of contention among I.T professional (being SCSI aborts) is described in the box labelled “550“.

If the SCSI command is an abort (which has no equivalent in the NFS protocol), the SCSI emulation process removes the corresponding request from the virtual SCSI request list created in the previous step (box labelled “540“).

The same is true if the SCSI command is a reset (which also has no equivalent in the NFS protocol), the SCSI emulation process removes the corresponding request from the virtual SCSI request list. This process is shown below in the box labelled “560”

US07865663-20110104-D00006

Next lets look at what happens if the SCSI “abort” or “reset” command is issued after the SCSI emulation process has passed on the command to the storage and is now receiving a reply to a command which the Guest OS / Application has aborted?

Its quite simple, the SCSI emulation process receives a reply from the NFS server, looks up the corresponding tag in the Virtual SCSI request list, and because this corresponding tag does not exist, the emulator drops the reply therefore emulating a SCSI abort command.

The process is shown below from box labelled “710” to “720” and finishing at “730“.

US07865663-20110104-D00007

In the patent, the above process is summed up nicely in the following paragraph.

Accordingly, a faithful emulation of SCSI aborts and resets, where the guest OS has total control over which commands are aborted and retried can be achieved by keeping a virtual SCSI request list of outstanding requests that have been sent to the NFS server. When the response to a request comes back, an attempt is made to find a matching request in the virtual SCSI request list. If successful, the matching request is removed from the list and the result of the response is returned to the virtual machine. If a matching request is not found in the virtual SCSI request list, the results are thrown away, dropped, ignored or the like.

So there we have it, that is how VMware’s patented SCSI Protocol emulation allows SCSI commands not supported natively by NFS to be honoured, therefore allowing applications dependant on Block based storage to be ran successfully within a VM where its VMDK is backed by NFS storage.

Let’s recap what we have learned so far.

1. The SCSI Commands, abort & reset have no equivalent in the NFS protocol.
2. The VMware SCSI Emulation process handles SCSI commands not supported natively by NFS thanks to the Virtual SCSI Request List.
3. Guest Operating Systems and Applications running in Virtual Machines on ESXi issue native SCSI commands to the NTFS volume, which is presented to the VM via a VMDK and housed on an NFS datastore.
4. The underlying NFS protocol is not exposed to the Guest OS, Application/s or Virtual Machine.
5. The SCSI Commands, abort & reset are emulated by the hyper visor through removing these requests from the Virtual SCSI emulation list.

In part two, I will discuss Forced Unit Access (FUA) & Write Through.

Integrity of Write I/O for VMs on NFS Datastores Series

Part 1 – Emulation of the SCSI Protocol
Part 2 – Forced Unit Access (FUA) & Write Through
Part 3 – Write Ordering
Part 4 – Torn Writes
Part 5 – Data Corruption

Nutanix Specific Articles

Part 6 – Emulation of the SCSI Protocol (Coming soon)
Part 7 – Forced Unit Access (FUA) & Write Through (Coming soon)
Part 8 – Write Ordering (Coming soon)
Part 9 – Torn I/O Protection (Coming soon)
Part 10 – Data Corruption (Coming soon)

Related Articles

1. What does Exchange running in a VMDK on NFS datastore look like to the Guest OS?
2. Support for Exchange Databases running within VMDKs on NFS datastores (TechNet)
3. Microsoft Exchange Improvements Suggestions Forum – Exchange on NFS/SMB
4. Virtualizing Exchange on vSphere with NFS backed storage?