Tag Archives: datastore

How to successfully Virtualize MS Exchange – Part 7 – Storage Options

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When virtualizing Exchange, we not only have to consider the Compute (CPU/RAM) and Network, but also the storage to provide both the capacity and IOPS required.

However before considering IOPS and capacity, we need to decide how we will provide storage for Exchange as storage can be presented to a Virtual Machine in many ways.

This post will cover the different ways storage can be presented to ESXi and used for Exchange while subsequent posts will cover in detail each of the options discussed.

First lets discuss Local Storage.

What I mean by Local Storage is SSD/HDDs within a physical ESXi hosts that is not shared (e.g.: Not accessible by other hosts).

This is probably the most basic form of storage we can present to ESXi and apart from the Hypervisor layer could be considered similar to a physical Exchange deployment.

UseLocalStorage

Next lets discuss Raw Device Mappings.

Raw Device Mappings or “RDMs” are where shared storage from a SAN is presented through the hypervisor to the guest as a native SCSI device and enables.

RDMs

For more information about Raw Device Mappings, see: About Raw Device Mappings

The next option is Presenting Storage direct to the Guest OS.

It is possible and sometime advantageous to presents SAN/NAS storage direct to the Guest OS via NFS , iSCSI or SMB 3.0 and bypasses the hyper-visor all together.

DirectInGuest

The final option we will discuss is “Datastores“.

Datastores are probably the most common way to present storage to ESXi. Datastores can be Block or File based, and presented via iSCSI , NFS or FCP (FC / FCoE) as of vSphere 5.5.

Datastores are basically just LUNs or NFS mounts. If the datastore is backed by a LUN, it will be formatted with Virtual Machine File System (VMFS) whereas NFS datastores are simply NFS 3 mounts with no formatting done by ESXi.

ViaDatastore

For more information about VMFS see: Virtual Machine File System Technical Overview.

What do all the above options have in common?

Local storage, RDMs, storage presented to the Guest OS directly and Datastores can all be protected by RAID or be JBOD deployments with no data protection at the storage layer.

Importantly, none of the four options on their own guarantee data protection or integrity, that is, prevent data loss or corruption. Protecting from data loss or corruption is a separate topic which I will cover in a non Exchange specific post.

So regardless of the way you present your storage to ESXi or the VM, how you ensure data protection and integrity needs to be considered.

In summary, there are four main ways (listed below) to present storage to ESXi which can be used for Exchange each with different considerations around Availability, Performance, Scalability, Cost , Complexity and support.

1. Local Storage (Part 8)
2. Raw Device Mappings  (Part 9)
3. Direct to the Guest OS (Part 10)
4. Datastores (Part 11)

In the next four parts, each of these storage options for MS Exchange will be discussed in detail.

Back to the Index of How to successfully Virtualize MS Exchange.

Integrity of I/O for VMs on NFS Datastores – Part 3 – Write Ordering

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This is the third part 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”.
As a reminder from the first post, this post is not talking about presenting NFS direct to Windows.

 

Write Ordering / Order Preservation

Another common concern when running business critical applications such as MS SQL and MS Exchange is Write Ordering and if/how this is handled by the SCSI protocol emulation process.

This requirement is described by Microsoft as:

The order of the I/O operations associated with SQL Server must be maintained. The system must maintain write ordering or it breaks the WAL protocol as described in this paper. (The log records must be written out in correct order and the log records must always be written to stable media before the data pages that the log records represent are written.) After a transaction log record is successfully flushed, the associated data page can then be flushed as well. If the subsystem allows the data page to reach stable media before the log record does, data integrity is breached.

Source: Microsoft SQL Server I/O basics.

VMware have released a Knowledge Base article specifically on this topic which states the following.

Write ordering and write-through integrity for NFS storage are both satisfied with NFS in an VMware ESX environment.
An NFS datastore, when mounted on an ESX host, goes through virtual SCSI emulation. A virtual machine disk (VMDK) file on an NFS datastore appears as a SCSI disk within the virtual machine’s guest operating system, which is no different than one residing on a VMFS volume over FCP or iSCSI protocol. Therefore, write ordering and write-through integrity are no different than those with block based storage (such as iSCSI or FCP protocol).
The above is the bulk of the article, but the full article can be found below.

Maintaining write ordering and write-through integrity using NFS in an ESX environment (KB1012143)

So as with Forced Unit Access (FUA) & Write-Through, Write Ordering is supported by VMware but even with this support, it is also a function of the underlying storage to honour the request and this process or even support may vary from storage vendor to storage vendor.

Again the point here is this process is delivered by the VMDK at the hypervisor level and passed onto the underlying storage, so regardless of the protocol being Block (iSCSI/FCP) or File based (NFS) it is the responsibility of the storage solution once the I/O is passed to it from the hypervisor.

In part four, I will discuss Torn I/O Protection.

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

Integrity of I/O for VMs on NFS Datastores – Part 2 – Forced Unit Access (FUA) & Write Through

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This is the second part of this series and the focus of this post is to cover a critical requirement for many applications including MS SQL and MS Exchange (which is designed to work with Block based storage) to operate as designed and to ensure data integrity is support for Forced Unit Access (FUA) & Write Through.

As a reminder from the first post, this post is not talking about presenting NFS direct to Windows.

The key here is for the storage solution to honour the “Write-to-stable” media intent and not depend on potentially vulnerable caching/buffering solutions using non persistent media which may require battery backing.

Microsoft have a Knowledge base article relating to the requirements for SQL Server, which details the FUA & Write Through requirements, along with other requirements covered in this series which I would recommend reading.

Key factors to consider when evaluating third-party file cache systems with SQL Server

Forced Unit Access (FUA) & Write-Through is supported by VMware but even with this support, it is also a function of the underlying storage to honour the request and this process or even support may vary from storage vendor to storage vendor.

A key point here is this process is delivered by the VMDK at the hypervisor level and passed onto the underlying storage, so regardless of the protocol being Block (iSCSI/FCP) or File based (NFS) it is the responsibility of the storage solution once the I/O is passed to it from the hypervisor.

Where a write cache on non persistent media (ie: RAM) is used, the storage vendor needs to ensure that in the event of a power outage there is sufficient battery backing to enable the cache to be de-staged to persistent media (ie: SSD / SAS / SATA).

Some solutions use Mirrored Write Cache to attempt to mitigate the risk of power outages causing issues but this could be argued to be not in compliance with the FUA which intends the Write I/O to be committed to stable media BEFORE the I/O is acknowledged as written.

If the solution does not ensure data is written to persistent media, it is not compliant and applications requiring FUA & Write-Through will likely be impacted at some point.

As I work for a storage vendor, I wont go into detail about any other vendor, but I will have an upcoming post on how Nutanix is in compliance with FUA & Write-Through.

In part three, I will discuss Write Ordering.

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