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

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

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