This is the fifth part of a series of posts covering how the Integrity of Write 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”.

This part will focus on Data Corruption.

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

So why am I covering data corruption? Simple, because there is a misconception that SCSI commands are not properly supported for VMs running on NFS datastores which leads to corruption. This was covered in Part 1, so Part 5 will focus on data corruption not specific to NFS, but which can effect all storage platforms and how it occurs, then how storage solutions can mitigate the risk of data corruption issues.

The following data is a summary of the data provided in An analysis of data corruption in the storage stack.

Netapp conducted a large scale study into data corruption, which covered >1 Million HDDs across tens of thousands of Netapp systems over 41 months (2004 – 2007) and long story short, Netapp detected a level of data corruption which surprised me and seems to disprove many things like advertised MTBF for HDDs.

The following shows a breakdown of the problems found.

The first thing I noticed in the above pie charts is the vast difference between the percentage of failures in Enterprise grade disks (left) and nearline based disks (right).

It also shows physical interconnects to be a large percentage of failures, which highlights the need for simplicity in the storage solution. In addition, one of the more surprising results in the level of storage protocol and performance based failures being the cause of corruption.

Note: In this study, the majority of systems deployed were FC (Block storage based) based, this highlights that a storage protocol itself regardless of being block or file based storage, can have issues if improperly implemented. So regardless of storage protocol, corruption can occur.

The below summary of corruption type and percentage of disks effected shows the dramatic 10x more issues with SATA drives compared to Enterprise grade drives.

The above also shows bit corruptions or Torn Writes effect more disks compared to lost or misdirected writes, which highlights the importance of Torn I/O Protection (covered in Part 4).

The article summarizes in the following points:

The main take away from my perspective is:

1. The requirement to have corruption handling mechanisms for any environment running workloads which require data integrity.

2. Data should be spread out (ideally across disks) to minimize the chance of issues.

The article went on to form these conclusions:

In Summary:

1. Data corruption can occur on JBOD , enterprise grade storage solutions and everything in between.

2. SATA drives have a much higher rate (~10x) of corruption.

3. Enterprise grade drives are much better from a data integrity perspective.

4. Corruption handling via sector and ideally block based checksums is essential on writes.

5. Using a checksum on Read helps detect corrupted data.

6. Corruption can occur even when no ECC errors are reported by a physical HDD.

7. Any storage protocol implementation can have bugs which can lead to corruption.

8. Backup / Recovery solutions are essential. Reliance solely on primary storage or application level backups using disks puts your data at risk.

9. Solutions solely dependant on application level data protection on disk are at risk of corrupted data being replicated to other active/passive or backup copies.

My final point, in an enterprise grade storage solutions which use checksums to verify data integrity on write and reads, have a much lower risk of data corruption regardless of media type and storage protocol.

JBOD style deployments using SATA drives have a significantly higher risk of data corruption which is contributed to by the SATA drives 10x higher corruption rates and the lack of enterprise grade checksum features found in some shared storage (SAN/NAS) solutions.

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)

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