Number of Disk Stripes Per Object/Stripe Width
Number Of Disk Stripes Per Object, commonly referred to as stripe width, is the setting that defines the minimum number of capacity devices across which each replica of a storage object is distributed. vSAN may actually create more stripes than the number specified in the policy for reasons will will discuss later. In the UI Striping will appear as “RAID 0” with the number of components being driven primarily by what the stripe width was set to.
Striping may help performance if certain virtual machines are I/O intensive and others are not. With striping, a virtual machines data is spread across more drives which all contribute to the overall storage performance experienced by that virtual machine. In the case of hybrid,this striping would be across magnetic disks. In the case of all-flash, the striping would be across whatever flash devices are making up the capacity layer.
However, striping may not help performance if (a) an application is not especially I/O intensive, or (b) a virtual machine’s data is spread across devices that are already busy servicing other I/O intensive virtual machines.
Hybrid Reads On a hybrid cluster when the read cache rate is low, and the back end capacity disks are reaching congestion, raising the stripe level can increase read cache miss performance by spreading out the workload across multiple capacity devices and cache devices.
Streaming Write performance
In Hybrid and All Flash use cases large streaming write workloads can be improved in some cases by increasing stripe width. By increasing the available write buffer (by striping across multiple disk group) to a given workload this can increase the size of a burst write workload before congestions and back pressure will be applied. On the back end, using more capacity devices can also increase the de-stage speed (assuming the cache tier is not the bottleneck).
However, for the most part, VMware recommends leaving striping at the default value of 1 unless performance issues that might be alleviated by striping are observed. The default value for the stripe width is 1 whereas the maximum value is 12. The most common case if it is being raised is in Hybrid clusters using slower or fewer capacity devices encountering a high cache miss rate.
Stripe Width – Sizing Consideration
There are two main sizing considerations when it comes to stripe width. The first of these considerations is if there are enough physical devices in the various hosts and across the cluster to accommodate the requested stripe width, especially when there is also a Number Of Failures To Tolerate value to accommodate.
The second consideration is whether the value chosen for stripe width is going to require a significant number of components and consume the host component count. Both of these should be considered as part of any vSAN design, although considering the increase in maximum component count in 6.0 with on-disk format v2, this realistically isn’t a major concern anymore for most deployments.
Automatic Striping
In cases where a disk group does not contain enough space to hold a VMDK, automatic striping of objects will happen. Also, in cases where an object is over 255GB, it will automatically seperated into a separate component. This component could end up on the same disk, or on a different disk group (passively giving striping) but there is no deterministic decision to do this as placement is driven by capacity and not performance reasons.
Deduplication and Compression Stripes
It should be noted that when Deduplication and Compression are enabled, data will be naturally stripped across a disk group, as block matches may happen on any disk within a disk group. The stripe policy will still allow you to stripe across different disk groups if you choose.
Wrapping Up
In most real world use cases, we do not see significant performance increases from changing the striping policy. It is available and you should weigh the added complexity against the need before changing it from the default.
