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How to analyze the storage strategy and application of vsan virtual machine? in view of this problem, this article introduces the corresponding analysis and solution in detail, hoping to help more partners who want to solve this problem to find a more simple and feasible method.

Virtual machine storage policy of VSAN

VSAN's virtual machine storage policy has five functions, or five rules (Rule). From the support of various disk array manufacturers to Virtual Volumes, we can see that the rules covered by VMware SPBM are much richer than the five rules of VSAN. With the continuous development of VSAN in data services (Data Services, that is, storage functions), more rules will be supported in the future. In the new version of VSAN, deduplication, erasure, and QoS (IOPS Limit) are also included in the storage policy.

In VSAN, each defined policy is actually a combination of five rules, namely a rule set (Rule-Set). We can see these five rules in the following figure, which will be described in detail in the top-to-bottom order of the drop-down list in the figure.

Five rules of virtual machine storage policy of VSAN

1) number of disk strips per object (SW)

Number of disk stripes per object: the number of disk stripes per object (Stripe Width, SW) refers to the number of disks across the persistence layer of each copy of the virtual machine object, that is, the stripe width of each copy. A value greater than 1 may produce better performance, but it can also result in the use of more system resources.

Stripe width of virtual machine storage policy

In a mixed configuration, the stripes are scattered across the disk. In an all-flash configuration, striping may occur in the SSD that makes up the persistence layer.

It is important to emphasize that VSAN currently relies mainly on SSD in the cache layer to ensure performance. All writes are written to the SSD of the cache layer first, so increasing the stripe width does not necessarily lead to a performance improvement. Only two cases in the mixed configuration can ensure that increasing the stripe width can increase performance: one is that during write operations, if there is a large amount of data from the SSD cache layer Destage (brush) to the HDD; second read operation, if there is a large amount of data missed in the SSD cache layer. Because concurrency of multiple blocks of HDD can improve performance in both cases.

The default value is 1. The maximum value is 12. VMware does not recommend changing the default stripe width.

2) Flash read cache reservation

Flash read cache reservation (%): flash read cache reservation refers to the flash capacity reserved as the read cache of the virtual machine object as a percentage of the logical size of the virtual machine disk (VMDK), which can be accurate to up to 4 decimal places, such as 2 TB VMDK. If the reserved percentage is 0.1%, the flash capacity reserved by the cache is 2.048 GB. The reserved flash capacity cannot be used by other objects. Unreserved flash memory is fairly shared among all objects. This option should only be used to resolve specific performance issues.

This rule is not supported in an all-flash configuration, so when defining a virtual machine storage policy, you should not change its default value. VSAN only supports the use of this property for mixed configurations. You do not need to set a reservation to get the cache. By default, VSAN dynamically allocates read cache for storage objects on demand. This is the most flexible and optimal use of resources. Therefore, there is usually no need to change the default value of 0 for this parameter.

If you want to increase this value when resolving performance issues, be careful. If you overallocate cache reservation space among multiple virtual machines, be careful whether it may cause SSD space to be wasted due to overreservation and cannot be used for workloads that require a certain amount of space at a given time. This may affect some performance. The default value is 0%. The maximum value is 100%.

3) allowed number of failures (FTT)

Number of failures to tolerate: the number of failures allowed (hereafter referred to as FTT) defines the number of host and device failures allowed by the virtual machine object. If FTT is n, the number of replicas of virtual machine objects created is n, and the number of witnesses is n, so the number of hosts required for storage is the number of replicas + witnesses = nroom1 + n = 2n+1.

The number of replicas mentioned several times earlier is 2, which means that at most one host is allowed to fail, that is, the FTT value is 1, and the minimum number of hosts is 3. As of VSAN version 6.1, the maximum value for FTT is 3, that is, a maximum of 4 copies.

"when assigning storage resources to a virtual machine, if no storage policy is selected, VSAN uses the default virtual machine storage policy, which specifies a FTT of 1."

Number of failures allowed by virtual machine storage policy

If you have configured failure domains, 2n+1 failure domains are required and there are hosts in these failure domains that can provide capacity. Hosts that do not belong to any failure domain are considered to be their own single host failure domain.

If you do not want VSAN to protect a single mirrored copy of the virtual machine object, you can specify FTT as 0. However, abnormal delays may occur when the host enters maintenance mode. The delay occurs because VSAN must expel the object from the host to successfully complete the maintenance operation. Setting FTT to 0 means that your data is not protected and you may lose data when the VSAN cluster encounters a device failure.

The default FTT value for VSAN is 1. The maximum value is 3.

4) mandatory provision

Force provisioning: if the mandatory setting is set to yes (yes), the object will be set even if the existing storage resource does not meet the storage policy. In this way, the virtual machine appears as Not Compliant in the virtual machine Summary page and the associated virtual machine storage policy view.

Mandatory setting of virtual machine storage policy, resulting in non-compliance (Not Compliant)

Forced provisioning allows VSAN to violate the policy requirements of FTT, stripe width, and flash read cache reservation during the initial deployment of the virtual machine. VSAN will try to find a location that meets all the requirements. If not, it will try to find a simpler location where the requirement is reduced to FTT=0, stripe width = 1, flash read cache reservation = 0. This means that VSAN will try to create an object with only one copy. However, the object still complies with the policy requirements of object space reservation (described in more detail below).

When looking for locations for objects, VSAN does not just reduce the requirements that cannot be met. For example, if an object requires FTT=2, but that requirement is not met, then VSAN will not try FTT=1, but will try FTT=0 directly. Similarly, if the requirement is FTT=1 with stripe width = 10, but VSAN does not have enough persistence disk to accommodate stripe width = 10, then it will fall back to FTT=0 with stripe width = 1, even though policy FTT=1, stripe width = 1 may succeed.

Administrators who use forced provisioning virtual machines need to be aware that once additional resources become available in the cluster, such as adding new hosts or disks, or hosts in failure or maintenance mode return to normal, VSAN may immediately consume those resources to try to meet the virtual machine's policy settings, that is, towards compliance.

The default value is no, which is acceptable for most production environments. When the policy requirements are not met, VSAN can successfully create a user-defined storage policy, but cannot set up virtual machines. The warning message in the figure below indicates that three hosts are required to provide storage, but only two are found in the cluster. Due to the mandatory setting of virtual machine storage policy, the storage capacity is insufficient to create a virtual machine.

5) object space reservation

Object space reservation (%): object space reservation is the logical size percentage of virtual machine disk (VMDK) objects that should be reserved or thickly set when deploying virtual machines. The default value of 0 means that all objects deployed on the VSAN are compact and do not take up any space at first, and only when the data is written will the vsanDatastore dynamically occupy the space according to the storage policy.

The default value is 0%. The maximum value is 100%. When the object space reservation is set to 100%, the space requirements for virtual machine storage are set to thick backup delay zeroing (LZT,Lazy Zeroed Thick) format.

Use of storage policies

1) default storage policy of the system

In the following figure, we can see what the five rules of VSAN represent by default, which are:

FTT=1, that is, the number of copies is 2, so the VMDK full of 100GB will actually consume the storage space of 200GB.

The stripe width is 1, that is, each copy spans only one persistent disk.

Force configuration to No

Object space reserved at 0% (that is, simplified configuration)

The flash read cache is reserved at 0.0000% (that is, no reservation).

Default value for VSAN virtual machine storage policy

2) Select storage policy when allocating virtual machines

The storage policy-based management of VMware enables administrators to pay more attention to business applications, centering on business applications / virtual machines, rather than automatically allocating storage resources from top to bottom around storage. Storage administrators can extricate themselves from the tedious and tedious work of volume management, LUN mapping, VMFS formatting, and building Datastore, and focus on more advanced tasks, that is, to create storage policies according to the requirements of storage performance, availability and capacity of different workloads. After the storage policy is created, it can be applied to different virtual machines of the same kind of workload.

As shown in the following figure, the storage policy created is Print Server,Tier 2 Apps,VDI-Desktops. When the vSphere administrator needs to create a virtual machine, or create a new VMDK for an existing virtual machine, you can choose according to the storage policy created by the storage administrator in advance, or the default storage policy of the system. In this way, the interaction time and workload of various administrators are greatly reduced, and the deployment of storage resources is very convenient.

3) it is very simple to change the storage policy

We know that users have many kinds of business applications, and some business applications may need to change storage resources in a specific period of time to cope with the high performance and high availability required at peak hours or critical moments. Traditional storage requires several steps, or even a pause in business, before changing the storage policy. VSAN is very simple, as long as you create a new storage policy and apply it to the (Apply) virtual machine.

This is the answer to the question on how to analyze the storage strategy and application of the vsan virtual machine. I hope the above content can be of some help to you. If you still have a lot of doubts to be solved, you can follow the industry information channel to learn more about it.

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