Tag Archives: FTT

Sizing for your workloads

When sizing a vSAN environment there are many considerations to take into account, and with the launch of the new vSAN Sizing tool I thought I would take time and write up what questions I commonly ask people in order to get an understanding of what they want to run on vSAN as well as a scope of requirements that meet that workload.

Capacity
Obviously capacity is going to be our baseline for any sizing activity, no matter what we achieve with the other requirements, we have to meet a usable capacity, remember we should always work off a usable capacity for any sizing, a RAW capacity does not take into account any Failure Tolerance Methodology, Erasure Coding or Dedupe/Compression, this is something we will cover a bit later in this article.

Capacity should also include the required Swap File space for each of the VMs that the environment is being scoped for.

IOPS
I have been involved in many discussions where it is totally unknown what the performance requirements are going to be, so many times I have been told “We want the fastest performance possible” without being told what the current IOPS requirement is, to put it into context what is the point in buying a 200mph sports car when the requirement is to drive at 70mph max!

IOPS requirement plays a key part in determining what level of vSAN Ready Node specification is required, for example if a total IOPS requirement is 300,000 IOPS out of a 10 Node cluster, is there much point spending more money on an All-Flash configuration that delivers 150,000 IOPS Per node?  Simple answer…No!  You could opt for a lower vSAN All-Flash Ready Node config that meets the requirements a lot closer and still offer
room for expansion in the future.

Workload Type
This is a pretty important requirement, for example if your workload is more of a write intensive workload then this would change the cache requirements, it may also require a more write intensive flash technology such as NVMe for example.  If you have different workload types going onto the same cluster it would be worthwhile categorizing those workloads into four categories:-

  • 70/30 Read/Write
  • 80/20 Read/Write
  • 90/10 Read/Write
  • 50/50 Read/Write

Having the VMs in categories will allow you to specify the workload types in the sizing tool (in the advanced options).

vCPU to Physical Core count
This is something that gets overlooked not from a requirement perspective, but people are so used to sizing based on a “VM Per Host” scenario which with the increasing CPU core counts does not fit that model any more, even the new sizing tool bases it on vCPU to Physical Core ratio which makes things a lot easier, most customers I Talk to who are performing a refresh of servers with either 12 or 14 core processors can lower the amount of servers required by increasing the core count on the new servers, thus allowing you to run more vCPUs on a single host.

List of questions for requirements for each workload type

  • Average VMDK Size per VM
  • Average number of VMDKs per VM
  • Average number of vCPU per VM
  • Average vRAM per VM
  • Average IOPS Requirement per VM
  • Number of VMs
  • vCPU to Physical Core Ratio

RAW Capacity versus Usable Capacity, how much do I actually need?
The new sizing tool takes all your requirements into account, even the RAID levels, Dedupe/Compression ratios etc and returns with a RAW Capacity requirement based on the data you enter, if you are like me and prefer to do it quick and dirty, below is table showing you how to work out based on a requirement of 100TB of usable (Including Swap File Space), based on a standard cluster with no stretched capacilities it looks like this:

FTT LevelFTT MethodMin number of hostsMultiplication FactorRAW Capacity Based on 100TB Usable
FTT=0NoneN/A1x100TB
FTT=1Mirror32x200TB
FTT=2Mirror53x300TB
FTT=3Mirror74x400TB
FTT=1RAID541.33x133TB
FTT=2RAID661.5x150TB

Now in vSAN 6.6 VMware introduced localized protection (Secondary FTT) and the ability to include or not include specific objects from the stretched cluster (Primary FTT), below is a table showing what the RAW Capacity requirements are based on the two FTT levels

Primary FTT LevelSecondary FTT LevelFTT MethodMin Number of hosts per siteRAW Capacity Based on 100TB Usable
PFTT=1SFTT=1Mirror2400TB
PFTT=1SFTT=1RAID54266TB
PFTT=2SFTT=2RAID66300TB

Mixed FTT levels and FTT Methods
Because vSAN is truly a Software Defined Storage Platform, this means that you can have a mixture of VMs/Objects with varying levels of protection and FTT Methods, for example for Read intensive workloads you may choose to have RAID 5 in the storage policy, and for more write intensive workloads a RAID 1 policy, they can all co-exist on the same vSAN Cluster/Datastore perfectly well, and the new sizing tool allows you to specify different Protection Levels and Methods for each workload type.

 

Managing Storage Policies using PowerCLI

It has been a personal project due to a few people asking me in working out how to create and manipulate vSAN Storage Policies from the new PowerCLI commandlets introduced in PowerCLi 6.5, the tasks I will cover here will be

  1. Creating a Brand New Storage Policy
  2. Changing an existing storage policy

For the purpose of this excersise I am using PowerCLI 6.5 Release 1 build 4624819 I owuld recommend you use that build or newer as some of the commands do not work as intended on previous PowerCLI builds.

 

Creating a Storage Policy

Before we proceed with creating a storage policy, you need to ensure you are logged into your vCenter server via PowerCLI and get a list of capabilities from the storage provider, for this we run the command Get-SpbmCapability and filter out the VSAN Policy capabilities

C:\> Get-SpbmCapability VSAN*
Name ValueCollectionType ValueType AllowedValue
---- ------------------- --------- ------------
VSAN.cacheReservation System.Int32
VSAN.checksumDisabled System.Boolean
VSAN.forceProvisioning System.Boolean
VSAN.hostFailuresToTolerate System.Int32
VSAN.iopsLimit System.Int32
VSAN.proportionalCapacity System.Int32
VSAN.replicaPreference System.String
VSAN.stripeWidth System.Int32

For the definitions of the values associated with each definition:-
System.Int32 = Numerical Value
System.Boolean = True or False
System.String = “RAID-1 (Mirroring) – Performance” or “RAID-5/6 (Erasure Coding) – Capacity”

So I want to create a Policy with the following Definitions

Policy Name: PowerCLI-RAID5
Policy Description: RAID5 Policy Created with PowerCLI
Failures to Tolerate: 1
Failures to Tolerate Method: RAID5
Stripe Width: 2
IOPS Limit: 1000

To create a policy with the above specification we need to run the following command:

New-SpbmStoragePolicy -Name PowerCLI-RAID5 -Description "RAID5 Policy Created with PowerCLI" `
-AnyOfRuleSets `
(New-SpbmRuleSet `
(New-SpbmRule -Capability (Get-SpbmCapability -Name "VSAN.hostFailuresToTolerate" ) -Value 1),`
(New-SpbmRule -Capability (Get-SpbmCapability -Name "VSAN.stripeWidth" ) -Value 2),`
(New-SpbmRule -Capability (Get-SpbmCapability -Name "VSAN.replicaPreference" ) -Value "RAID-5/6 (Erasure Coding) - Capacity"),`
(New-SpbmRule -Capability (Get-SpbmCapability -Name "VSAN.iopsLimit" ) -Value 1000)`
)

We can then check for our policy by running the following command:

C:\> Get-SpbmStoragePolicy -Name "PowerCLi-RAID5" |Select-Object -Property *

CreatedBy : Temporary user handle
CreationTime : 11/04/2017 13:42:36
Description : RAID5 Policy Created with PowerCLI
LastUpdatedBy : Temporary user handle
LastUpdatedTime : 11/04/2017 13:42:36
Version : 0
PolicyCategory : REQUIREMENT
AnyOfRuleSets : {(VSAN.hostFailuresToTolerate=1) AND (VSAN.stripeWidth=2) AND (VSAN.replicaPreference=RAID-5/6 (Erasure Coding) - Capacity) AND (VSAN.iopsLimit=1000)}
CommonRule : {}
Name : PowerCLI-RAID5
Id : 9f8f61f4-24db-4513-b48f-a48c584f0eac
Client : VMware.VimAutomation.Storage.Impl.V1.StorageClientImpl

And from the UI We can see our storage policy:

That’s our policy created, now we’ll move onto changing an existing policy.

Changing an existing storage policy

The policy we created earlier, if we suddenly decide that we want RAID 6 rather than RAID 5, we have to change the Failure To Tolerate number to 2, and we will also want to change our policy name and description as it will no longer be RAID5. In order to change a policy, we have to ensure we specify the other values for the other policy definitions, if we do not specify the policy definitions that we do not want to be changed, they will be removed from the storage policy, so first we run the command that tells us what our policy definition is:

C:\> Get-SpbmStoragePolicy -Name "PowerCLi-RAID5" |Select-Object -Property *

CreatedBy : Temporary user handle
CreationTime : 11/04/2017 13:42:36
Description : RAID5 Policy Created with PowerCLI
LastUpdatedBy : Temporary user handle
LastUpdatedTime : 11/04/2017 13:42:36
Version : 0
PolicyCategory : REQUIREMENT
AnyOfRuleSets : {(VSAN.hostFailuresToTolerate=1) AND (VSAN.stripeWidth=2) AND (VSAN.replicaPreference=RAID-5/6 (Erasure Coding) - Capacity) AND (VSAN.iopsLimit=1000)}
CommonRule : {}
Name : PowerCLI-RAID5
Id : 9f8f61f4-24db-4513-b48f-a48c584f0eac
Client : VMware.VimAutomation.Storage.Impl.V1.StorageClientImpl

So we need to make the following changes to the policy

  • Change the Failure to Tolerate value from 1 to 2
  • Change the Name of the Policy from PowerCLI-RAID5 to PowerCLI-RAID6
  • Change the Description of the policy from “RAID5 Policy Created with PowerCLI” to “RAID6 Policy Changed with PowerCLI”

We run the following command which specified the new values, and also the values we do not want to change:

Set-SpbmStoragePolicy -StoragePolicy PowerCLI-RAID5 -Name PowerCLI-RAID6 -Description "RAID6 Policy Changed with PowerCLI" `
-AnyOfRuleSets `
(New-SpbmRuleSet `
(New-SpbmRule -Capability (Get-SpbmCapability -Name "VSAN.hostFailuresToTolerate" ) -Value 2),`
(New-SpbmRule -Capability (Get-SpbmCapability -Name "VSAN.stripeWidth" ) -Value 2),`
(New-SpbmRule -Capability (Get-SpbmCapability -Name "VSAN.replicaPreference" ) -Value "RAID-5/6 (Erasure Coding) - Capacity"),`
(New-SpbmRule -Capability (Get-SpbmCapability -Name "VSAN.iopsLimit" ) -Value 1000)`
)

We can verify the policy has been changed by running the command with the new policy name:

C:\> Get-SpbmStoragePolicy -Name "PowerCLi-RAID6" |Select-Object -Property *

CreatedBy : Temporary user handle
CreationTime : 11/04/2017 13:42:36
Description : RAID6 Policy Changed with PowerCLI
LastUpdatedBy : Temporary user handle
LastUpdatedTime : 11/04/2017 14:03:51
Version : 2
PolicyCategory : REQUIREMENT
AnyOfRuleSets : {(VSAN.hostFailuresToTolerate=2) AND (VSAN.stripeWidth=2) AND (VSAN.replicaPreference=RAID-5/6 (Erasure Coding) - Capacity) AND (VSAN.iopsLimit=1000)}
CommonRule : {}
Name : PowerCLI-RAID6
Id : 9f8f61f4-24db-4513-b48f-a48c584f0eac
Client : VMware.VimAutomation.Storage.Impl.V1.StorageClientImpl

And again in the UI we can see the changes made

That’s how you change an existing storage policy, I hope this blog helps you manage your storage policies via PowerCLI, thanks go out to Pushpesh in VMware for showing me the error of my ways when trying to figure this out myself.

 

 

 

 

 

Cache Sizing

Since vSAN was released in 2014 there has been a bit of confusion as to how much cache should be sized for the cluster, this article is intended to clear that up and provide direction for both Hybrid and All-Flash configurations.? The reason there are differences in the recommendations is primarily because in Hybrid the cache is a Read Cache as well as a Write Buffer and in All-Flash it’s just serving as a Write Buffer, so the sizing is not a “one size fits all”.

Capacity Definitions:

  • RAW Capacity – This is the amount of capacity the vSAN Datastore will provide
  • Usable Capacity – This is the amount of capacity that can be provided based on the FTT level specified in storage policies
  • Provisioned / Deployed Capacity – This is the amount of space taken by objects before FTT is taken into account
  • Consumed Capacity – This is the amount of space that has been consumed by objects taking into account FTT, for example a 100GB Object with FTT=1 will consume 200GB of storage space

 

Hybrid Cache Sizing
In Hybrid the recommendation has always been 10% of Usable Capacity or Deployed capacity, if we have a 3-Node vSAN cluster, each host has two disk groups and each disk group has 7×1.2TB 10K SAS Drives, that means each host has 16.8TB of RAW Capacity and our 3-Node cluster has 50.4TB of RAW Capacity, based on the following FTT Values in our storage policy this means our total Usable Capacity is:

FTT=0 – 50.4TB
FTT=1 – 25.2TB
FTT=2 – 16.8TB
FTT=3 – 12.6TB

Based on the 10% rule our cache requirements are as follows:

FTT=0 – 5.4TB which equates to 1.8TB Per node which equates to 900GB Per Disk Group
FTT=1 – 2.52TB which equates to 0.84TB Per node which equates to 420GB Per Disk Group
FTT=2 – 1.68TB which equates to 0.56TB Per node which equates to 280GB Per Disk Group
FTT=3 – 1.26TB which equates to 0.42TB Per node which equates to 210GB Per Disk Group

The above sizing is all well and good if you are only using a single FTT method, however vSAN allows you to define policies with different FTT levels which means you can have objects on vSAN that have varying levels of protection, this makes sizing using the above method all the more difficult.

The best way to size the cache in a Hybrid cluster is to base it on your deployed or provisioned capacity, for example in the above RAW capacity of 50.4TB you may choose to have the following as an example

10 Objects based on FTT=0 of 500GB which totals 5TB of Provisioned Capacity and 5TB of Consumed Capacity
10 Objects based on FTT=1 of 500GB which totals 5TB of Provisioned Capacity and 10TB of Consumed Capacty
10 Objects based on FTT=2 of 500GB which totals 5TB of Provisioned Capacity and 15TB of Consumed Capacity
10 Objects based on FTT=3 of 500GB which totals 5TB of Provisioned Capacity and 20TB of Consumed Capacity

If you total up the above, our Provisioned Capacity is 20TB but our Consumed Capacity is 50TB, based on the Provisioned Capacity of 20TB, 10% of this is 2TB which equates to 0.67TB Per Node, or 333GB per Disk Group, this is how your cache in Hybrid should be sized.

All-Flash Cache Sizing
All flash has a lot more factors to consider, Erasure Coding, Dedupe and Compression and the fact that the cache is purely a Write Buffer so we have to take into account write endurance so the usual 10% sizing does not apply here.? In reality the typical 70% Read / 30% Write workload means that a lot of the requests are coming from the Capacity Tier which in this case is flash based anyway, so this means that the cache layer can be much smaller than it would have been in Hybrid for the same RAW capacity, however there is the write endurance factor to take into account.? We all know there is a write buffer limit in vSAN but that does not mean you should limit the size of the SSD drives based on that, the main reason is to increase the endurance of the drive, vSAN will cycle through all the cells on the drive irrelevant of the Write Buffer Limit.? VMware recently published a new sizing guide for All-Flash which is shown below

 

There we have it!