Frequently asked questions about Virtual SAN / VSAN

After I published the vSphere Flash Read Cache FAQ many asked if I would also do a blog post for frequently asked questions about Virtual SAN / VSAN. I guess it makes sense considering Virtual SAN / VSAN being such a hot topic. So here are the questions I have received so far, followed by the answers of course. If you have a question do not hesitate to leave a comment.

** updated to reflect VSAN GA **

  • Can I add a host to a VSAN cluster which does not have local disks?
    • Yes a VSAN cluster can consist of hosts which are not contributing to VSAN storage. You will need to create a VSAN VMkernel and simply add it to the cluster. Note that you will need at a minimum 3 hosts which contribute storage to VSAN
  • VSAN requires an SSD, what is it used for?
    • The SSD is used for read caching (70%) and write buffering (30%). Every write will go to SSD first and will be destaged to HDD later.
  • When creating my VSAN VM Storage Policy, when do I use “failures to tolerate” and when do I use “stripe width”?
    • Failures to tolerate is all about availability, this is what you define when your virtual machine will need to be available when a host or disk group has failed. So if you want to take 1 host failure in to account, you define the policy to 1. This will then create 2 data objects and 1 witness in your cluster. Stripe width is about performance (read performance when not in cache and write destaging). Setting it to two or higher will result in data being striped across multiple disks. When used in conjunction with “failures” to tolerate this could potentially result in data of a single VM stored on multiple disks on multiple hosts.
  • Is there a default storage policy for VSAN?
    • Yes there is a policy applied by default to all VMs on a VSAN datastore but you cannot see this policy within the vSphere UI. You can see that a default policy is defined to various classes using the following command: esxcli vsan policy getdefault. By default an N+1 failures to tolerate policy is applied so that even in the case where user forgets to create and set a policy objects are made resilient. It is not recommended to change the default policy.
  • How is data striped across multiple disks on a host when stripe width is set to 2?
    • When stripe width is set to 2 first of all there is no guarantee that the data is striped across disks within a host. VSAN has it’s own algorythm to determine where data should be placed and as such it could happen that although you have sufficient disks in all host your data is striped across multiple hosts instead of disks within a host. When data is striped this is done in chunks of 1MB.
  • What is the purpose of “disk groups” since VSAN will create one datastore anyway?
    • A disk group defines the SSD that is used for caching/buffering in front of a set of HDDs. Basically a disk groups is a way of mapping HDDs to an SSD. Each disk group will have 1 SSD and a maximum of 7 disks.
  • How many disks can a single host contribute to VSAN?
    • Max 5 diskgroup
    • Each disk group needs 1 SDD and 1 HDD at a mininum and 7 HDDs at a maximum
    • HDD count max per host = 5 x 7 = 35
    • SSD count max per host = 5 x 1 = 5
  • Are both SSD and PCIe Flash cards supported?
    • Yes both are supported but check the HCL for more details around this as there are guidelines and requirements
  • Is 10GbE a hard requirement for VSAN?
    • 10GbE is not a hard requirement for VSAN. VSAN works perfectly fine in smaller environments, including labs, with 1GbE. Do note that 10GbE is a recommendation.
  • Why is it recommended for HA’s isolation response to be configured to “powered-off”?
    • When VSAN is enabled vSphere HA uses the VSAN VMkernel network for heartbeating. When a host does not receive any heartbeats, it is most likely that the host is also isolated/partitioned from a VSAN perspective from the rest of the cluster. In this state it is recommended to power-off the virtual machine as a new copy will be powered-on by HA on the remaining hosts in the cluster automatically. This way when the host comes out of isolation the situation where 2 VMs with the same identity are on the network does not occur.
  • Can I partition my SSD or disks so that I can use them for other (install ESXi / vFlash) purposes?
    • No you cannot partition your SSD or HDD(s). Virtual SAN will only, and always, claim entire disks. With VSAN it probably makes most sense to install ESXi on an internal USB/SD card, this to maximize the capacity for VSAN.
  • Does VSAN support deduplication or compression?
    • In the current version VSAN does not support deduplication or compression. The most expensive resource in your VSAN cluster is SSD/Flash, hence duplication of data is most relevant on that layer. While having multiple copies of your data results in two copies on HDDs, and two temporary copies in the distributed write buffer (30% of the SSDs), the distributed read cache portion of the Flash (70%) will only contain a single copy of any cached data.
  • Can VSAN leverage SAN/NAS datastores?
    • VSAN currently does not support the use of SAN/NAS datastores. Disks will need to be “local” and directly passed to the host.
  • I was told VSAN does thin disks by default, if I set Object Space Reservation to 100% does that mean the VMDK will be eager zero thick provisioned?
    • No it does not mean the VM will be thick provisioned, or a portion for that matter, when you define Object Space Reservation. Object Space Reservation is all about the numbers used by VSAN when calculation used disk space / available disk space etc. When Object Space Reservation is set to 100% on a disk of 25GB then this disk will be a thin provisioned disk but VSAN will do its math with 100% used of 25GB. I guess you can compare it to a memory reservation.
  • Does VSAN use iSCSI or NFS to connect hosts to the datastore?
    • VSAN does not use either of these two to connect hosts to a datastore. It uses a proprietary mechanism.
  • What is the impact of maintenance mode in a VSAN enabled cluster?
    • There are three ways of placing a host which is providing storage to your VSAN datastore in maintenance mode:
      1) Full Data Migration – All data residing on the host will be migrated. Impact: Could take a long time to complete.
      2) Ensure accessibility – VSAN ensures that all VMs will remain accessible by migrating the required data to other hosts. Impact: Potentially availability policies are violated.
      3) No Data Migration – No data will be migrated. Impact: Depending on the “failures to tolerate” policy defined some VMs might become unusable.
      The safest option is option 1, with option 2 being the preferred and default as it is the fastest to complete. I guess the question is why you are placing the host in maintenance mode and how fast it will become available again. Option 3 is a fall back, in caseyou really need to get into maintenance mode fast and don’t care about potential data loss.
  • Are there any features of vSphere which aren’t supported/compatible with VSAN?
    • Currently vSphere Distributed Power Management, Storage DRS and Storage IO Control are not supported with VSAN.
  • How do I add a Virtual SAN / VSAN license?
    • VSAN licenses are applied on a cluster level. Open the Webclient click on your VSAN enabled cluster, click the “Manage” tab followed by “Settings”. Under “Configuration” click “Virtual SAN Licensing” and then click “Assign License Key”.
  • How will Virtual SAN be priced / licensed?
    • VSAN is licensed per socket, the price is $ 2495 per socket or $ 50,- per VDI user. Note that the license includes the Distributed Switch and VM Storage Policies, even when using a vSphere license lower than Enterprise Plus!
  • If a host has failed and as such data is lost and all VMs were protected N+1, how long will it take before VSAN starts rebuilding the lost data?
    • VSAN will identify which objects are out of compliance (those which had N+1 and were stored on that host) and starts a time-out period of 60 minutes. It has a time-out period to avoid an unnecessary and costly full sync of data. If the host returns within those 60 minutes then the differences will copied to that host. When a VM has multiple mirrors it doesn’t notice the failure, this 60 minute period is all about going back to full policy compliance, i.e. being able to satisfy additional failures may they occur.
  • When a virtual machines moves around in a cluster will its objects follow to keep IO local?
    • No, objects (virtual disks for instance) do not follow the virtual machine. Just imagine what the cost/overhead of moving virtual disks between hosts would be each time DRS suggests a migration. Instead IO can be done remotely. Meaning that although your virtual machine might run on host-1 from a CPU/Mem perspective, its virtual disks could be physically located on host-2 and host-3.
  • When a Virtual Machine is migrated to another host,  is the situation such that after a vMotion the SDD cache is lost (temporary performance hit) and the cache will be rebuilt over time?
    • No cache will not be lost and there is no need to rebuilt/warm the cache up again. Cache will be accessed remotely when needed.
  • Does VSAN support Fault Tolerance aka FT?
    • No, VSAN does not support Fault Tolerance in this release.
  • The SSD in my host is being reported in vSphere as “non-SSD”. According to support this is a known issue with the generation of server I am using. Will this “mis-reporting” of the disk type affect my ability to configure a VSAN?
    • Yes it will, you will need to tag the SSD as local using (example below is what I use in my lab, your identifier will be different). And in this case I claim it as being “local” and as “SSD”.
      esxcli storage nmp satp rule add –satp VMW_SATP_LOCAL –device mpx.vmhba2:C0:T0:L0 –option “enable_local enable_ssd”
  • It was mentioned that it will take 60 minutes after a failure before VSAN starts the automatic repair. Is it possible to shorten this time-out value?
    • **disclaimer: Although I do not recommend changing this value, I was told it is supported**
      Yes it is possible to shorten this time-out value by configuring the advanced setting named “VSAN.ClomRepairDelay” on every host in your VSAN cluster.
  • Why can’t I use datastore heartbeat functionality in VSAN only cluster?
    • There is no requirement for heartbeat datastores. The reason you do not have this functionality when you only have a VSAN datastore is because HA will use the VSAN network for heartbeats. So if a host is isolated from the VSAN network and cannot send heartbeats, it is safe to say that it will also not be able to update a heartbeat region remotely as such making it pointless to enable this feature in a VSAN only environment.
  • Are there specific Best Practices around deploying View on VSAN?
  • Can the VSAN VMkernel of hosts in a cluster be part of a different subnet?
    • VSAN VMkernel’s need to be part of the same subnet. Different subnet for one (or multiple) hosts within a VSAN cluster is not supported. When using multiple VMkernel interfaces per host each interface needs to be part of a different subnet!
  • Does VSAN support being stretched across multiple geographical locations?
    • In the current version VSAN will not support “metro” clustering.
  • Is there a difference between a host failing and a disk gradually failing?
    • Yes there is a difference. There are various failure stated and depending on the state it also determines how fast VSAN will spin up a new mirror. The two failure states are “absent” and “degraded”. Degraded is where a disks has failed and the system has recognized this as such and knows it isn’t coming back. In this case VSAN recognizes this “degraded” state and will create a new mirror of the impacted objects immediately, as there is no point in waiting for 60 minutes when you know it isn’t coming back soon. The “absent” state means that VSAN doesn’t know if it is coming back any time soon, this could be a host that has failed or for instance when you yank a disk, in this case the 60 minute time-out starts.
  • Is there any explanation around how VSAN handles disk failures or host failures?
  • What happens when an SSD fails in a VSAN cluster?

    • An SSD sits in front of a Disk Group as the read cache / write buffer. When the SSD fails then the disk group and all the components stored on it are marked as degraded. VSAN will then instanties new mirror copies where applicable and when sufficient disk capacity is available. For more details read this post.
  • Does vSphere support TRIM for SSDs?
    • No, TRIM is currently not supported/leveraged.
  • What are the Maximum Numbers for Virtual SAN GA?
    • 32 hosts per cluster
    • 100 VMs per host maximum
    • 3200 VMs per cluster maximum
    • 2048 VMs HA protected per cluster maximum
    • 2 million IOPS tested
  • How do I size a VSAN datastore / cluster?
  • How do I monitor VSAN performance?
    • Performance can easily be monitored using the VSAN Observer tool. This has been discussed by various people: here, here and here, here.
  • What’s likely to affect VSAN performance ?
    • Performance is most likely affected by leveraging cheap flash devices or incorrectly configured policies. In the case a workload is highly random and has a large “working set” it could be that many of the IOs will need to come from disk, this can also impact performance depending on the disk type used and the number of disk stripes.
  • Why is  Storage DRS not supported in VSAN ?
    • VSAN only provides a single datastore and has its own placement and balancing algorithms.
  • What will happen when the whole environment goes down and power back on again ? Do we run some sort of integrity check ?
  • Is VSAN dependent on vCenter ? Can I configure VSAN if vCenter is down ?
    • VSAN is not dependent on vCenter. It can be configured from the console using “esxcli” and can even be configured and used before vCenter is up and running. William Lam wrote two articles around how to bootstrap vCenter on a single host running VSAN. (here and here)
  • Could you have locality in VSAN ? Does locality make sense at all compared to other solutions ?
    • By default VSAN does not have a “data locality” concept as I explained here. However, for View environments CBRC is fully supported and that provides a local read cache for desktops.
  • Is vCops aware of VSAN datastore?
    • The current version of VC Ops has limited functionality in its current release. The upcoming version of VC Ops will include more statistics and ways of monitoring a VSAN datastore.
  • How do you backup your VM’s in VSAN ? Just usual existing backup procedures ?
    • VDP supports VSAN and various storage vendors are going through testing/releasing a new version of their product as we speak. VMs stored on a VSAN database should not be treated differently then regular VMs.
  • Does VSAN support any data reduction mechanisms like deduplication or compression?
    • In the current version deduplication or compression is not included.
  • x

If you have a question, please don’t hesitate to ask… Over time I will add more and more to this list so come back regularly.

vSphere HA Futures: Restart Order

At VMworld I hosted a group discussion together with Keith Farkas (HA Lead Engineer) on the topic of HA Futures. Based on this discussion group session Keith and I decided to gather more feedback from the field, this post will hopefully help us with that. Please do not hesitate to comment. I will have a couple of articles following this one, but lets get started with HA futures for the Restart Order first.

A topic that has come up at various sessions is HA restart ordering / priorities. Today HA provides four levels of restart priority: High, Medium, Low, Disabled. The thing to note with the current restart priority though is that there is no guarantee VMs are actually restarted in that order when the VMs are started on more than one host. Even when HA would restart them in the right order there is also no guarantee around when the boot cycle completes. Typically large virtual machines with for instance a database will take longer to boot than a server just running DNS. So what do we propose? We propose restart orders instead of restart priority. What does this mean, and what would we like to now from you?

There are two complementary ways of implementing this and we would like your feedback including which one you think would be most useful.

  1. Global Restart Order aka Bucketing
  2. VM to VM dependency Chains

Lets explain these two options and then I let you guys chime in.

Global Restart Order aka Bucketing is basically what you have today with “restart priorities” only it will actually enforce the restart order and it will allow for more flexibility. So with this option you could for instance create 5 buckets, and then add virtual machines to these buckets appropriately. These buckets could be: Priority 1, Priority 2 and so on. When a failure has occurred vSphere HA would then restart all VMs in the bucket “Priority 1″ first and when that bucket has finished starting (e.g., wait for VMware Tools Heartbeat to report “alive” for each VM) vSphere HA would continue with the next bucket and so on.  Waiting for VMtools to report “alive” is one way to determine that a VM is “ready”. We are thinking of providing three other “wait” options —  wait for an application heartbeat, wait a certain amount of time after the VM powers on, or today’s behavior, wait for the power on task to complete”.

I guess a couple of questions we have:

  1. How many levels would you like to see?
  2. Which of the wait conditions (e.g., wait on VMtools) are most useful for you?
  3. Suppose HA could not power on a “Priority 1″ VM. Do you want HA to stop powering on the “Priority 2″ etc VMs until it can, move to the “Priority 2″ group after a timeout, or something else?
The second option is VM to VM dependency Chains. These can be seen as an explicit restart order for a specific group of VMs which typically would form a service. I guess not unlike the vApp construct today, but then without all the caveats and restrictions around this. (vApps are essential resource pools, and we don’t want resource management in this case… just restart orderering.) In the simplest form, you could imagine specifying ordered lists of VMs, each list specifying the restart order for that set — the VMs in a list would be powered on sequentially. For example, something like the following:

Database VM –> Application Server –> Web Server

As you can see that would offer a significant amount of granularity, but also potentially a lot of operational complexity. How far would you like to go I guess is the question? Questions we have for you:

  1. Is an ordered list sufficient to express dependencies in a chain of VMs or do you need more sophistication?
  2. A VM with a dependent fails, do you expect HA to restart that child VM even though the previous has failed?
  3. What if HA could not be able to restart a VM with dependents — should HA restart these dependent VMs after a delay or only after the first VM is restarted?
A final question. We think bucketing will be easier to manage operationally but it introduces artificial dependencies between VMs and will make it take much longer to restart all VMs after a failure. How significant are these limitations?

That is it for now… Please chime in, as your response will help us define the future of vSphere HA.

VMware vSphere Virtual SAN design considerations…

I have been playing a lot with vSphere Virtual SAN (VSAN) in the last couple of months… I figured I would write down some of my thoughts around creating a hardware platform or constructing the virtual environment when it comes to VSAN. There are some recommended practices and there are some constraints, I aim to use this blog post to gather all of these Virtual SAN design considerations. Please read the VSAN introduction, how to install VSAN in your virtual lab and “How do you know where an object is located” to get a better understanding of the product. There is a long list of VSAN blogs that can be found here:

The below is all based on vSphere 5.5 Virtual SAN (public) Beta and my interpretation and thoughts based on various conversations with colleagues, engineering and reading various documents.

  • vSphere Virtual SAN (VSAN) clusters are limited to a maximum total of 32 hosts and there is a minimum of 3 hosts. VSAN is also currently limited to 100 VMs per host, resulting in a maximum of 3200 VMs in a 32 host cluster. Please note that HA currently has a limit of 2048 protected VMs in a single Datastore.
  • It is recommended to dedicate a 10GbE NIC port to your VSAN VMkernel traffic, although 1GbE is fully supported it could be a limiting factor in I/O intensive environments. Both VSS and VDS are supported.
  • It is recommended to have a VSAN VMkernel on every physical NIC! Ensure to configure them in a “active/standby” configuration so that when you have 2 physical NIC ports and 2 VSAN VMkernel’s each of them will have its own port. Do note that multiple VSAN VMkernel NICs on a single host on the same subnet is not a supported configuration, in  different subnets it is supported.
  • IP Hash Load Balancing is supported by VSAN, but due to limited number of IP-addresses between source/destination load balancing benefits could be limited. In other words, an etherchannel formed out of 4x1GbE NIC will most likely not result in 4GbE.
  • Although Jumbo Frames are fully supported with VSAN they do add a level of operational complexity. When Jumbo Frames are enabled ensure these are enabled end-to-end!
  • VSAN requires at a minimum 1 SSD and 1 Magnetic Disk per diskgroup on a host which is contributing storage. Each diskgroup can have a maximum of 1 SSD and 7 magnetic disks. When you have more than 7 HDDs or two or more SSDs you will need to create additional diskgroups.
  • Each host that is providing capacity to the VSAN datastore has at least one local diskgroup. There is a maximum of 5 disk groups per host!
  • It can beneficial to create multiple smaller disk groups instead of larger diskgroups. More diskgroups means smaller failure domains and more cache drives / queues.
  • Ensure when sizing your environment to take data replicas in to account. If your environment needs N+1 or N+2 (etc) resiliency factor this in accordingly.
  • SSD capacity does not count towards total VSAN datastore capacity. When sizing your environment, do not include SSD capacity in your totalized capacity calculation.
  • It is a recommended practice to have a minimum 1:10 ratio of SSD capacity to HDD capacity in each disk group. In other words, when you have 1TB of HDD capacity, it is recommended to have at least 100GB of SSD capacity. Note that VMware’s recommendation has changed since BETA, new recommendation is:
    • 10 percent of the anticipated consumed storage capacity before the number of failures to tolerate is considered
  • By default, 70% of the available SSD capacity will be used as read cache and 30% will be used as a write buffer. As in most designs, when it comes to cache/buffer –> more = better.
  • Selecting the SSD with the right performance profile can make a 5x-10x difference in VSAN performance easily, chose carefully and wisely. Both SSD and PCIe flash solutions are supported, but there are requirements! Make sure to check the HCL before purchasing new hardware. My tip Intel S3700, great price/performance balance.
  • VSAN relies on VM Storage Policies for policy based management. There is a default policy under the hood, but you cannot see this within the UI. As such it is a recommended practice to create a new standard policy for your environment after VSAN has been configured. It is recommended to start with all settings set to default, ensure “Number of failures to tolerate” is configured to 1. This guarantees that when a single host fails virtual machines can be restarted and recovered from this failure with minimal impact on the environment. Attach this policy to your virtual machines when migrating them to VSAN or during virtual machine provisioning.
  • Configure vSphere HA isolation response to “power-off” to ensure that virtual machines which reside on an isolated host can be safely restarted.
  • Ensure vSphere HA admission control policy (“host failures to tolerate” or the “percentage based) aligns with your VSAN availability strategy. In other words, ensure that both compute and storage are configured using the same “N+x” availability approach.
  • When defining your VM Storage Policy avoid unnecessary usage of “flash read cache reservation”. VSAN has internal read cache optimization algorithms, trust it like you trust the “host scheduler” or DRS!
  • VSAN does not support virtual machine disks greater than 2TB-512b, VMs which require larger VMDKs are not suitable candidates at this point in time for VSAN.
  • VSAN does not support FT, DPM, Storage DRS or Storage I/O Control. It should be noted though that VSAN internally takes care of scheduling and balancing when required. Storage DRS and SIOC are designed for SAN/NAS environments.
  • Although supported by VSAN, it is recommended practice to keep the hosts/disk configuration for a VSAN cluster similar. Non-uniform cluster configuration could lead to variations in performance and could make it more complex to stay compliant to defined policies after a failure.
  • When adding new SSDs or HDDs ensure these are not pre-formatted. Note that when VSAN is configured to “automatic mode” disks are added to existing disk groups or new disk groups are created automatically.
  • Note that vSphere HA behaves slightly different in a VSAN enabled cluster, here are some of the changes / caveats
    • Be aware that when HA is turned on in the cluster, FDM agent (HA) traffic goes over the VSAN network and not the Management Network. However, when an potential isolation is detected HA will ping the default gateway (or specified isolation address) using the Management Network.
    • When enabling VSAN ensure vSphere HA is disabled. You cannot enable VSAN when HA is already configured. Either configure VSAN during the creation of the cluster or disable vSphere HA temporarily when configuring VSAN.
    • When there are only VSAN datastores available within a cluster then Datastore Heartbeating is disabled. HA will never use a VSAN datastore for heartbeating.
    • When changes are made to the VSAN network it is required to re-configure vSphere HA.
  • VSAN requires a RAID Controller / HBA which supports passthrough mode or pseudo passthrough mode. Validate with your server vendor if the included disk controller has support for passthrough. An example of a passthrough mode controller which is sold separately is the LSI SAS 9211-8i.
  • Ensure log files are stored externally to your ESXi hosts and VSAN by leveraging vSphere’s syslog capabilities.
  • ESXi can be installed on: USB, SD and Magnetic Disk. Hosts with 512GB or more memory are only supported when ESXi is installed on magnetic disk.

That is it for now. When more comes to mind I will add it to the list!

vSphere 5.5 nuggets: High Availability Enhancement

There aren’t a lot of changes in 5.5 when it comes to vSphere High Availability aka HA, but one is worth noting. As most of you are probably aware of, vSphere HA in the past did nothing with VM to VM Affinity or Anti Affinity rules. Typically for people using “affinity” rules this was not an issue, but those using “anti-affinity” rules did see this as an issue. They created these rules to ensure specific virtual machines would never be running on the same host, but vSphere HA would simply ignore the rule when a failure had occurred and just place the VMs “randomly”. With vSphere 5.5 this has changed! vSphere HA is now “anti affinity” aware. In order to ensure anti-affinity rules are respected you will need to set an advanced setting:

das.respectVmVmAntiAffinityRules - Values: "false" (default) and "true"

Now note that this also means that when you configure anti-affinity rules and have this advanced setting  configured to “true” and somehow there aren’t sufficient hosts available to respect these rules… then rules will be respected and it could result in HA not restarting a VM. Make sure to understand this potential impact when configuring this setting and configuring these rules.

vSphere 5.5 nuggets: Change Disk.SchedNumReqOutstanding per device!

Always wanted to change Disk.SchedNumReqOutstanding per device instead of per host? Well now with vSphere 5.5 you can! I didn’t know about this either, but my colleague Paudie pointed this out. Useful feature when you have several storage arrays and you need to tweak these values, now lets be clear… I do not recommend tweaking this, but in the case you need to you can now do it per device using esxcli.

Get the current configured value for a specific device:
esxcli storage core device list --device <device>

Set the value for a specific device::
esxcli storage core device set -d <device> -O <value between 1-256>.