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VMware vSphere storage, datastores, snapshotsIn this article, byAbhilash G B, author of the book,VMware vSphere 6.5 CookBook – Third Edition, we will cover the following:Managing VMFS volumes detected as snapshotsCreating NFSv4.1 datastores with Kerberos authenticationEnabling storage I/O control

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IntroductionStorage is an integral part of any infrastructure. It is used to store the files backing your virtual machines. The most common way to refer to a type of storage presented to a VMware environment is based on the protocol used and the connection type. NFS are storage solutions that can leverage the existing TCP/IP network infrastructure. Hence, they are referred to as IP-based storage. Storage IO Control (SIOC) is one of the mechanisms to use ensure a fair share of storage bandwidth allocation to all Virtual Machines running on shared storage, regardless of the ESXi host the Virtual Machines are running on.
Managing VMFS volumes detected as snapshotsSome environments maintain copies of the production LUNs as a backup, by replicating them. These replicas are exact copies of the LUNs that were already presented to the ESXi hosts. If for any reason a replicated LUN is presented to an ESXi host, then the host will not mount the VMFS volume on the LUN. This is a precaution to prevent data corruption. ESXi identifies each VMFS volume using its signature denoted by aUniversally Unique Identifier (UUID). The UUID is generated when the volume is first created or resignatured and is stored in the LVM header of the VMFS volume. When an ESXi host scans for new LUN ;devices and VMFS volumes on it, it compares the physical device ID (NAA ID) of the LUN with the device ID (NAA ID) value stored in the VMFS volumes LVM header. If it finds a mismatch, then it flags the volume as a snapshot volume.Volumes detected as snapshots are not mounted by default. There are two ways to mount such volumes/datastore:Mount by Keeping the Existing Signature Intact - This is used when you are attempting to temporarily mount the snapshot volume on an ESXi that doesn’t see the original volume. If you were to attempt mounting the VMFS volume by keeping the existing signature and if the host sees the original volume, then you will not be allowed to mount the volume and will be warned about the presence of another VMFS volume with the same UUID:Mount by generating a new VMFS Signature - This has to be used if you are mounting a clone or a snapshot of an existing VMFS datastore to the same host/s. The process of assigning a new signature will not only update the LVM header with the newly generated UUID, but all the Physical Device ID (NAA ID) of the snapshot LUN. Here, the VMFS volume/datastore will be renamed by prefixing the wordsnap followed by a random number and the name of the original datastore:

Getting readyMake sure that the original datastore and its LUN is no longer seen by the ESXi host the snapshot is being mounted to.
How to do it…The following procedure will help mount a VMFS volume from a LUN detected as a snapshot:Log in to the vCenter Server using the vSphere Web Client and use the key combination Ctrl+Alt+2 to switch to the Host and Clusters view.Right click on the ESXi host the snapshot LUN is mapped to and go to Storage | New Datastore.On the New Datastore wizard, select VMFS as the filesystem type and click Next to continue.On the Name and Device selection screen, select the LUN detected as a snaphsot and click Next to continue:On the Mount Option screen, choose to either mount by assigning a new signature or by keeping the existing signature, and click Next to continue:On the Ready to Complete screen, review the setting and click Finish to initiate the operation.
Creating NFSv4.1 datastores with Kerberos authenticationVMware introduced support for NFS 4.1 with vSphere 6.0. The vSphere 6.5 added several enhancements:It now supports AES encryptionSupport for IP version 6Support Kerberos’s integrity checking mechanismHere, we will learn how to create NFS 4.1 datastores. Although the procedure is similar to NFSv3, there are a few additional steps that needs to be performed.

Getting readyFor Kerberos authentication to work, you need to make sure that the ESXi hosts and the NFS Server is joined to the Active Directory domainCreate a new or select an existing AD user for NFS Kerberos authenticationConfigure the NFS Server/Share to Allow access to the AD user chosen for NFS Kerberos authentication
How to do it…The following procedure will help you mount an NFS datasture using the NFSv4.1 client with Kerberos authentication enabled:Log in to the vCenter Server using the vSphere Web Client and use the key combination Ctrl+Alt+2 to switch to the Host and Clusters view, select the desired ESXi host and navigate to it  Configure | System | Authentication Services section and supply the credentials of the Active Directory user that was chosen for NFS Kerberon Authentication:Right-click on the desired ESXi host and go to Storage | New Datastore to bring-up the Add Storage wizard.On the New Datastore wizard, select the Type as NFS and click Next to continue.On the Select NFS version screen, select NFS 4.1 and click Next to continue. Keep in mind that it is not recommended to mount an NFS Export using both NFS3 and NFS4.1 client. On the Name and Configuration screen, supply a Name for the Datastore, the NFS export’s folder path and NFS Server’s IP Address or FQDN. You can also choose to mount the share as ready-only if desired:On the Configure Kerberos Authentication screen, check the Enable Kerberos-based authentication box and choose the type of authentication required and click Next to continue:On the Ready to Complete screen review the settings and click Finish to mount the NFS export.
Enabling storage I/O controlThe use of disk shares will work just fine as long as the datastore is seen by a single ESXi host. Unfortunately, that is not a common case. Datastores are often shared among multiple ESXi hosts. When datastores are shared, you bring in more than one local host scheduler into the process of balancing the I/O among the virtual machines. However, these lost host schedules cannot talk to each other and their visibility is limited to the ESXi hosts they are running on. This easily contributes to a serious problem called thenoisy neighbor situation. The job of SIOC is to enable some form of communication between local host schedulers so that I/O can be balanced between virtual machines running on separate hosts. 

How to do it…The following procedure will help you enable SIOC on a datastore:Connect to the vCenter Server using the Web Client and switch to the Storage view using the key combination Ctrl+Alt+4.Right-click on the desired datastore and go to Configure Storage I/O Control:On the Configure Storage I/O Control window, select the checkbox Enable Storage I/O Control, set a custom congestion threshold (only if needed) and click OK to confirm the settings: With the Virtual Machine selected from the inventory, navigate to its Configure | General tab and review its datastore capability settings to ensure that SIOC is enabled: 
How it works…As mentioned earlier, SIOC enables communication between these local host schedulers so that I/O can be balanced between virtual machines running on separate hosts. It does so by maintaining a shared file in the datastore that all hosts can read/write/update. When SIOC is enabled on a datastore, it starts monitoring the device latency on the LUN backing the datastore. If the latency crosses the threshold, it throttles the LUN’s queue depth on each of the ESXi hosts in an attempt to distribute a fair share of access to the LUN for all the Virtual Machines issuing the I/O.The local scheduler on each of the ESXi hosts maintains an iostats file to keep its companion hosts aware of the device I/O statistics observed on the LUN. The file is placed in a directory (naa.xxxxxxxxx) on the same datastore.For example, if there are six virtual machines running on three different ESXi hosts, accessing a shared LUN. Among the six VMs, four of them have a normal share value of 1000 and the remaining two have high (2000) disk share value sets on them. These virtual machines have only a single VMDK attached to them. VM-C on host ESX-02 is issuing a large number of I/O operations. Since that is the only VM accessing the shared LUN from that host, it gets the entire queue’s bandwidth. This can induce latency on the I/O operations performed by the other VMs: ESX-01 and ESX-03. If the SIOC detects the latency value to be greater than the dynamic threshold, then it will start throttling the queue depth: The throttled DQLEN for a VM is calculated as follows:DQLEN for the VM = (VM’s Percent of Shares) of (Queue Depth)Example: 12.5 % of 64 → (12.5 * 64)/100 = 8The throttled DQLEN per host is calculated as follows:DQLEN of the Host = Sum of the DQLEN of the VMs on itExample: VM-A (8) + VM-B(16) = 24The following diagram shows the effect of SIOC throttling the queue depth:
SummaryIn this article we learnt, how to mount a VMFS volume from a LUN detected as a snapshot, how to mount an NFS datasture using the NFSv4.1 client with Kerberos authentication enabled, and how to enable SIOC on a datastore.

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