IT Certification Exam Questions & Answers

Storage I/O Control v2
Storage I/O Control (SIOC) was initially introduced in vSphere 4.1 to provide I/O prioritization of virtual machines running on a cluster of ESXi hosts that had access to shared storage. It extended the familiar constructs of shares and limits, which existed for CPU and memory, to address storage utilization through a dynamic allocation of I/O queue slots across a cluster of ESXi servers. The purpose of SIOC is to address the ‘noisy neighbor’ problem, i.e. a low priority virtual machine impacting other higher priority virtual machines due to the nature of the application and its I/O running in that low priority VM.
vSphere 5.0 extended SIOC to provide cluster-wide I/O shares and limits for NFS datastores. This means that no single virtual machine should be able to create a bottleneck in any environment regardless of the type of shared storage used. SIOC automatically throttles a virtual machine which is consuming a disparate amount of I/O bandwidth when the configured latency threshold has been exceeded. To allow other virtual machines receive their fair share of I/O bandwidth on the same datastore, a share based fairness mechanism has been created which now is supported on both NFS and VMFS.
vSphere 5.1 introduced a new SIOC feature called Stats Only Mode. When enabled, it doesn’t enforce throttling but gathers statistics to assist Storage DRS. Storage DRS now has statistics in advance for new datastores being added to the datastore cluster & can get up to speed on the datastores profile/capabilities much quicker than before.
Another 5.1 feature was Automatic Threshold Computation. The default latency threshold for SIOC is 30ms. Not all storage devices are created equal so this default was chosen as a sort of “catch-all”. There are certain devices which will hit their natural contention point much earlier than others, for example All Flash Arrays, in which case the threshold should be lowered by the user. However, manually determining the correct latency can be difficult for users. This gave rise to the need for the latency threshold to get automatically determined at a correct level for each device. Using the I/O injector modeling of SIOC, peak throughput and corresponding latency of a datastore is measured. The latency threshold value at which Storage I/O Control will kick in is then set to 90% of this peak value (by default). vSphere administrators can change this 90% to another percentage value or they can still input a millisecond value if they so wish.
The default latency threshold for SIOC can be reduced to as low as 5ms.
SIOC V1 Overview
SIOC V1 is disabled by default. It needs to be enabled on a per datastore level, and it is only utilized when a specific level of latency has been reached. By default, the latency threshold for a datastore is set to 30ms, as mentioned earlier. If SIOC is triggered, disk shares (aggregated from all VMDKs using the datastore) are used to assign I/O queue slots on a per host basis to that datastore. In other words, SIOC limits the number of IOs that a host can issue. The more VMs/VMDKs that run on a particular host, the higher the number of shares, and thus the higher the number of IOs that that particular host can issue. The throttling is done by modifying the device queue depth of the various hosts sharing the datastore. When the period of contention passes, and latency returns to normal values, the device queue depths are allowed to return to default values on each host.
SIOC V2 Introduction
Before describing SIOC V2, it should be highlighted that SIOC V1 and SIOC V2 can co-exist on vSphere 6.5. This makes it much simpler when considering upgrades, or migrations between versions. With that in mind, SIOC V2 is considerably different from a user experience perspective when compared to V1. SIOCv2 is implemented using IO Filter framework Storage IO Control category. SIOC V2 can be managed using SPBM Policies. What this means is that you create a policy which contains your SIOC specifications, and these policies are then attached to virtual machines.
Creating an SIOC policy based
Creating an SIOC policy is done is exactly the same way as building a storage policy for VSAN or Virtual Volumes. Select the VM Storage Policy from the vSphere client home page, and from there select the option to create a new VM Storage Policy. VM Storage Policies in vSphere 6.5 has a new option called “Common Rules”. These are used for configuring data services provided by hosts, such as Storage I/O Control and Encryption.
Use common rules in the VM storage policy
The first step is to click on the check box to enabled common rules. This will then allow you to add components, such as SIOC, to the policy.

Add Component – Storage I/O Control
In vSphere 6.5, there are two components available for common rules, Encryption and Storage I/O Control. Select Storage I/O Control in this case. Now you can select Normal, High, Low or Custom shares allocation.

This table describes the different Limits,Shares and Reservations associated with each setting:
HIGH
NORMAL
LOW
Limits
100,000
10,000
1,000
Reservation
100
50
10
Shares
2,000
1,000
500
When the policy has been created, it may be assigned to newly deployed VMs during provisioning,or to already existing VMs by assigning this new policy to the whole VM (or just an individual VMDK) by editing its settings. One thing to note is that IO Filter based IOPS does not look at the size of the IO. For example, there is no normalization so that a 64K IOP is not equal to 2 x 32K IOPS. It is a fixed value of IOPS irrespective of the size of the IO.
Custom Allocation
If neither of the values in the Normal, High, Low allocations is appropriate, there is the ability to create custom settings for these values. In a custom setting, IOPS limit and IOPS reservation are both set to -1, implying unlimited. These may be modified as required.

Advanced Options
SchedCostUnit
This is an advanced parameter that was created for SIOC V1 only. SIOC V2 does not have SchedCostUnit implemented. For V1, SchedCostUnit determines the unit size (normalized size) of an IO operation for scheduling, and it is currently a constant value of 32K. This constant value, however, may not satisfy different requirements from different customers. Some customers may want to set this unit size to 4K. Other customers may want to set it up to 256K.
To satisfy these different requirements, SchedCostUnit is now configurable. It defaults to an IO size value of 32K, and allowable values range between 4K to 256K.
The SchedCostUnit dictates how requests are counted. A request with size <= SchedCostUnit counts as a single I/O. Anything greater than SchedCostUnit will be counted as 2 or more requests.
For example, by changing the SchedCostUnit from 32K to 64K, the number of IOPS observed will halve. The size of the IO can be set using the:
“esxcli system settings advanced set -o /Disk/SchedCostUnit -i 65536”
and verified by using the”
“esxcli system settings advanced list -o /Disk/SchedCostUnit”
command. SIOC V2 counts guest IO directly. IOPS will be counted based on IO count, regardless of the IO size.
SchedReservationBurst
When limits are set on VMDKs, requests could have high average latency because the limit was enforced at a high (per request) granularity. This was due to the strict enforcement on a VM getting its share of IOs in interval of 1 second/L, where L is the user specified limit. The issue is more visible in fast storage, such as flash arrays. It was noted that SIOC V2 did not perform well when presented with a “bursty” workload on fast storage.
This SchedReservationBurst setting relaxes that constraint so a VM get its share of IOs at any time during a 1 second window, rather than enforce strict placement of IOs in intervals of 1/L. BURST option is turned-on by default.
SIOC V2 Limitations
In this initial release of SIOC V2 in vSphere 6.5, there is no support for vSAN or Virtual Volumes. SIOC v2 is only supported with VMs that run on VMFS and NFS datastores.

Cannot create a new datastore from vCenter Server or directly from the vSphere Client Creating a new datastore fails The Add Storage Wizard reports the error: An error occurred during host configuration. Call “HostDatastoreSystem.QueryVmfsDatastoreCreateOptions” for object “ha-datastoresystem” on ESXi “xxx.xxx.xxx.xx” failed. An internal error occurred in the vsphere client.
Cause
This issue may occur if:
The size of the LUN is more than 2TB – 512 bytes. The maximum LUN size in vSphere 4 is 2TB – 512 bytes. For more information on this limitation, see Troubleshooting a LUN that is extended in size past the 2TB/2047GB limit (1004230).
The LUN being presented was used as an RDM earlier and does not have a valid partition table.
The LUN being presented was used as a disk device on an other operating system and does not have a valid partition table.
The LUN contains a GPT partition that cannot be removed.
Resolution
To prevent this issue:
Ensure that the LUN size is less than 2TB – 512 bytes or 2047GB. If the LUN is larger then 2TB – 512 bytes, delete it from your Storage Array and recreate it with a size less than the 2TB – 512 bytes limit.
If the disk was used by another operating system in the past (I.E.: it was an RDM, or Linux, FreeBSD, or other filesystem) or contains a GPT partition, you must delete the partition information.For more information

VMware vSphere ESXi can use locally attached SSDs (Solid State Disk) and flash devices in multiple ways. Since SSDs offer much higher throughput and much lower latency than traditional magnetic hard disks the benefits are clear. While offering lower throughput and higher latency, flash devices such as USB or SATADOM can also be appropriate for some use cases. The potential drawback to using SSDs and flash device storage is that the endurance can be significantly less than traditional magnetic disks and it can vary based on the workload type as well as factors such as the drive capacity, underlying flash technology, etc.
This KB outlines the minimum SSD and flash device recommendations based on different technologies and use case scenarios.
SSD and Flash Device Use Cases
A non-exhaustive survey of various usage models in vSphere environment are listed below.
Host swap cache
This usage model has been supported since vSphere 5.1 for SATA and SCSI connected SSDs. USB and low end SATA or SCSI flash devices are not supported.
The workload is heavily influenced by the degree of host memory over commitment.
Regular datastore
A (local) SSD is used instead of a hard disk drive.
This usage model has been supported since vSphere 7.0 for SATA and SCSI connected SSDs.
There is currently no support for USB connected SSDs or for low end flash devices regardless of connection type.
vSphere Flash Read Cache (aka Virtual Flash)
This usage model has been supported since vSphere 5.5 for SATA and SCSI connected SSDs.
There is no support for USB connected SSDs or for low end flash devices.
vSAN
This usage model has been supported since vSphere 5.5 for SATA and SCSI SSDs. For more information, see the vSAN Hardware Quick Reference Guide.
vSphere ESXi Boot Disk
A USB flash drive or SATADOM or local SSD can be chosen as the install image for ESXi, the vSphere hypervisor, which then boots from the flash device.
This usage model has been supported since vSphere 3.5 for USB flash devices and vSphere 4.0 for SCSI/SATA connected devices.
Installation to SATA and SCSI connected SSD, SATADOM and flash devices creates a full install image which includes a logging partition (see below) whereas installation to a USB device creates a boot disk image without a logging partition.
vSphere ESXi Coredump device
The default size for the coredump partition is 2.5 GiB which is about 2.7 GB and the installer creates a coredump partition on the boot device device for vSphere 5.5 and above. After installation the partition can be resized if necessary using partedUtil. For more information, see the vSphere documentation.
Any SATADOM or SATA/SCSI SSD may be configured with a coredump partition.
This usage model has been supported from vSphere 3.5 for boot USB flash devices and since vSphere 4.0 for any SATA or SCSI connected SSD that is local.
This usage model also applies to Autodeploy hosts which have no boot disk.
vSphere ESXi Logging device
A SATADOM or local SATA/SCSI SSD is chosen as the location for the vSphere logging partition (/scratch partition). This partition may be but need not be on the boot disk and this applies to Autodeploy hosts which lack a boot disk.
This usage model has been supported since vSphere 7.0 for any SATA or SCSI connected SSD that is local. SATADOMs that meet the requirement set forth in Table 1 are also supported.
This usage model can be supported in a future release of vSphere for USB flash devices that meet the requirement set forth in Table 1.
SSD Endurance Criteria
The flash industry often uses Tera Bytes Written (TBW) as a benchmark for SSD endurance. TBW is the number of terabytes that can be written to the device over its useful life. Most devices have distinct TBW ratings for sequential and random IO workloads, with the latter being much lower due to Write Amplification Factor (WAF) (defined below). Other measures of endurance commonly used are DWPD (Drive Writes Per Day) and P/E (Program/Erase) cycles.
Conversion formulas are provided here:
Converting DWPD (Drive Writes Per Day) to TBW (Terabytes Written):
TBW = DWPD * Warranty (in Years) * 365 * Capacity (in GB) / 1,000 (GB per TB) Converting Flash P/E Cycles per Cell to TBW (Terabytes Written):
TBW = Capacity (in GB) * (P/E Cycles per Cell) / (1,000 (GB per TB) * WAF) WAF is a measure of the induced writes caused by inherent properties of flash technology. Due to the difference between the storage block size (512 bytes), the flash cell size (typically 4KiB or 8KiB bytes) and the minimum flash erase size of many cells one write can force a number of induced writes due to copies, garbage collection, etc. For sequential workloads typical WAFs fall in the range of single digits while for random workloads WAFs can approach or even exceed 100. Table 1 contains workload characterization for the various workloads excepting the Datastore and vSphere Flash Read Cache workloads which depend on the characteristics of the Virtual Machines workloads being run and thus cannot be characterized here. A WAF from the table can be used with the above P/E to TBW formula.