Category Archives: VMFS

ESXi Storage Device Naming Convention for a LUN : Part-1

This blog post came as a result of my VMware community interaction, so the questions are simple !!

1- What is the need for such a Storage Device Naming Convention for a LUN and the theory behind this.

2- Who is responsible for assigning an Unique Storage Device Name for the LUN in an ESX/ESXi host.

3- For a LUN why you need an unique and same LUN ID across the ESX/ESXi hosts in a cluster.

4- How an ESX/ESXi host can uniquely identify a LUN in a Storage Area Network.

5- What are the different types of naming standards or convention for a LUN in an ESX/ESXi host.

As we all know, for the ESX/ESXi hosts & clusters we have to create/present LUN’s from the Storage Array, to get the features like VMotion,HA,DRS etc. Now let’s see the answers for the above:

1- What is the need for such a naming convention and the theory behind this.

The need for a standard

But here comes a basic problem. If I can expose the same LUN to one or more machines, then how could I address it? In other words, how can safely I distinguish between one LUN and another? This seems to be a really trivial problem. Just stick a a unique GUID to each LUN and you are done! Or, stick a unique number. Or… a string… but hold on, things are not that easy. What if storage array maker ABC assigns GUIDs to each LUN and another vendor assigns 32-bit numbers? We have a complete mess.

To add to the confusion, we have this other concept – the serial number attached to a SCSI disk. But this doesn’t work all the time. For example, some vendors assign a serial number for each LUN, but this serial number is not guaranteed to be unique. Why, some SCSI controllers are even returning the same serial number for all exposed LUNs!

Every hardware vendor had a more-or-less proprietary method to identify LUNs exposed to a system. But if you wanted to write an application that tried to discover all the LUNs you had a hard time since your code was tied to the specific model of each array.  What if two vendors had a conflicting way to assign IDs to LUNs? You could end up with two LUNs having the same ID !!

We all know the Storage devices, I/O interfaces, SAS disks are basically used to send and receive Data by using SCSI commands and they all has to follow the SCSI (Small Computer System Interface) standards.

T10 develops standards and technical reports on I/O interfaces, particularly the series of SCSI (Small Computer System Interface) standards. T10 is a Technical Committee of the InterNational Committee on Information Technology Standards (INCITS, pronounced “insights”). INCITS is accredited by, and operates under rules that are approved by, the American National Standards Institute (ANSI).

T10 operates under INCITS and is responsible for setting standards on SCSI Storage Interfaces, SCSI architecture standards (SAM), SCSI command set standards.  As per T10, SCSI Primary Commands – 3 (SPC-3) contains the third-generation definition of the basic commands for all SCSI devices. As of now all the major Storage array vendors like EMC, Netapp, HP, DELL, IBM, Hitachi and many others follows these T10 standards, all these arrays follow the SPC-3 standards during the LUN creation, presentation, and communication to the hosts etc. Similarly the ESXi storage stack and other latest Operating systems also uses these standards to communicate to the Storage array, Access the luns etc.

So in short these are industry standards and vendor neutral so that ISV, OEM and other software/hardware vendors can develop solutions and products inside single frame work.

2- Who is responsible for assigning an Unique Storage Device Name for the LUN to an ESX/ESXi host.

There are 2 people assigns and maintain an unique name for a LUN one is the Storage Array and other is the Host, both assigns and maintains at their own level.  But what ever luns created and given from a Storage it will be unique, and it is the responsibility of the Storage to maintain the Uniqueness the array uses the T10/SPC-3 standards to maintain the Uniqueness.

That is from the SAN when we create a LUN with a LUN ID the SAN itself make sure it is unique, and we will give LUN name to understand easily. Once that LUN is presented the ESX/ESXi host will make this volume unique with UUID, and in particularly with ESX/ESXi it has different types of multiple naming conventions and representations.

So the Storage array is responsible for this and the ESX/ESXi just uses the LUN, but they follow the guidelines of T10/SPC-3 standards to maintain the Uniqueness.


Magic Effect of SIOC (VMware Storage Input Output Control) in vSphere 5 : A practical study.

Recently I got a chance to implement and test the Enterprise Plus feature in vSphere called SIOC. It is really an amazing feature…..


Application performance can be impacted when servers contend for I/O resources in a shared storage environment. There is a crucial need for isolating the performance of critical applications from other, less critical workloads by appropriately prioritizing access to shared I/O resources. Storage I/O Control (SIOC), a new feature offered in VMware vSphere 4 & 5, provides a dynamic control mechanism for managing I/O resources across VMs in a cluster.

Datacenters based on VMware’s virtualization products often employ a shared storage infrastructure to service clusters of vSphere hosts. NFS, ISCSI, and Storage area networks (SANs) expose logical storage devices (LUNs) that can be shared across a cluster of ESX hosts.  Consolidating VMs’ virtual disks onto a single VMFS datastore, or NFS datastore, backed by a higher number of disks has several advantages—ease of management, better resource utilization, and higher performance (when storage is not bottlenecked).

With vSphere 4.1 we can only use SIOC with FC/ISCSI datastores, but with vSphere 5 we can use NFS.
However, there are instances when a higher than expected number of I/O-intensive VMs that share the same storage device become active at the same time. During this period of peak load, VMs contend with each other for storage resources. In such situations, lack of a control mechanism can lead to performance degradation of the VMs running critical workloads as they compete for storage resources with VMs running less critical workloads.
Storage I/O Control (SIOC), provides a fine-grained storage control mechanism by dynamically allocating portions of hosts’ I/O queues to VMs running on the vSphere hosts based on shares assigned to the VMs. Using SIOC, vSphere administrators can mitigate the performance loss of critical workloads during peak load periods by setting higher I/O priority (by means of disk shares) to those VMs running them. Setting I/O priorities for VMs results in better performance during periods of congestion.

So What is the Advantage for an VM administrator/Organization? Now a days, with vSphere 5 we can have 64TB of single LUN and with high end SAN with FLASH we are achieving more consolidation. That’s great !! but when we do this…. just like the CPU/MEMORY resource pools ensure the Computing SLA, the SIOC ensures the virtual disk STORAGE SLA and its response time.

In short, below are the benefits;

– SIOC prioritizes VMs’ access to shared I/O resources based on disk shares assigned to them. During the periods of I/O congestion, VMs are allowed to use only a fraction of the shared I/O resources in proportion to their relative priority, which is determined by the disk shares.
– If the VMs do not fully utilize their portion of the allocated I/O resources on a shared datastore, SIOC redistributes the unutilized resources to those VMs that need them in proportion to VMs’ disk shares. This results in a fair allocation of storage resources without any loss in their utilization.
– SIOC minimizes the fluctuations in performance of a critical workload during periods of I/O congestion, as much as a 26% performance benefit compared to that in an unmanaged scenario.

How Storage I/O Control Works
SIOC monitors the latency of I/Os to datastores at each ESX host sharing that device. When the average normalized datastore latency exceeds a set threshold (30ms by default), the datastore is considered to be congested, and SIOC kicks in to distribute the available storage resources to virtual machines in proportion to their shares. This is to ensure that low-priority workloads do not monopolize or reduce I/O bandwidth for high-priority workloads. SIOC accomplishes this by throttling back the storage access of the low-priority virtual machines by reducing the number of I/O queue slots available to them. Depending on the mix of virtual machines running on each ESX server and the relative I/O shares they have, SIOC may need to reduce the number of device queue slots that are available on a given ESX server.

It is important to understand the way queuing works in the VMware virtualized storage stack to have a clear understanding of how SIOC functions. SIOC leverages the existing host device queue to control I/O prioritization. Prior to vSphere 4.1, the ESX server device queues were static and virtual-machine storage access was controlled within the context of the storage traffic on a single ESX server host. With vSphere 4.1 & 5, SIOC provides datastore-wide disk scheduling that responds to congestion at the array, not just on the hostside HBA.. This provides an ability to monitor and dynamically modify the size of the device queues of each ESX server based on storage traffic and the priorities of all the virtual machines accessing the shared datastore. An example of a local host-level disk scheduler is as follows:

Figure 1 shows the local scheduler influencing ESX host-level prioritization as two virtual machines are running on the same ESX server with a single virtual disk on each.


Figure 1. I/O Shares for Two Virtual Machines on a Single ESX Server (Host-Level Disk Scheduler)

In the case in which I/O shares for the virtual disks (VMDKs) of each of those virtual machines are set to different values, it is the local scheduler that prioritizes the I/O traffic only in case the local HBA becomes congested. This described host-level capability has existed for several years in ESX Server prior to vSphere 4.1 & 5. It is this local-host level disk scheduler that also enforces the limits set for a given virtual-machine disk. If a limit is set for a given VMDK, the I/O will be controlled by the local disk scheduler so as to not exceed the defined amount of I/O per second.

vSphere 4.1 onwards it has added two key capabilities: (1) the enforcement of I/O prioritization across all ESX servers that share a common datastore, and (2) detection of array-side bottlenecks. These are accomplished by way of a datastore-wide distributed disk scheduler that uses I/O shares per virtual machine to determine whether device queues need to be throttled back on a given ESX server to allow a higher-priority workload to get better performance.

The datastore-wide disk scheduler totals up the disk shares for all the VMDKs that a virtual machine has on the given datastore. The scheduler then calculates what percentage of the shares the virtual machine has compared to the total number of shares of all the virtual machines running on the datastore. As described before, SIOC engages only after a certain device-level latency is detected on the datastore. Once engaged, it begins to assign fewer I/O queue slots to virtual machines with lower shares and more I/O queue slots to virtual machines with higher shares. It throttles back the I/O for the lower-priority virtual machines, those with fewer shares, in exchange for the higher-priority virtual machines getting more access to issue I/O traffic.

However, it is important to understand that the maximum number of I/O queue slots that can be used by the virtual machines on a given host cannot exceed the maximum device-queue depth for the device queue
of that ESX host.

What are the conditions required for the SIOC to work ?
– A large datastore, and lot of VMs in it and this datastore is shared between multiple ESX hosts.
– Need to set disk shares, for the vm’s inside the datastore.
– Based on the LUN/NFS type (made of SSD,SAS,SATA), select the SIOC threshold, and enable SIOC.
– The SIOC will monitor the datastore IOPS usage, latency of VM’s and monitor overall STORAGE Array performance/Latency.

SIOC don’t check the below;

How much Latency created by other physical systems to the array, that is the Array is shared for the PHYSICAL hosts also, and other backup applications/jobs etc. So it gives false ALARMS like “VMware vCenter – Alarm Non-VI workload detected on the datastore] An unmanaged I/O workload is detected on a SIOC-enabled datastore”

Tricky Question – Will it work for one ESXi host ? Simple answer – SIOC is for multiple ESXi hosts, Datastore is exposed to many hosts and in that Datastore contains a lot of VM’s. For single host also we can enable the SIOC if we have the license, but it is not intended for this CASE.

So what is the solution for a single ESXi host ? – Simple just use shares for the VMDK and the ESXi host use Host-Level Disk Scheduler and ensures the DISK SLA.

So is there any real advantage of the SIOC in Real time scenario ? the below experience of me PROVES this..

My scenario: I have 4 ESXi 5 hosts in the cluster, and many FC LUNS from the HP 3PAR storage arrays is mounted. In one or the LUN we have put around 16 VMs and this includes our vCenter and it Database, and inside that Datastore we have a less critical VM (Analytics engine, which is from the vCOPS appliance) and this consumes a lot of IOPS from the storage and eventually effects other critical VM. We enabled the SIOC and monitored for 10 days then we compared the performance of the VMDK and its LATENCY during the PEAK hours.

Below is the Datastore with many VMS and it is shared across 4 HOSTS.


With SIOC disabled.



With SIOC enabled.



So what is the Take away with SIOC ?

Better I/O response time, reduced latency for critical VMS and ensured disk SLA for critical VM’s. So bottom line is the crtical VM’s wont get affected during the event of storage contention and in highly consolidated environment.



Partition alignment in VMware vSphere 5, a DeepDrive, Part-2

In continuation to my part-1, we will see how the Virtual machine disk alignment effects the Virtualized world and eventually the performance.

Here we need to know how to align the disk partitions in OS like windows 2003, XP, RHEL 5.x etc in VMware vSphere VMFS 5 Datastore.

Let us assume the VMFS is residing above a RAID volume or a LUN from a storage array, in either case the RAID STRIP size will be from 4KB to 256KB depending upon the ARRAY and RAID levels. So now we are deployed the above mentioned operating systems on the VMFS,

What is the ISSUE with the GUEST OS disk misalignment ?

This issue is applicable to the PHSICAL and VIRTUAL also and it is not because of the VMFS layer or any other means. It is the limitation of these OS and how they do the partition in the given HDD, just like to the PHYSICAL world, in VIRTUAL world the Hypervisor also gives an HDD but it is a VIRTUAL HDD. But the OS doesn’t know if it is a virtual HDD or Actual HDD. So eventually the OS wont do the partition with correct alignment.

Leave the VITRUL world and go to PHYSICAL world,  let us assume the Guest filesystem Block/Cluster is 4KB, and it writes or reads a 4KB to the hard disk, and in turn it goes to the HDD PHYSICAL sector of 4KB. Because the partition is not ALIGNED it uses 2 PHYSICAL sectors of 4KB for this operation, here the amount of Data read/write is same but it uses 2 PHYSICAL sectors.


So the Harddisk HEAD need to do more work to fetch the Data to and from the PHYSICAL sectors. So for a single IO for the same 4KB data from the OS, the HDD or Array need to use 2 PHYSICAL sectors, so the IOPS response time and latency will be affected. So we will get a POOR performance. This is applicable in VIRTUAL world also,

As of now the VMFS3 and VMFS5 are already aligned to the underlying storage. The below give more info how the VMFS is aligned, open an ESXi shell or SSH and type the below;

~ # partedUtil get /vmfs/devices/disks/naa.600508b1001030394330313737300300
71380 255 63 1146734896

1 2048 1146719699 0 0
~ #

The first line displayed is disk geometry information (cylinders, heads, sectors per track and LBA [Logical Block Address] count). The second line is information about the partitions. There is only 1 partition; it starts at LBA 2048 and ends at LBA 1146719699.That’s something else to be aware of – newly created VMFS-5 partitions start at LBA 2048. This is different to previous versions of VMFS:

  • VMFS-2 created on ESX 2.x; starting LBA 63
  • VMFS-3 created on ESX 3 & 4; starting LBA 128
  • VMFS-5 created on ESXi 5; starting LBA 2048

So the VMFS is aligned with the 1MB boundary (starts from the LBA 2048), and as we all know the VMFS 5 the block size is 1MB. So the VMFS and the underlying storage is already aligned. So when a misaligned GUEST OS send’s a READ/WRITE request to the HYPERVISOR layer, that is from the VMDK to the VM SCSI controller to the VMFS and finally to the STORAGE, the STORAGE has to look for more than one PHYSICAL sector or CHUNK, this is an over head for the storage and of course it will be an over head to the VMkernel, because the VMkernel has to wait until the array does the task.


If the GUEST OS is aligned, for one 4KB write/read it will use only one single CHUNK from the storage, this will give GOOD Response time and LOW latency for the IOPS operation.


Now How to align the Guest OS;

For windows follow the below –

1- Add the required virtual HDD to the Guest OS

2- Verify the HDD is visible in the OS



3- Open Command prompt and use the Command Line Syntax below


DISKPART> list disk

DISKPART> select disk 2

Disk 2 is now the selected disk.

DISKPART> create partition primary align=1024




After this format the PARTITON, Windows MSSQL databases and Exchange servers, it is recommended to format the NTFS cluster size as 64K (64 kilobytes) and for other normal server we can use 32K.


Generally to check a partition is aligned or not use the below command, then refer the PART 1 of this blog to do the MATH.

wmic partition get BlockSize, StartingOffset, Name, Index


In my case this shows both my disks having partitions that are aligned to 1024KB or 1MB …or sector2048.

(1048576 bytes)/(512 bytes/sector) = 2048 sector

To check File Allocation Unit Size – Run this command for each drive to get the file allocation unit size:

fsutil fsinfo ntfsinfo c:


Steps to align the partition for Linux.

To check that your existing partitions are aligned, issue the command:

fdisk –lu

The output is similar to:


1. Enter fdisk /dev/sd<x> where <x> is the device suffix.

2. Type n to create a new partition.

3. Type p to create a primary partition.

4. Type 1 to create partition No. 1.

5. Select the defaults to use the complete disk.


1. Type t to set the partition’s system ID.

2. Type 8e to set the partition system ID to 8e (Linux LVM)

3. Type x to go into expert mode.

4. Type b to adjust the starting block number.

5. Type 1 to choose partition 1.

6. Type 2048 to set it to start with the sector 2048.

7. Type w to write label and partition information to disk.


To check that your existing partitions are aligned, issue the command:

fdisk –lu


Now you can see the partition are aligned, that is started from the sector 2048 that is 1MB boundary.


Now in the internet there are many methods to automate the process during the TEMPLATE deployment, one method is add few 1 GB VMDK and do the partition alignment and make a template and after that when the template deployment is over, increase the VMDK for the guest OS. This will work fine for the thin disk and lazy zeroed disks, but if you do the same process for a EAGAR ZEROED disk, then we all know the outcome. Once you increase the EAGAR ZEROED disk it will become LAZY zeroed disk, so for FT and windows clusters, oracle clusters it will be a problem. Else after the VMDK increase you need to use the VMKFSTOOLS to change the VMDK from LAZY zeroed disk to EAGAR ZEROED disk. So again a management overhead, so its your decision !!



Partition alignment in VMware vSphere 5, a DeepDrive, Part-1

This topic has been discussed seriously for a long time… in the virtualization domain. I would like to add few insights in to this topic, with the new vSphere 5.x release there are lot of changes happened in the VMFS filesystem and also with the release of Windows 2008, 2008 R2, 2012 and RHEL 6.x, Ubuntu 12.x there is no need of doing partition alignment, but it would be good to know how these OS do the partition and handle the disk volumes.

Also the legacy OS like Windows 2003, RHEL 5.x still need partition alignment with vSpehere 5 & VMFS 5 filesystem. In short, generally this topic applies to physical, other hypervisor vendors and to VMWARE also.

There are many outdated articles in the web and below info will give a good insight to this topic.

Theory & History

As we all know a physical server or Storage array need physical HDD’s, with virtual machine it is a virtual HDD. The below image shows the hard disk geometry


In the case of early IDE/ATA hard disks the BIOS provides access to the hard disk through an addressing mode called Cylinder-head-sector, also known as CHS, was an early method for giving addresses to each physical block of data on a hard disk drive. CHS addressing is the process of identifying individual sectors on a disk by their position in a track, where the track is determined by the head and cylinder numbers.

In old computer system the maximum amount of addressable data was very limited – due to limitations in both the BIOS and the hard disk interface. The legacy OS like NT, DOS etc uses this method.

Modern hard disks use a recent version of the ATA standard, such as ATA-7. These disks are accessed using a different addressing mode called: logical block addressing or LBA involves a totally new way of addressing sectors. Instead of referring to a cylinder, head and sector number, each sector is instead assigned a unique “sector number”. In essence, the sectors are numbered 0, 1, 2, etc. up to (N-1), where N is the number of sectors on the disk. So all modern disk drives are now accessed using Logical Block Addressing (LBA) scheme where the sectors are simply addressed linearly from 0 to some maximum value and disk partition boundaries are defined by the start and end LBA address numbers. In LBA addressing system with each cylinder standardized to 255 heads, each head has one track with 63 logical blocks or sectors and each one has 512 bytes. You can see this info in linux


All the modern OS uses the LBA method to read/write data to the Harddisk.

In olden days the physical sector of the HDD is 512-byte, and this has been the standard for over 30 years. This physical sector size will match with the size of one Logical block or sector of the OS that is 512 byte, so no issue. During 2009, IDEMA (The International Disk Drive Equipment and Materials Association) and leading data storage companies introduced Advanced Format (AF) Technology in the HDD, so that the physical sector size will be 4K (4,096 bytes). Disk drives with larger physical sectors allow enhanced data protection and correction algorithms, which provide increased data reliability. Larger physical sectors also enable greater format efficiencies, thereby freeing up space for additional user data.

One of the problems of introducing this change in the media format is the potential for introducing compatibility issues with existing software and hardware. As a temporary compatibility solution, the storage industry is initially introducing disks that emulate a regular 512-byte sector disk, but make available info about the true sector size through standard ATA and SCSI commands. As a result of this emulation, there are, in essence, two sector sizes:

Logical sector: The unit that is used for logical block addressing for the media. We can also think of it as the smallest unit of write that the storage can accept. This is the “emulation.”

Physical sector: The unit for which read and write operations to the device are completed in a single operation. That is the Actual physical sector size of storage data on a disk.

Initial types of large sector media

The storage industry is quickly ramping up efforts to transition to this new Advanced Format type of storage for media having a 4 KB physical sector size. Two types of media will be released to the market:

4 KB native: Disks that directly report a 4 KB logical sector size and have a physical sector size of 4 KB – The disk can accept only 4 KB IOs to the disks. However, the software stack can provide 512-byte logical sector size support through RMW support. This media has no emulation layer and directly exposes 4 KB as its logical and physical sector size. The overall issue with this new type of media is that the majority of apps and operating systems do not query for and align I/Os to the physical sector size, which can result in unexpected failed I/Os.

512-byte emulation (512e): Disks that directly report a 512-byte logical sector size but have a physical sector size of 4 KB – Firmware translate 512 byte writes to 4k writes RMW (Read Modify Write). In today’s drives, this translation introduces a performance penalty. This media has an emulation layer as discussed in the previous section and exposes 512-bytes as its logical sector size (similar to a regular disk today), but makes its physical sector size info (4 KB) available.

Overall Windows support for large sector (4KB) media

This table documents the official Microsoft support policy for various media and their resulting reported sector sizes. See this KB article for details.


Windows 8, windows server 2012 and Starting with Linux Kernel Version 2.6.34 has full support to read and write 4K for the LBA by the OS. Operating systems like Windows 7, 2008, 2008 R2 still uses 512 Bytes for each logical block or sector, so one physical sector of 4K contains 8 logical sectors of size 512 Bytes.


512-byte emulation at the drive interface



To maintain compatibility, Western Digital, Hitachi, Toshiba emulates a 512-byte device by maintaining a 512-byte sector at the drive interface that is the firmware inside the HDD controller will do the conversion; and Seagate uses SmartAlign technology, for this emulation (firmware level). These drives are also called Advanced Format 512e. Let see how this works !!!

512-byte Read

When the host requests to read a single 512-byte logical block, the hard drive will actually read the entire 4K physical sector containing the requested 512 bytes. The 512-byte block is extracted and sent to the host. This can be done very quickly.


512-byte Write (Read-Modify-Write)

When the host attempts to write a single 512-byte logical block, the hard drive will first read the 4K physical sector containing the 512 bytes that are to be overwritten. Next, it will insert the 512 bytes of new data and write the entire 4K block of data back to the media. This process is called a “Read-Modify-Write”. The drive must read the existing data, modify a subset, and then write the data back to the disk. This process can require additional revolutions of the hard disk.


How Does Advanced Format Technology Maintain Performance?

In order to maintain top performance, it is important to ensure that writes to the disk are aligned. Ideally, writes should be done in 4K blocks, and each block will then be written to a physical 4K sector on the drive. This can be accomplished by ensuring that the OS and applications write data in 4K blocks, and that the drive is partitioned correctly. Most modern operating systems use a file system that allocates storage in 4K blocks or clusters (NTFS Cluster/File allocation unit or EXT3/4 file system block size). In a traditional hard drive, the 4K block is made up of eight 512-byte sectors (see Figure 4: 512-byte Emulated Device Sector Size).


In production, the RAID layer will come in to picture, these 4KB physical sectors are again combined to form a RAID STRIP size, this may vary from 4KB to 256 KB depending upon the RAID level. In the storage array also this is the same. So the RAID array controller like PERC, Intel, LSI, Smart Array etc will handle and gives a RAID volume to the OS. In both cases the OS partition needs to be aligned.

Since most modern operating systems will write in 4K blocks, it is important that each 4K logical block is aligned to a physical 4K block on the disk (see Figure 5). This is especially important because the 512e feature of the drive cannot prevent a partitioning utility from creating a misaligned partition. When misalignment occurs, a logical 4K block will reside on two physical sectors.


In this case, a single read or write of a 4K block will result in a read/write of two physical sectors. The impact of a “read” is minimal, whereas a single write will cause two “Read-Modify-Writes” to occur, potentially impacting performance (see Figure 6).


So now what will happen with the OS like XP, windows 2003, RHEL5.x etc and why we need disk Alignment in PHYSICAL world or in VIRTUAL world ?

As I mentioned the physical disks has 4KB physical sectors and RAID volume or a LUN from storage array  has 4KB to 256 KB STRIP SIZE, that is a multiple of 4KB, and the operating system has 512 bytes of logical sector. When we install the operating system, like 2003, RHEL 5.x. in a HDD or a LUN, in these OS the first 62 sectors (first track) of the HDD is reserved for BOOT area and it is hidden to the OS.

That is sectors from 0 to 62 and reserved (hidden), the master boot record (MBR) resides within these hidden sectors. The master boot record (MBR) resides within these hidden sectors. It uses the first sector of the first track for MBR data (LBA 0) and the first partition begins in the last sector of the first track, which is from (logical) block address 63. You can see this in the below;


Here in RHEL 5.x/LINUX older version, we can see the first partition starts from sector/LBA 63, and if you add another HDD or a LUN this host, and when you create a partition, the partition tool of these linux versions again create partition starting from sector 63.


Here in the above info from Windows 2003, the first partition starts from the offset 32256, in windows it won’t show the LBA/sector number, instead it shows the values in Bytes. That is an offset of 32256 means (32256/512) = 63 LBA/sector, so the partition starts from sector 63. Below is the detailed way of confirming this;

Essential Correlations: Partition Offset, File Allocation Unit Size, and Stripe Unit Size

Use the information in this section to confirm the integrity of disk partition alignment configuration for existing partitions and new implementations.

There are two correlations which when satisfied are a fundamental precondition for optimal disk I/O performance. The results of the following calculations must result in an integer value:

Partition_Offset ÷ Stripe_Unit_Size (Disk physical sector size or RAID strip size)

Stripe_Unit_Size ÷ File_Allocation_Unit_Size

Of the two, the first is by far the most important for optimal performance. The following demonstrates a common misalignment scenario: Given a starting partition offset for 32,256 bytes (31.5 KB) and stripe unit size of 4086 bytes (4 KB), the result is 31.5/4 = 7.894273127753304. This is not an integer; therefore the offset & strip unit size are not correlated.

In the second one, the NTFS cluster size or file allocation unit the default value is 4086, and we can give 32KB, 64KB etc. If it is MSSQL and EXCHANGE It is recommended to give 64KB during the formatting time, and this value is not an issue and it will be an integer. But the first one is the crucial !!!

So we have MISALIGNMENT, and we need to realign the partition, the below diagrams show pictorially;


So Windows 7, 8, 2008, 2008 R2, 2012, RHEL 6, Debian 6, Ubuntu 10, 11, 12, SUSE 11 onwards, automatically aligns partitions during installation. You can see this in the below;





In Windows case the partition alignment defaults to 1024 KB or 1MB boundary (that is, startingoffset 1,048,576 bytes = 1024KB). It correlates well (as described in the previous section, 1024KB/4KB = 256 an integer) with common stripe unit sizes such as 4KB, 64 KB, 128 KB, and 256 KB as well as the less frequently used values of 512 KB and 1024 KB. That is simply the windows partition tool begin the first partition at LBA/sector 2048 (1,048,576/512 = 2048). So here not need to manual alignment and if we add another disk also it will do auto alignment.




In RHEL and latest linux cases, the first partition starts from 2048 that is LBA 0 to 2047 is reserved. That is the OS is aligned to 1MB boundary, if we do the math the sector 2048 is at the offset 1,048,576 bytes (1,048,576 bytes/512 = 2048) and if we add another HDD or LUN it will do alignment automatically.

My next post will be discussing how to do the disk alignment in vSphere or any other hypervisor.



ESXi VMFS Heap size Blockade – For Monster Virtual Machines in Bladecenter Infrastructure

Recently I faced a big issue with my client, we are migrating VM’s from old HP Proliant servers to HP Bladecenter, we are using
vSphere5, HP BL680c G7 Blades (Full-height, double-wide blade – 512GB RAM, Quad socket), HP 3PAR V400 FC SAN

The VM’s are very huge in VMDK level, some of them are 10TB and most of the others are in the range 2 to 3 TB VMDK. We have moved around 140 VM’s (35 per ESXi host).
After that we are not able to do VMotion, DRS, SVMotion and we are not able to create new VM’s.

I believe those who are are going/planning to achieve more consolidation ratio in their datacenter with vSphere, will definitely face this issue. Now a days the HP/IBM Bladecenters/ IBM PureFlex etc dominates the big Datacenters and also the Specs of the Rack servers, RAM size etc are huge. So while doing the new vSphere implementation or Design, this factor has to be considered well in advance.The errors we got are;

During SVmotion –

Relocate virtual machine lf0hrap01p.xxxx A general system error occurred: Storage VMotion failed to copy one or more of the VM ‘s disks.  Please consult the VM’s log for more details, looking for lines starting with “SVMotion”.


During VMotion –

Migrate virtual machine wfowsus1p.xxxx A general system error occurred: Source detected that destination failed to resume.


During HA –

After virtual machines are failed over by vSphere HA from one host to another due to a host failover, the virtual machines fail to power on with the error:
vSphere HA unsuccessfully failed over this virtual machine. vSphere HA will retry if the maximum number of attempts has not been exceeded. Reason: Cannot allocate memory.

During manual VM migration –

When you try to manually power on a migrated virtual machine, you may see the error:
The VM failed to resume on the destination during early power on.
Reason: 0 (Cannot allocate memory).
Cannot open the disk ‘<<Location of the .vmdk>>’ or one of the snapshot disks it depends on.

You see warnings in /var/log/messages or /var/log/vmkernel.log similar to:

vmkernel: cpu2:1410)WARNING: Heap: 1370: Heap_Align(vmfs3, 4096/4096 bytes, 4 align) failed. caller: 0x8fdbd0
vmkernel: cpu2:1410)WARNING: Heap: 1266: Heap vmfs3: Maximum allowed growth (24) too small for size (8192)
cpu4:1959755)WARNING:Heap: 2525: Heap vmfs3 already at its maximum size. Cannot expand.
cpu4:1959755)WARNING: Heap: 2900: Heap_Align(vmfs3, 2099200/2099200 bytes, 8 align) failed. caller: 0x418009533c50
cpu7:5134)Config: 346: “SIOControlFlag2” = 0, Old Value: 1, (Status: 0x0)

Resolution :-

The reason for this issue is, with the default installation/configuration of ESXi host, there is a limitation in the VMkernel, to handle the Opened VMDK files in the VMFS file system.

The default heap size in ESXi/ESX 3.5/4.0 for VMFS-3 is set to 16 MB. This allows for a maximum of 4 TB of open VMDK capacity on a single ESX host.
The default heap size has been increased in ESXi/ESX 4.1 and ESXi 5.x to 80 MB, which allows for 8 TB of open virtual disk capacity on a single ESX host.

From the VMware KB article 1004424 it is explained and the steps to resolve this issue.

We need to change the VMFS heap size of the ESXi host to 256 MB, and reboot the host.


– In the article it is mentioned VMFS3 and in the ESXi5 host advance configuration also you only see the VMFS3, but this applies to VMFS5 also. We confirmed from the VMware technical support.

– The VMFS Heap is a part of the kernel memory, so increasing this will increase memory consumption of the kernel which results in shortage of memory for other VM’s on the system.

Format a ESX & ESXi VMFS file system manually

Recently I came across a query in the VMware communities, the query was “How to reformat a VMFS file system or Datastore manuallly”

The preferred method of reformat the VMFS file system is from a console or SSH session as you can simply recreate the file system without having to make any changes to the disk partition.

Note: All data on a VMFS volume is lost when the datastore is recreated. Migrate or move all virtual machines and other data to another datastore. Back up all data before proceeding.
Note: This procedure should not be performed on a local datastore on an ESX host where the Operating System is located as it may remove the Service Console privileged virtual machine which is located there.


From the ESX/ESXi console:

Use the vmkfstools command to format the VMFS-5 or VMFS-3 datastore (Partition), specifying a Block Size of 1 to 8MB, a friendly name for the datastore, and the chosen disk device and the created partition.

vmkfstools -C vmfs3 -b BlockSize -S DatastoreVolumeName /vmfs/devices/disks/ DeviceName:Partition

vmkfstools -C vmfs5 -b BlockSize -S DatastoreVolumeName /vmfs/devices/disks/ DeviceName:Partition



1, Storage vMotion, or move the virtual machines located on the datastore you would like to re format.

2, Log in to the Local Tech Support Mode console of the ESX/ESXi host. For more information, see Unable to connect to an ESX host using Secure Shell (SSH) (1003807), Tech Support Mode for Emergency Support (1003677), or Using Tech Support Mode in ESXi 4.1 and ESXi 5.0 (1017910).

3, Use the esxcfg-scsidevs command to obtain the disk identifier (mpx or naa) for the datastore you want to re format.
# esxcfg-scsidevs –m


4, Use vmkfstools to create a new VMFS datastore file system with the corresponding block size.

Note: vSphere 5.0 and later releases have a block size of 1MB only. For ESX/ESXi in Vsphere 4 it is from 1MB to 8MB

# vmkfstools -C VMFS-type -b Block-Size -S Datastore-Name /vmfs/devices/disks/Disk-Identifier:Partition-Number

For Vsphere 5x
vmkfstools -C vmfs5 –b 1m -S Storage1 /vmfs/devices/disks/mpx.vmhba1:C0:T0:L0:3

For Vsphere 4x
vmkfstools -C vmfs3 –b 8m -S Storage1 /vmfs/devices/disks/mpx.vmhba1:C0:T0:L0:3


5, Performing a rescan of the storage on an ESX/ESXi host

Using the ESXi 5.x

To rescan all HBAs:
esxcli storage core adapter rescan –all

To rescan a specific HBA:
esxcli storage core adapter rescan –adapter <vmkernel SCSI adapter name>

Where <vmkernel SCSI adapter name> is the vmhba# to be rescanned. To get a list of all adapters, run the esxcli storage core adapter list command.


esxcfg-rescan <vmkernel SCSI adapter name>

esxcfg-rescan –all

Where <vmkernel SCSI adapter name> is the vmhba# to be rescanned.

In Vsphere 4x

esxcfg-rescan <vmkernel SCSI adapter name>

esxcfg-rescan –all

Where <vmkernel SCSI adapter name> is the vmhba# to be rescanned.

Or simply in Vsphere 4x and 5x

To search for new VMFS datastores, run this command:
vmkfstools –V

Note: This command does not generate any output.



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