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Myth Busting Goes Virtual. Todd Mera - [email protected] Systems Engineer | Quest Software Server Virtualization Management Group (541) 314-3302 . Mattias Sundling Evangelist, Quest Software Eric Sloof VMware Certified Instructor, NTPRO.NL. Just in case you are curious…. - PowerPoint PPT Presentation

Text of Myth Busting Goes Virtual


Myth Busting Goes VirtualTodd Mera - [email protected] Engineer | Quest SoftwareServer Virtualization Management Group(541) 314-3302

Mattias SundlingEvangelist, Quest Software

Eric SloofVMware Certified Instructor, NTPRO.NL

2010 Quest Software, Inc. ALL RIGHTS RESERVED#Just in case you are curious2010 With the second launch of the SpaceX Dragon, SpaceX becomes the first privately held company to successfully launch, orbit and recover a spacecraft.

1995 - The Grateful Dead announced they were breaking up after 30 years of making music. The news came four months after the death of lead guitarist Jerry Garcia

1886 Diego Rivera, Mexican painter (d. 1957)

Eli Whitney (December 8, 1765 January 8, 1825) was an American inventor best known for inventing the cotton gin

1966 Sinad O'Connor, Irish musician#2IntroductionvSphere evolves with every release.Things that used to be true aren't true anymore.Engage in virtualization communities and social media to get up to speed.#People are still making decisions based on ESX 2 and 3 guidelines

Virtualization Communities:Popular websites: VMTN, Yellow-Bricks, NTPRO.NL (maybe mention a few more?)Attend VMUGs

Social Media:Twitter3Agenda/MythsiSCSI RDMs have superior performance over VMFS.CBT causes significant overhead on your VMs.Resource Pools should always be used to categorize and allocate resources to VMs.LSI SCSI is always better than Paravirtual SCSI.#4

Myth 1:iSCSI RDMs have superior performance over VMFS

#Quick Poll: Raise your hand if you think RDMs provides better performance than VMFS 5

What is a RDMA RDM is a symbolic link. When a LUN is opened for access, the mapping file is read to obtain the reference to the raw LUN.Thereafter, reads and writes go directly to the raw LUN rather than going through the mapping file.


#A RDM is a symbolic link from a VMFS volume to a raw LUN. The mapping makes LUNs appear as files ina VMFS volume. The mapping file, not the raw LUN, is referenced in the virtual machine configuration.When a LUN is opened for access, the mapping file is read to obtain the reference to the raw LUN. Thereafter,reads and writes go directly to the raw LUN rather than going through the mapping file.6VMkernel and StoragePhysical Compatibility mode RDM

Virtual mode RDM

Regular VMDK on VMFS

Logical device IO scheduler

vStorage APIs

Adapter IO schedulerand Linux emulation

#Raw Device Mapping CharacteristicsA RDM is a special mapping file in a VMFS volume that manages metadata for its mapped device. The mapping file is presented to the management software as an ordinary disk file, available for the usual filesystemoperations. To the virtual machine, the storage virtualization layer presents the mapped device as a virtual SCSI device. Key contents of the metadata in the mapping file include the location of the mapped device (name resolution), the locking state of the mapped device, permissions, and so on.

RDM Virtual and Physical Compatibility ModesYou can use RDMs in virtual compatibility or physical compatibility modes. Virtual mode specifies full virtualization of the mapped device. Physical mode specifies minimal SCSI virtualization of the mappeddevice, allowing the greatest flexibility for SAN management software. In virtual mode, the VMkernel sends only READ and WRITE to the mapped device. The mapped device appears to the guest operating system exactly the same as a virtual disk file in a VMFS volume. The real hardware characteristics are hidden. If you are using a raw disk in virtual mode, you can realize the benefits of VMFS such as advanced file locking for data protection and snapshots for streamlining development processes. Virtual mode is also more portable across storage hardware than physical mode, presenting the same behavior as a virtual disk file.

In physical mode, the VMkernel passes all SCSI commands to the device, with one exception: the REPORT LUNs command is virtualized so that the VMkernel can isolate the LUN to the owning virtual machine.Otherwise, all physical characteristics of the underlying hardware are exposed. Physical mode is useful to run SAN management agents or other SCSI target-based software in the virtual machine. Physical mode also allowsvirtual-to-physical clustering for cost-effective high availability. VMFS5 supports greater than 2TB disk size for RDMs in physical compatibility mode. The followingrestrictions apply:

- You cannot relocate the RDMs to datastores other than VMFS5.- You cannot convert the RDMs to virtual disks, or perform other operations that involve RDM to virtual disk conversion. Such operations include Storage VMotion, migration, and cloning.7The iSCSI LaboratoryIomega StorCenter px6-300d with 6 SATA 7200 Disks

Windows 2008 R24096 MB 1 vCPUHardware Version 8VMware vSphere 5Single Intel 1GB EthernetCisco 2960 switchMTU Size 1500

#8Results Passmark PerformanceTestRDM PhysicalRDM VirtualVMDK on VMFSMicrosoft software ISCSI initiator

#Myth Busted1 Gigabit/sec (Gbps) = 112.16 MB/s.Use VMFS datastores whenever possible.VMFS is optimized for storing and accessing large files.

Only Use RDMs if your VM

is performing SAN clustered to a physical machine using MSCS.has large amounts of data (64 TB) that you do not want to convert into a VMDK (2 TB).

#Please explain to me why you have so much data on network speeds on this page.

VMware ESX Server offers three options for disk access managementVMFS, RDM (Virtual), and RDM (Physical). All the options provide distributed file system features like userfriendly persistent names, distributed file locking, and file permissions. Both VMFS and RDM allow you to migrate a virtual machine using vMotion. This study compares the three options and finds similar performance for all three.

For random workloads, VMFS and RDM produce similar I/O throughput. For sequential workloads with small I/O block sizes, RDM provides a small increase in throughput compared to VMFS. However, the performance gap decreases as the I/O block size increases. VMFS and RDM operate at similar CPU efficiency levels for random workloads at smaller I/O block sizes. For sequential workloads, RDM shows improved CPU efficiency.

10Myth 2:CBT causes significant overhead on your VMs

#Quick Poll: Who have noticed a slowdown on VMs with CBT enabled?11What is CBT?Driver inside VMkernelIdentifies change blocks within virtual disksBlock size based on VMDK sizeBackup window and host resources significantly reducedRequirements: vSphere 4+ and Virtual HW v7+Limitations: pRDM, iSCSI within VMEnable through vCenter or backup application (per VM)

CBT Driver#Backup reduction: No need to scan for changes, only backup allocated areas of virtual disk, ignoring unallocated areas (of sparse disk)

Enable CBT: Poweredon virtual machines must go through a stununstun cycle (power on, resume after suspend, migrate, or snapshot create/delete/revert) before the reconfiguration takes effect.

To enable Changed Block Tracking with the VMware vSphere Client:Select the virtual machine and ensure that Summary > VM Version says 7 for virtual hardware version. In the Summary tab, click Edit Settings > Options > Advanced > General. In the right side of the dialog box, click Configuration Parameters... In the new dialog box, locate the row for name ctkEnabled, and change its value from false to true. (scsi0:0.ctkEnabled),and set its value to true. One for each diskSee above concerning the stununstun cycle.

To enable Changed Block Tracking and back up with the VMware vSphere API:1 Query change tracking status of the virtual machine. If false, activate Changed Block Tracking.2 Create a snapshot of the virtual machine. The snapshot operation causes a stununstun cycle.

12CBT overheadMemoryMax 256 KB / Disk (2TB)1.25 KB / 10 GB VMDKCPUTurning on a bit in bitmap when an I/O request completesStorageSpaceOne .ctk file per VMDK.ctk file 0.5 MB / 10 GB VMDKI/OEvery time disk gets closed, change tracking info written to disk

#CBT driver uses 1 bit for each block which has variable size based on VMDK size.Lets assume 1 bit represent 16 KB: 64 bits per MB equals 8KB per 1GB VMDK, max 256KB per VMDK (2TB)

13Test/ResultVery little overhead CPU, Mem, StorageCould not measure it, except .ctk fileNo negative impact on disk I/O

CBT DisabledCBT Enabled#14Myth BustedCBT overhead is very smallBackup window and host resources reduced significantAlways use CBT if your backup solution supports it


Myth 3:Resource Pools should always be used to categorize and allocate resources to VMs

#See comment on Page 3

Quick Poll: Who use Resource Pools? What for?Try to get some interaction with audience.16Resource SettingsCPU Shares, Reservation, Limit.Memory Shares, Reservation, Limit.

vApp Resource Pool VM

#Change resource settings of a resource pool or virtual machine Changes the following resource settings:

- CPU Shares, Reservation, Limit.- Memory Shares, Reservation, Limit.

17Sibling CPU Shares ValuevApp = 4000

Resource Pool= 4000

VM = number of vCPU * 1000

#Sibling Memory Shares ValuevApp = 163840

Resource Pool = 163840

VM = Mem MB * 10


The Laboratory

Root Resource Pool 24 Ghz6 Ghz6 Ghz6 Ghz6 GhzVM 4000 Shares6 Ghz1.5 Ghz0.5 Ghz0,75 Ghz6 GhzRP 4000 Shares6 GhzRP 4000 Shares6 GhzvApp 4000 Shares6 Ghz

#Myth BustedIf you want to

use Resource Pools, do your math!

group VMs in a container use the blue folders!

#Myth 4:LSI Logic SCSI is always better than PVSCSI

#Quick Poll: Who

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