© 2010 Quest Software, Inc. ALL RIGHTS RESERVED

Myth Busting Goes Virtual

Todd Mera - Todd.Mera@Quest.comSystems Engineer | Quest SoftwareServer Virtualization Management Group(541) 314-3302

Mattias SundlingEvangelist, Quest Software

Eric SloofVMware Certified Instructor, NTPRO.NL

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Just in case you are curious…• 2010 – 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 – Sinéad O'Connor, Irish musician

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Introduction• vSphere 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.

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Agenda/Myths

1) iSCSI RDMs have superior performance over VMFS.2) CBT causes significant overhead on your VMs.3) Resource Pools should always be used to categorize and allocate

resources to VMs.4) LSI SCSI is always better than Paravirtual SCSI.

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Myth 1:iSCSI RDMs have superior performance over VMFS

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What is a RDM

• A 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.

RAW LUN

VMFS volume

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VMkernel and Storage

• Physical Compatibility mode RDM

• Virtual mode RDM

• Regular VMDK on VMFS

• Logical device IO scheduler

• vStorage APIs

• Adapter IO schedulerand Linux emulation

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The iSCSI Laboratory• Iomega StorCenter px6-300d with 6 SATA

7200 Disks

• Windows 2008 R24096 MB – 1 vCPUHardware Version 8

• VMware vSphere 5• Single Intel 1GB Ethernet• Cisco 2960 switch

MTU Size 1500

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Results Passmark PerformanceTest• RDM Physical• RDM Virtual• VMDK on VMFS• Microsoft software

ISCSI initiator

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Myth Busted

• 1 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 snapshotting.• is 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).

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Myth 2:CBT causes significant overhead on your VMs

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What is CBT?

• Driver inside VMkernel• Identifies change blocks within virtual

disks• Block size based on VMDK size• Backup window and host resources

significantly reduced• Requirements: vSphere 4+ and Virtual

HW v7+• Limitations: pRDM, iSCSI within VM• Enable through vCenter or backup

application (per VM)

CBT Driver

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CBT overhead

• Memory– Max 256 KB / Disk (2TB)– 1.25 KB / 10 GB VMDK

• CPU– Turning on a bit in bitmap when an I/O request completes

• Storage– Space

• One .ctk file per VMDK• .ctk file 0.5 MB / 10 GB VMDK

– I/O• Every time disk gets closed, change tracking info written to disk

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Test/Result• Very little overhead CPU, Mem, Storage• Could not measure it, except .ctk file• No negative impact on disk I/O

CBT Disabled CBT Enabled

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Myth Busted• CBT overhead is very small• Backup window and host resources reduced

significant• Always use CBT if your backup solution

supports it

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

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Resource Settings

• CPU – Shares, Reservation, Limit.• Memory – Shares, Reservation, Limit.

vApp Resource Pool VM

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Sibling CPU Shares Value• vApp = 4000

• Resource Pool= 4000

• VM = number of vCPU * 1000

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Sibling Memory Shares Value• vApp = 163840

• Resource Pool = 163840

• VM = Mem MB * 10

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The Laboratory

Root Resource Pool 24 Ghz

6 Ghz6 Ghz6 Ghz6 Ghz

VM 4000 Shares6 Ghz

1.5 Ghz0.5 Ghz0,75 Ghz6 Ghz

RP 4000 Shares6 Ghz

RP 4000 Shares6 Ghz

vApp 4000 Shares6 Ghz

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Myth Busted

If you want to

• use Resource Pools, do your math!

• group VMs in a container use the blue folders!

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Myth 4:

LSI Logic SCSI is always better than PVSCSI

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What is PVSCSI?

• Paravirtual SCSI– Introduced in vSphere 4, improved in vSphere 4.1– Designed for high performance (+12%)– Requires less resources on vSphere Host (-18% CPU)

– Supports only Win 2003+, RHEL5+, SUSE Linux Enterprise 11 SP1+, Ubuntu 10.04 +, Linux 2.6.33+

– Virtual HW v7+

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Test Result LSI Logic vs. PVSCSILSI Logic PVSCSI

Host CPU utilization90MB/s – 60 000 IOPS

• Very similar disk throughput

• Lower Host CPU Utilization for PVSCSI

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PVSCSI Limitations• PVSCSI controllers have the following limitations:

– Disks on PVSCSI controllers might not experience performance gains if they have snapshots or if memory on the ESXi host is over committed.

– MSCS clusters are not supported.– PVSCSI controllers do not support boot disks, the disk that contains

the system software, on Red Hat Linux 5 virtual machines. Attach the boot disk to the virtual machine by using any of the other supported controller types.

Source: http://pubs.vmware.com/vsphere-50/index.jsp?topic=/com.vmware.vsphere.storage.doc_50/GUID-8AE88758-20C1-4873-99C7-181EF9ACFA70.html

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Myth Busted• PVSCSI is equal or faster than LSI SCSI• PVSCSI requires less Host resources• Used to have more limitations • PVSCSI is better in all ways so why aren´t we

using it on all supported VMs?• Takes time to change behaviour

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Summary• VMFS is better than RDMs• CBT has very little overhead• Do your math when using sibling RPs• PVSCSI is equal or faster and requires less resources than LSI

SCSI

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Thanks!