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Virtualization has made shared storage a new reality in data centres. Learn which technologies offer the best fit and how to pinpoint the source of possible problems.
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Storage: The Backbone ofyour Virtual Environment
q EVALUATING STORAGEFOR VIRTUAL ENVIRONMENTS
q TESTING STORAGEFOR VIRTUALIZATION
q EXPLORING ADVANCEDSTORAGE FEATURES
q TROUBLESHOOTING STORAGE
EVALUATING STORAGE FORVIRTUAL ENVIRONMENTS
TESTING STORAGEFOR VIRTUALIZATION
EXPLORING ADVANCEDSTORAGE FEATURES
TROUBLESHOOTINGSTORAGE
2 STORAGE : THE BACKBONE OF YOUR V IRTUAL ENV IRONMENT
Evaluating Storagefor Virtual Environments
with server virtualization, operating systems and applications are no
longer tied to one specific piece of hardware. They canmove frommachine to
machine—either manually or automatically—and their data must be able to
follow. Shared storage makes that happen.
So what exactly is shared storage? It refers to a device or devices that store
data for and are accessible by multiple systems over a network. In addition to
providing the flexibility needed to support virtualization, shared storage also
increases efficiency.When servers don’t have to spend the resources to store
data locally or transmit data to attached devices, they can use that processing
power to handle more workloads—or more intensive workloads—which helps
server consolidation.
There are twomain storage approaches for virtualization: storage area net-
works (SANs), which typically use iSCSI or Fibre Channel technologies, and
network-attached storage (NAS). Each technology has its pros and cons. Your
organization’s goals, size and budget—among other factors—will determinewhich
technology or blend of technologies is a good fit for your virtualization project.
FIBRE CHANNEL SAN
A SAN uses block-level storage, which relies on a filing system to write and re-
trieve data. It provides high performance, but it can be expensive and complex.
Cost and complexity can be prohibitive for small andmedium-sized busi-
nesses for whom the costs of buying, installing andmanaging a SANmay out-
weigh the savings that would result from consolidating a few physical servers.
Despite these issues, SAN technology is still a popular choice for virtualization
storage, especially among larger organizations. That’s because many SANs rely
on Fibre Channel networking, which provides high-speed data transmission.
EVALUATINGSTORAGE FOR
VIRTUALENVIRONMENTS
The speed of data transmission is essential in virtual environments, espe-
cially when it comes to performance. After all, it doesn’t matter how fast a phys-
ical server and its virtual machines (VMs) can process data if the data is slow in
arriving to those systems in the first place. For some workloads, such as test
and development projects, a certain amount
of lag timemay be acceptable. But for oth-
ers, such as databases orWeb servers, max-
imum performance is a must.
When calculating potential return on in-
vestment (ROI) in a Fibre Channel SAN, or-
ganizations should consider speed because
it will affect backup and recovery. Some
businesses start losing money the second a
certain application goes down, and the
bleeding doesn’t stop until the application comes back up. In these cases, speed
is of the utmost importance, and the ROI of Fibre Channel is clear. But for or-
ganizations that can tolerate some downtime, Fibre Channel might not be
worth the money.
FIBRE CHANNEL OVER ETHERNET
For organizations looking to realize the benefits of Fibre Channel with less cost
and complexity, Fibre Channel over Ethernet (FCoE) is an emerging alternative.
The FCoE protocol lets organizations use their existing Ethernet infrastruc-
tures to transmit data across a SAN. Because of that, it requires fewer cables
and network cards, which can reduce the cost of a SAN implementation.
Despite these benefits, FCoE does have some drawbacks. The speed of exist-
ing Ethernet networks has raised concerns among some administrators. But as
more organizations move from 1-Gigabit Ethernet to 10-Gigabit Ethernet, that
becomes less of an issue.
Other businesses are hesitant to add storage traffic to their existing Ethernet
3 STORAGE : THE BACKBONE OF YOUR V IRTUAL ENV IRONMENT
EVALUATING STORAGE FORVIRTUAL ENVIRONMENTS
TESTING STORAGEFOR VIRTUALIZATION
EXPLORING ADVANCEDSTORAGE FEATURES
TROUBLESHOOTINGSTORAGE
EVALUATINGSTORAGE FOR
VIRTUALENVIRONMENTS
The speed of datatransmission isessential in virtualenvironments, espe-cially when it comesto performance.
networks because of the addedmanagement it requires—and the friction it can
cause between storage administrators and network administrators.
iSCSI SAN
Another SAN option is iSCSI, a protocol that arrived after many organizations
had already adopted Fibre Channel. Like FCoE, iSCSI can run over Ethernet
networks, and that can result in cost savings and reducedmanagement com-
plexity.
Organizations that choose iSCSI can use their existing networks without
having to buy the expensive switches, adapters and other equipment required
to support Fibre Channel. Plus, administrators don’t have to learn how to man-
age a whole new network.
Some experts say that performance takes a hit on an iSCSI SAN compared to
Fibre Channel. Others say that a properly configured iSCSI SAN can reach sim-
ilar performance levels.
NETWORK-ATTACHED STORAGE
The SAN versus NAS debate is similar to the Fibre Channel versus iSCSI de-
bate in that each centers on both cost and performance.
Unlike a SAN, NAS takes a file-level approach to storage for virtualization.
The additional layer of abstraction that a SAN provides isn’t there with NAS, so
there’s less complexity and, therefore, lower costs.
But security is an added concern when using NAS for virtualization storage.
Because NAS stores the file systems, anyone who can access the NAS device
can access those files if they have the proper privileges. SANs control security
themselves, regardless of the security levels of individual file systems.
Having options can help organizations come up with the best solutions. In
the next segment, learn how to test each approach and determine which is best
for your environment. �
4 STORAGE : THE BACKBONE OF YOUR V IRTUAL ENV IRONMENT
EVALUATING STORAGE FORVIRTUAL ENVIRONMENTS
TESTING STORAGEFOR VIRTUALIZATION
EXPLORING ADVANCEDSTORAGE FEATURES
TROUBLESHOOTINGSTORAGE
EVALUATINGSTORAGE FOR
VIRTUALENVIRONMENTS
Testing Storage for Virtualization
you can do all the research in the world, but the only way to make sure you
have the right storage for your virtual environment is to test it. It’s important to
test your storage to find out which results require the most attention.
As of now, the use of solid-state drives (SSD) is not commonplace. Even if it
were, all the available shared storage technologies communicate over a wire
from the virtualization host to a storage array. This wire either communicates
using Fibre Channel protocol, iSCSI, a network file system or Fibre Channel
over Ethernet.
These protocols and the speeds at which the data is transmitted across the
wire are often seen as limiting factors in storage. Also consider all the devices
on the path to the disk where the data will eventually be saved. This includes
such things as host bus adapters, local cache, Ethernet or Fibre Channel
switches, storage processors, disk controllers and the spindles on each of the
disks, if you are not using SSD.
STORAGE AS A SHARED RESOURCE
In addition to the hardware considerations, remember that storage in a virtual
environment is a shared resource—shared not only among VMs running on a
host but also among hosts. So any technology used for storage—whether file
sharing or block storage—must support clustered file systems. That implies
some ability to minimally lock the file system table of contents to ensure up-
dates are serialized across all hosts.
This type of locking often adds quite a bit of complexity into a seemingly sim-
ple storage system. Now add the ability to have multiple storage paths from the
VM to the storage device, and you will see that real-world testing is often re-
quired in the form of a bake-off between storage vendors.
The tests are intended to determine two things: one is performance of reads
5 STORAGE : THE BACKBONE OF YOUR V IRTUAL ENV IRONMENT
EVALUATING STORAGE FORVIRTUAL ENVIRONMENTS
TESTING STORAGEFOR VIRTUALIZATION
EXPLORING ADVANCEDSTORAGE FEATURES
TROUBLESHOOTINGSTORAGE
TESTINGSTORAGE FOR
VIRTUALIZATION
and writes through the virtual environment—often referred to as latency—and
the other is the maximum number of I/O operations per second (IOPS) that can
take place to and from the storage device without increasing latency. IOPS can
also be seen as a measure of the number of VMs a particular storage device can
handle per logical unit, or LUN, of the storage device.
In some cases, using more LUNswill decrease lock operations. It will also in-
crease IOPSwhen a LUN is locked because no operations can take place for the
length of the lock, which is usually less than 10 microseconds. But because
quite a fewwrites or reads can take
place during 10 microseconds, it’s im-
portant to remember that IOPS and la-
tency values will suffer if the lock
extends past the desired time.
CREATING A REAL-WORLD
TEST ENVIRONMENT
When setting up a test for your storage
subsystem, create as close to a real-
world test as possible. Ideally, you
would put the new storage system into
your QA lab and thenmove via Stor-
age vMotion or otherwise migrate
your real test environment to the new storage device.
If your QA environment matches your production environment, you have a
ready-built test environment. If your QA environment does not match your
production environment, then you have your work cut out for you.
For better results, youmay consider trying to match your QA environment as
closely as possible to your production environment in your count of application
instances, quantity of transactions and data per transaction. If your QA envi-
ronment does not match your production environment, your storage latency
6 STORAGE : THE BACKBONE OF YOUR V IRTUAL ENV IRONMENT
EVALUATING STORAGE FORVIRTUAL ENVIRONMENTS
TESTING STORAGEFOR VIRTUALIZATION
EXPLORING ADVANCEDSTORAGE FEATURES
TROUBLESHOOTINGSTORAGE
TESTINGSTORAGE FOR
VIRTUALIZATION
If your QA environmentmatches your productionenvironment, you havea ready-built test envi-ronment. If your QAenvironment does notmatch your productionenvironment, then youhave your work cut outfor you.
and IOPS values as well as counts of VMs per LUNwill not be accurate and
may actually be quite a bit worse than expected when you finally place the cho-
sen storage device into production.
TWO TESTS TO CONSIDER
There are at least two tests to consider: those that have many small virtual disks
per LUN and those that have a few large virtual disks per LUN. If you are test-
ing Tier 1 applications, youmay also have raw disk maps for very large LUNs to
hold data such as a database or an extremely large file share.
Ensure that your test mimics the real
world as much as possible. Determine
your max throughput by inputting trans-
actions in your environment as fast as
you possibly can. Next, use a real-world
quantity of transactions to determine
your steady-state numbers.
It is important to use the proper tools
to determine latency and IOPS values.
Such tools will be able to get performance
data directly from the array as well as de-
termine how your virtual environment
sees the performance—both are required. Once you have completed your test-
ing, you can then choose the proper layout of your storage within the virtual en-
vironment and the proper storage subsystem. The key for this type of testing is
good planning, suitable documentation and a good test suite that is as real-
world as possible. �
7 STORAGE : THE BACKBONE OF YOUR V IRTUAL ENV IRONMENT
EVALUATING STORAGE FORVIRTUAL ENVIRONMENTS
TESTING STORAGEFOR VIRTUALIZATION
EXPLORING ADVANCEDSTORAGE FEATURES
TROUBLESHOOTINGSTORAGE
TESTINGSTORAGE FOR
VIRTUALIZATION
Once you havecompleted your testing,you can then choosethe proper layout ofyour storage withinthe virtual environ-ment and the properstorage subsystem.
Exploring Advanced Storage Features
storage can enhance your virtual environment. Adding advanced features
such as data deduplication, thin provisioning and others can actually improve
virtualization performance and reduce your storage footprint.
Storage is one of the more expensive parts of any virtual environment. Be-
cause of that, use this limited resource sparingly. The good news is that today’s
technology helps administrators use storage more efficiently.
Some technologies that aid efficiency are in hardware, such as data dedupli-
cation. Others are not, such as thin provi-
sioning and linked clones. All these
technologies are designed to allow ad-
mins to use limited storage capabilities
better.
HOW DATA DEDUPLICATION WORKS
Data deduplication allows a storage de-
vice to condense identical blocks of data
down to one block with a link back to
that block per LUN and sometimes
across LUNs. This happens within the storage processor and is possible only
because of advancements in processor capabilities such as multiple cores.
In some cases, data deduplication happens after the fact as a background
task. Until the deduplication task is completed, the number of actual blocks of
disk being usedmay change.
Deduplication is extremely useful for unencrypted data because once the
data is encrypted, deduplication would be minimal.While already removing
like blocks from a file, compressed data may still participate in deduplication
because many compressed files of similar construct can still be deduplicated.
8 STORAGE : THE BACKBONE OF YOUR V IRTUAL ENV IRONMENT
EVALUATING STORAGE FORVIRTUAL ENVIRONMENTS
TESTING STORAGEFOR VIRTUALIZATION
EXPLORING ADVANCEDSTORAGE FEATURES
TROUBLESHOOTINGSTORAGE
EXPLORINGADVANCEDSTORAGE
FEATURES
Data deduplicationallows a storage deviceto condense identicalblocks of data down toone block with a linkback to that block perLUN and sometimesacross LUNs.
Encryption attempts to avoid having the data appear the same twice, regardless
of similarities.
On the other hand, thin provisioning is a technology that allocates only the
in-use blocks of a file or virtual disk instead of the completely requested alloca-
tion of blocks. Thin provisioning could work with data deduplication to use
even fewer blocks of disk than previous thought because instead of allocating
blocks to participate in deduplication, the blocks are not allocated until needed.
Unfortunately, there is no way to
prevent a thin provisioned file or vir-
tual disk from allocating all its blocks.
Because of that, either could easily use
muchmore storage than originally en-
visioned.
ADVANTAGES OF LINKED CLONES
Linked clones provide the ability to
link one virtual disk to another virtual disk in such a way that only the changes
between the original virtual disk and the cloned virtual disk are recorded
within the clone. This is an extremely useful construct if a base part of a virtual
disk does not change, such as in the operating system of a VM.
The linked clone would instead contain only those changes required for a
given application, user or service. Linked clones tied to thin provisioning and
data deduplication could lead to quite a few storage blocks saved on a storage
device.
But a linked clone can grow to the full size of the original disk if the blocks of
the original disk are changed in any way, such as the case of an operating sys-
tem patch or disk optimization performed fromwithin the linked clone.
Linked clones have a further limitation in the depth of clones. The more
depth to the clones, the slower the virtual disk will be because of the block fix-
ups that are required.
9 STORAGE : THE BACKBONE OF YOUR V IRTUAL ENV IRONMENT
EVALUATING STORAGE FORVIRTUAL ENVIRONMENTS
TESTING STORAGEFOR VIRTUALIZATION
EXPLORING ADVANCEDSTORAGE FEATURES
TROUBLESHOOTINGSTORAGE
EXPLORINGADVANCEDSTORAGE
FEATURES
Linked clones tied tothin provisioning anddata deduplicationcould lead to quite a fewstorage blocks saved ona storage device.
WEIGHING STRENGTHS AND WEAKNESSES
If used correctly, all of these technologies could reduce the overall quantity
of storage required for your virtual environment, but each has its ownweak-
nesses and strengths. One weakness is that to use vSphere Fault Tolerance, the
virtual disk must not be thin provisioned or a link cloned.What is required is a
thickly allocated disk, meaning that all blocks are allocated and in use.
Data deduplication will not produce the same results if data is encrypted
before being written to the storage
array. That is the only way it would hap-
pen with today’s technology.
Always use care with tools such as
thin provisioning and linked clones be-
cause these technologies can easily be
abused. Any savings you thought you
would receive would be incorrect be-
cause of sudden growths of the virtual
disks to full sizes.
The truly inconsiderate component of
these sudden growths is that technology
does not yet exist to tell you that this is eminent. The virtual environments re-
port the full size of the virtual disks instead of the allocated sizes, and the stor-
age devices report only on what space is left, not on what space is available on a
per-file basis.
So use extreme care and audit results when using these technologies. Some
form of continual monitoring may be required if space on your storage arrays
suddenly grows.
Thin provisioning and linked clones are ways to fit more virtual disks per
LUN and to make better use of your storage device. Alternately, data deduplica-
tion has no such limits unless the data is encrypted. The bottom line is to use
caution when choosing these technologies, and know their limitations. �
1 0 STORAGE : THE BACKBONE OF YOUR V IRTUAL ENV IRONMENT
EVALUATING STORAGE FORVIRTUAL ENVIRONMENTS
TESTING STORAGEFOR VIRTUALIZATION
EXPLORING ADVANCEDSTORAGE FEATURES
TROUBLESHOOTINGSTORAGE
EXPLORINGADVANCEDSTORAGE
FEATURES
Data deduplicationwill not produce thesame results if data isencrypted before beingwritten to the storagearray. That is the onlyway it would happenwith today’s technology.
Troubleshooting Storage
problems are bound to pop up nomatter howwell you have researched and
implemented storage in your virtual environment. Uptime and I/O speed can
be challenging to maintain. The best defense is to find out how to locate the
source of possible problems and fix them.
In general, once you have a properly configured storage subsystem that is
built to meet the requirements of your virtual environment, there is often no
need to modify the environment unless something changes. Those changes can
include adding more applications to the original mix or adding new application
functionalities that require more storage.
Some changes could be more subtle. Connection issues happen occasionally,
and usually the first to fail is the virtualization host. Modern virtualization
hosts are good at connecting to storage that has been properly presented and
zoned to the virtual environment.
If there is a connectivity issue, it is essential to first check the hardware in-
volved for proper connections—such as making sure lights are properly lit—
and then check the zoning and presentation of the LUN to the virtualization
host. It should be noted that some hypervisors limit the number of LUNs that
can be used.
LOCKING STORAGE SUBSYSTEMS
Locking is required when dealing with files that are shared and that cross host
boundaries. So, at a minimum, the table of contents of each file system should
be locked.
When virtualization first started, locking storage subsystems was a prevalent
problem. But with modern hypervisors, many of these issues have been solved
by applying evenmore technology such as segmented table of contents, in-
creased lock time-outs and increased number of lock retries.
1 1 STORAGE : THE BACKBONE OF YOUR V IRTUAL ENV IRONMENT
EVALUATING STORAGE FORVIRTUAL ENVIRONMENTS
TESTING STORAGEFOR VIRTUALIZATION
EXPLORING ADVANCEDSTORAGE FEATURES
TROUBLESHOOTINGSTORAGE
TROUBLESHOOTINGSTORAGE
Storage subsystems have also improved, but problems still arise occasionally.
Amore prevalent issue is SCSI reservation time-out, which means the lock
failed so the data was only partly written or not written at all.
No one wants to see a time-out error message in a vSphere log file because it
implies something went seriously wrong. Before reviewing all disk write ac-
tions that have taken place, don’t forget to inspect the hardware.
Because most people assume the hardware is not at fault, a hardware inspec-
tion is often performed last. But, for locking issues, the reverse is often true.
After inspecting the hardware, review other types of actions that may cause
locks. Investigate anything that changes the table of contents of the clustered
file system.
Performance issues are those stemming from the measured or perceived
slowness of an application. The first thing to understand about performance is
that it may not be the storage subsystem, but you’ll need the proper tools to de-
termine if this is actually true.
WHICH TOOLS TO USE
Modern virtual environments have several built-in management tools that will
show the LUN latency values as well as the number of IOPS in use. Many third-
party tools also give the same data. Getting this data as close to the hardware is
often paramount to determining what is actually going on.
One helpful tool is NetApp Balance, which correlates VM activity with the
activity on the LUN of the questionable storage array. If latency is high, then re-
balance the data across LUNs and perhaps across storage paths until there is a
better latency score.
Changing storage paths could also imply moving the load from one virtual-
ization host to another. For these tasks, vSphere Storage vMotion and vMotion
or Xen/Hyper-V LiveMigration tools can be extremely useful.
There is no easy fix for a large number of IOPS unless you have control
over the application and can spread the load across multiple LUNswith data-
1 2 STORAGE : THE BACKBONE OF YOUR V IRTUAL ENV IRONMENT
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TESTING STORAGEFOR VIRTUALIZATION
EXPLORING ADVANCEDSTORAGE FEATURES
TROUBLESHOOTINGSTORAGE
TROUBLESHOOTINGSTORAGE
splitting or caching technologies or can use massively parallel databases such
as Greenplum. But even with technologies like these, the solution is often to re-
vamp an application instead of bolting on a solution.
And vSphere’s Storage IO Control (SIOC) functionality could also be used to
give higher priority to writes made by those applications marked as critical or
those that have an increased amount of shares of the storage path than other
virtual disks on the same path.
But SIOC is a contention-based tech-
nology based on LUN latency values. If
the latency values are below 30mi-
croseconds, then SIOC is not in use.
Multipath-plugin tools are also help-
ful. They allow you to use tools provided
by the storage vendors to aggregate, load
balance and better control how data is
written to the storage array.
There is also an increase in using lo-
calized SSD drives and storage cache
cards to improve overall write and read times to and from the storage array.
Writes hit the local cache and then are forwarded over the wire. Local reads of
the same data are then accelerated, as is the response time for writes.
Other issues can cause a malfunction in storage environments, such as a sud-
den loss of connectivity or storage processor failures. These failures often show
up as virtual environment issues andmay be investigated within the virtual en-
vironment first.
Any failure will adversely affect your virtual environment. Storage failures
can show up as other issues that suddenly increase latency and affect perform-
ance It’s always a good idea to investigate the storage subsystem at the same
time. �
1 3 STORAGE : THE BACKBONE OF YOUR V IRTUAL ENV IRONMENT
EVALUATING STORAGE FORVIRTUAL ENVIRONMENTS
TESTING STORAGEFOR VIRTUALIZATION
EXPLORING ADVANCEDSTORAGE FEATURES
TROUBLESHOOTINGSTORAGE
TROUBLESHOOTINGSTORAGE
Multipath-plugintools allow you to usetools provided by thestorage vendors toaggregate, load balanceand better control howdata is written to thestorage array.
Edward L. Haletky is the authorofVMware vSphere and VirtualInfrastructure Security: Securingthe Virtual Environment andVM-ware ESX and ESXi in the Enter-prise: Planning Deployment ofVirtualization Servers, 2nd Edi-tion. He owns AstroArch ConsultingInc., which provides virtualization,security, network consulting and de-velopment, and The VirtualizationPractice, where he is an analyst.Haletky is the moderator and host ofthe Virtualization Security Podcastand moderator for the VMware Com-munities Forums.
Colin Steele is senior site editorfor SearchServerVirtualization.comand SearchVMware.com, which pub-lish expert content for IT profession-als working with server virtualiza-tion technology. Before joiningTechTarget in 2007, he was a news-paper reporter for The Eagle-Tribune in North Andover, Mass.Steele has previously written for theGloucester Daily Times, Tri-TownTranscript and The Daily Orangeat Syracuse University, where hereceived his bachelor’s degree innewspaper journalism in 2002.
1 4 STORAGE : THE BACKBONE OF YOUR V IRTUAL ENV IRONMENT
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TESTING STORAGEFOR VIRTUALIZATION
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TROUBLESHOOTINGSTORAGE
EXECUTIVE EDITORJo Maitland
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