IBM DS8700 users now have the tools in Easy Tier and its associated software tool Storage Tier Advisor to evaluate the benefits of, plan for, deploy, and manage on an ongoing basis SSDs in combination with existing HDDs. It’s not for nothing that IBM calls its product Easy Tier.

1. Mesabi Group
Commentary
April 12, 2010
IBM Easy Tier Enables DS8700 Users to
Make Good Use of Solid State Drives
Solid state drives (SSDs) are a hot topic of interest in IT organizations because of their potential
to turbocharge storage performance. One inhibitor to adoption has been the high cost of SSDs
relative to hard disk drives (HDDs) and a second has been the problem of when and how to use
SSDs efficiently and effectively. IBM DS8700 users now have the tools in Easy Tier and its as-
sociated software tool Storage Tier Advisor to evaluate the benefits of, plan for, deploy, and
manage on an ongoing basis SSDs in combination with existing HDDs. It’s not for nothing that
IBM calls its product Easy Tier.
they can justify an investment in SSDs.
The Promise of Solid State Drives And that means getting just the right
Flash-memory-based solid state drives amount of SSDs as a blend with HDDs,
(SSDs) offer a storage Tier 0 comple- to avoid over-provisioning the more
ment to Tier 1 Fibre Channel (FC) costly SSDs.
drives in an enterprise-class disk
array. SSDs promise: IBM’s System Storage DS8700 users
now have the tools — with the combi-
• Opening up additional revenue nation of Storage Tier Advisor and Easy
possibilities by increasing the Tier — to be able to plan for and use
transactions per second or by re- SSDs appropriately in their distinctive
ducing elapsed time of now- workload environments. The Storage
performance-bound revenue- Tier Advisor software tool offers
generating applications. guidance to users on how existing
• Saving money by reducing hard- workloads can benefit by moving
ware resources, since there is less certain data to SSDs. Easy Tier, free
need for data to be duplicated firmware functionality in a DS8700
across disks in order to reduce I/O system, is the operational tool that
wait time. manages the proper placement of data
These benefits are potentially alluring between SSDs and HDDs on a
to IT organizations. However, because continuous basis. Its continuous
of the much higher relative cost of workload monitoring capability
SSDs as compared to traditional hard dynamically determines whether or not
disk drives (HDDs), cost-conscious IT data should be on an SSD or an HDD,
organizations have to make sure that and, as necessary, automatically moves
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2. Easy Tier Enables DS8700 Users to Make Good Use of Solid State Drives
Commentary Page 2
data non-disruptively from one tier to short stroking is used, SSDs may very
the other. well offer a cost-justified alternative.
Before examining IBM’s solution, an
understanding of HDD performance SSDs Overcome HDD Performance
issues and how SSD solves them — but Problems, but at a Price
at a price — is necessary. Non-volatile semiconductor (NAND
flash) chips are embedded in an SSD
The Trouble with HDD Performance drive that has the same traditional
The fastest HDD drives have been form factor as an HDD. The SSD
stuck at 15,000 rpm for some time now appears to a disk storage system (and
and, for technical reasons, are likely to to the server) as a random access HDD.
stay more or less that way. Processor Reliability is no longer an issue with
capabilities have not been similarly today’s enterprise-class SSDs, so our
constrained. The result is that, despite discussion focuses on price and
the continued shrinkage in disk performance.
storage space per datum, the perform- SSD drives are available in 73 and 146
ance gap between processors and the GB capacities versus 73, 146, 300,
fastest HDDs is growing. 450, and 600 GB capacities for the
This widening performance gap leads to highest-speed HDDs on the DS8700.
an increased I/O wait time, which is The rough tale of the tape (Table 1)
the time that a CPU spends waiting for shows that SSDs are an order of
data from a storage system. Not only is magnitude more expensive for the
that a problem with single OS-based same capacity than an HDD. However,
physical servers, but the problem may SSDs are much faster than HDDs and
be compounded when multiple virtual the result is that the SSDs are
machines (VMs) all want to access dramatically cheaper for each IOPS.
data. (However, note that performance
delays will not scale linearly with the
Table 1: Enterprise-Class Comparison
number of VMs, because a disk array
can service multiple VMs more or less Between SSDs and HDDs
in parallel.) Having expensive servers
unnecessarily idle is an investment Measure SSDs vs. HDDs
waste to which the proper use of SSDs
Relative Cost 10:1
offer a possible solution.
Ratio
One HDD workaround to increase disk
Input/Outputs Thousands to tens of
performance is short stroking. In short
Per Second thousands vs. tens to
stroking, only the outside tracks on an
(IOPS) possibly hundreds
HDD have data. This significantly
reduces the access seek time for a IOPS/$ 20:1
disk’s head to find a requested track.
Source: Objective Analysis April 2010
The downside is that much of the
disk’s capacity is not used and many This comparison highlights relative
more disks have to be used. When differences, which are important when
a lot of money is involved. A formal cost
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3. Easy Tier Enables DS8700 Users to Make Good Use of Solid State Drives
Commentary Page 3
analysis — involving the use of disks, Naturally, an analysis has to be
the I/O wait impact upon servers, and conducted initially to determine
power (SSDs are more efficient) — may whether or not using one or more SSDs
also need to be done. makes sense. If one or more SSDs are
But costs can also be viewed as inserted into a storage array configura-
absolute, i.e., viewed independently tion, then all three functions work
and not in a comparative or relative together in an iterative and continuous
sense. Here absolute costs mean the fashion.
actual out-of pocket-costs, and tell us
whether a solution is affordable at all. Getting the Right Balance Between SSDs
And with the fast decline in SSD and HDDs
prices, quite frankly even the starting Before the amount of SSD capacity can
point of a mini-pack of 8 drives may be determined for a given workload and
not be out of reach economically with put to correct use with the right data,
even a simple cost analysis. some “rule of thumb” and “best
practice” approaches can always be
Starting small and trying SSDs gives applied to guesstimate based on
real-world experience that can give application performance characteristics
more information as to whether further observed and the understanding of
SSDs can be justified. And Easy Tier storage system architecture. To
(along with the Storage Tier Advisor) improve the accuracy and prediction
enables planning and managing SSDs process, information has to be collected
in the most cost efficient manner, on that shows what data could benefit by
an ongoing basis, to complement the being moved to SSDs. Only then can a
use of HDDs. determination be made to acquire
SSDs as well as to put in place an
Easy Tier’s Basic Capabilities ongoing data monitoring and migra-
The combination of Easy Tier and the tion. And it all starts with the analysis
Storage Tier Advisor performs the of existing data via a “heat map.”
following three basic functions:
• Workload hotspot analysis — Heat Mapping
reports can provide information on The critical construct for being able to
data access patterns that can pro- determine what data is best suited to
vide SSD capacity planning recom- which tier is called a “heat” map. Of
mendations. course, all the allusions to “heat,”
• Continuous workload monitoring — “temperature.” “hot,” and “cold” are
this dynamically determines metaphors that relate to the frequency
whether data on the HDD tier needs of access patterns to different sets of
to be promoted to the SSD tier or data rather than the typical physical
whether data on the SSD needs to interpretation of those words.
be demoted to the HDD tier. Candidates for movement to an SSD
• Smart data placement — this from an HDD are evaluated at the
automates the process of relocating extent or sub-volume level, which in
data to the appropriate tier (either this case is a contiguous set of disk
SDD or HDD). sectors that add up to 1 GB. Given that
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4. Easy Tier Enables DS8700 Users to Make Good Use of Solid State Drives
Commentary Page 4
this is really at the sub-volume level, The user takes into account all of this
even with the smallest SSD (73 GB, information and gets a better feel for
which means that 73 extents can be whether or not using SSDs make
chosen), this level of granularity should sense, including how much data (from
be suitable for all practical purposes. a little to a lot) would benefit from
Initially, the frequency of access for all moving to SSDs. If the decision is made
the extents being evaluated is exam- to use SSDs, Easy Tier functionality in
ined over a period of, say, 24 hours. At the form of smart placement and
that point a continuum can be continuous workload monitoring can
generated from the least frequently then take place.
accessed extents on the “cold” side to
the most frequently accessed extents Smart Placement
on the “hot” side. Of course, the Smart placement is the name for the
“temperature” is relative (there is no process that Easy Tier goes through in
freezing point!). moving selected extents from HDDs to
SSDs (Figure 1). But it offers much
Workload Hotspot Analysis more than doing this non-disruptively
Information collected from the heat as well as bi-directionally (SSD to HDD
distribution can be used to generate and vice versa) and dynamically over
reports that can be used to determine time.
how SSDs might best be used. Note the
analysis does not presume that one or Figure 1: Tier Managed Extent Pool
more SSDs are already in place.
This analysis looks across all physical
SSD Arrays
volumes and their associated capacity
that are considered part of the
monitoring process. Among the report
outputs are the following:
• The heat distribution for each Cold Extents Hot Extents
volume, which is the capacity in Migrate Down Migrate Up
each volume that would be targeted
to SSDs (if not already there) or that
has already been moved to an SSD.
• The estimated migration time to
move the data from HDDs to SSDs,
if or when SSDs became available. HDD Arrays
• The percentage of hot capacity as a Source: IBM and Mesabi Group April 2010
fraction of the total capacity.
• The predicted performance im- Smart placement is about automating
provement for a particular SSD a manual process as well. Traditional
configuration over existing HDDs as disk storage mapping is about creating
well as the expected improvement if rigid logical volumes. For example, a
additional SSD capacity is added. high-performance-requiring application
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5. Easy Tier Enables DS8700 Users to Make Good Use of Solid State Drives
Commentary Page 5
is typically locked to Tier 1 drives that a good candidate for server virtual-
may be underutilized (i.e., over- ization. Server virtualization, in
provisioned) in order to improve turn, means that the server can be
performance, as in the previously more effectively utilized by running
discussed short-stroking. more applications.
Easy Tier uses the concept of hybrid
storage volumes, where the physical Continuous Workload Monitoring
volumes in both Tier 0 and Tier 1 or The temperature of at least some
Tier 2 (SATA) logically appear homoge- extents change over time, so a one-time
nous to an application, which means migration of the hottest data from
that an application is unaware of any HDDs to SSDs would eventually lead to
movement of hot or cold data that suboptimal performance from the SSD
exploits the appropriate storage tier. configuration, as the SSDs would
Letting the storage system do it contain some “colder” data than would
automatically, reduces reconfiguration now be available on HDDs. Easy Tier
time and errors compared to a manual solves this problem through continu-
configuration process. ous monitoring of the workloads on
extents.
Note that the Easy Tier algorithm is
multi-dimensional. For example, the Performance data is collected on every
algorithm calculates the “cost and extent that has active I/O on every
benefit” of promoting an extent before monitored logical device in the system.
it actually does so. The algorithm The I/O statistics collected for each
calculates whether the cost of promot- extent include read and write time,
ing an extent is greater than the benefit amount of data transferred, and
to performance of having that extent on cumulative response time. Those
SSD. Thus, there is a check and statistics enable a more sophisticated
balance on the movement of extents. analysis of performance ranking than
just IOPS alone.
Note that after the migration has taken
place, an IT organization may be able These statistics are collected at fixed
to take one or both of the following periods of approximately 5 minutes
actions that it couldn’t have taken and then are reported on at 15 minute
before (and help economically justify intervals. The results are accumulated
the use of SSDs if that is necessary): over a period of time, say, 24 hours. At
the end of that time, the data access
• Consolidate selected applications to pattern of all the extents will be
fewer HDDs and use the freed up examined to create a new I/O heat map
HDDs to meeting growing storage (i.e., iteratively go back to the workload
requirements (which means delay- hotspot analysis phase) and from that
ing the need to purchase additional map a new migration plan can be put
HDD capacity). in place to promote or demote individ-
• Decrease the I/O wait time on an ual extents (i.e., iteratively go back to
existing physical server, which the smart placement step).
means that either a smaller (and By the way, note that when Easy Tier is
less costly) physical server might be on it will migrate data automatically,
used or the existing server might be
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26 Country Lane Fax: 781 326 0038
Westwood, Massachusetts 02090 www.mesabigroup.com

6. Easy Tier Enables DS8700 Users to Make Good Use of Solid State Drives
Commentary Page 6
but users have the option to use the neously in tandem, has been an
manual volume relocation compo- ongoing challenge.
nent of Easy Tier to move a volume For DS8700 users, that challenge
out of the Easy Tier managed extent has been resolved with the combina-
pool at the volume level. tion of Easy Tier and Storage Tier
Advisor. Now, users have the tools
Conclusion and capabilities to manage the
Performance — by one definition, the balance between SSDs and HDDs
efficiency with which something successfully, and to give the
fulfills a purpose — is something performance that an application
that appears desirable in all cases. needs while not breaking the budget
But is more “performance” always bank. So Easy Tier is something that
necessary? The answer for many IT every DS8700 user should consider
applications, such as response time carefully when SSDs are a part of the
in sending e-mails, is that existing picture.
performance is good enough. Still,
some growing IT applications
(especially revenue-generating ones) David Hill
would clearly benefit from improved
performance in most cases (if we
define adequate performance as
satisfactory response time for a
larger number of transactions per
second). Moreover, adequate
performance does not mean cost-
effective performance: Even applica-
tions that deliver satisfactory
performance may do so by way of
unnecessary hidden costs, such as
added disk space because of short-
stroking, or excess server capacity
due to underutilization of servers
caused by excessive I/O wait times.
Enter SSDs as Tier 0 in disk storage
systems. But understanding where
SSDs can best be used in conjunc-
tion with HDDs, and managing the
two tiers transparently and simulta-
Analyst Name: David Hill Mesabi Group LLC is an affiliate of Valley View Ventures that
Topic Area: Storage Tiering aims to provide thought leadership and sound advice to both
vendors and users of information technology. .
Mesabi Group LLC
26 Country Lane Phone: (781) 326-0038
Westwood, MA 02090 email the author: davidhill@mesabigroup.com
www.mesabigroup.com
The information contained in this publication has been obtained from sources Mesabi Group LLC believes to be reliable, but is not
warranted by Mesabi Group LLC. Commentary opinions reflect the analyst’s judgment at the time and are subject to change without
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