Summary
How
do you design a computing system to provide continuous service and to ensure
that any failures interrupting service do not result in customer safety issues
or loss of customers due to dissatisfaction?
Historically,
system architects have taken two approaches to answer this question: building
highly reliable, fail-safe systems with low probability of failure, or building
mostly reliable systems with quick automated recovery.
The
RAS (Reliability, Availability, Serviceability) concept for system design
integrates concepts of design for reliability and for availability along with
methods to quickly service systems that can't be recovered automatically. This
approach is fundamental to systems where the concern is quality of service,
customer retention, and due diligence for customer safety.
This article was compiled from
Wikipedia:
IBM:
What
is Reliability, Availability, Performance and Serviceability?
Reliability
is evaluated by mean time to failure, and availability is measured by service
uptime over a year of continuous operation.
Reliability is a measure of the ability of a system to function
correctly, including avoiding data corruption, whereas availability measures
how often it is available for use, even though it may not be functioning
correctly. For example, a server may run forever and so have ideal
availability, but may be unreliable, with frequent data corruption.
Reliability
of a system can be defined as the probability that it will produce correct
outputs up to some given time t. Reliability is enhanced by features that help
to avoid, detect and repair hardware faults. A reliable system does not
silently continue and deliver results that include uncorrected corrupted data.
Instead, it detects and, if possible, corrects the corruption, e.g., by
retrying an operation for transient (soft) or intermittent errors, or else, for
uncorrectable errors, isolating the fault and reporting it to higher level
recovery mechanisms (which may failover to redundant replacement hardware,
etc.), or else by halting the affected program or the entire system and
reporting the corruption. Reliability is often characterized in terms of mean
time between failures (MTBF), with reliability = exp(-t/MTBF).
Availability
is the probability a system is operational at a given time, i.e. the amount of
time a device is actually operating as the percentage of total time it should
be operating. In high availability applications, availability may be reported
as minutes or hours of downtime per year. Availability features allow the
system to stay operational even when faults do occur. A highly available system
would disable the malfunctioning portion and continue operating at a reduced
capacity. In contrast, a less capable system might crash and become totally
nonoperational. Availability is typically given as a percentage of the time a
system is expected to be available, e.g., 99.999 percent ("five
nines").
|
Availability %
|
Downtime
per year |
Downtime
per month |
Downtime
per week |
|
90%
|
36.5 days
|
72 hours
|
16.8 hours
|
|
95%
|
18.25 days
|
36 hours
|
8.4 hours
|
|
98%
|
7.30 days
|
14.4 hours
|
3.36 hours
|
|
99%
|
3.65 days
|
7.20 hours
|
1.68 hours
|
|
99.5%
|
1.83 days
|
3.60 hours
|
50.4 minutes
|
|
99.8%
|
17.52 hours
|
86.23 minutes
|
20.16 minutes
|
|
99.9% ("three nines")
|
8.76 hours
|
43.2 minutes
|
10.1 minutes
|
|
99.95%
|
4.38 hours
|
21.56 minutes
|
5.04 minutes
|
|
99.99% ("four nines")
|
52.6 minutes
|
4.32 minutes
|
1.01 minutes
|
|
99.999% ("five nines")
|
5.26 minutes
|
25.9 seconds
|
6.05 seconds
|
|
99.9999% ("six nines")
|
31.5 seconds
|
2.59 seconds
|
0.605 seconds
|
Serviceability or
maintainability is the simplicity and speed with which a system can be repaired
or maintained; if the time to repair a failed system increases, then
availability will decrease. It includes various methods of easily diagnosing
the system when problems arise. Early detection of faults can decrease or avoid
system downtime. For example, some enterprise systems can automatically call a
service center without human intervention when the system experiences a system
fault. The traditional focus has been on making the correct repairs with as
little disruption to normal operations as possible.
Performance defines
the ability of the system deliver within design limits. For example, in ERP
software applications, this can be expressed as the measure = end user click to
click time for a transaction is within 10 seconds; design limits: a maximum of
50000 order lines per year, 250 named users with a maximum 20% concurrency i.e.
50 concurrent users.
Why
does this matter?
From the end-user
standpoint, computing systems provide services, and any outage in that service
can mean lost revenue, lost customers due to dissatisfaction, and, in extreme
cases, loss of life and possible legal repercussions. For example, with cell phone
services, if it's only in rare cases that I can't get a signal or make a
connection, I'll stick with my service provider. But if this occurs too often
or in critical locations like my office or home, I'll most likely switch
providers. The end result is loss of revenue and loss of a customer. Embedded
systems not only provide value added services such as communications, but they
also provide critical services for human safety. For example, my anti-lock
braking system is provided by a digital control service activated by my
ignition. My expectation is that this service will work without failure once
ignition is completed. Any system fault that might interrupt service should
prevent me from using the vehicle before I start to drive. A failure during operation
could result in loss of life and product liability issues.
Ideally, service
outages would not be an issue at all, but experienced system architects know
they must analyze, predict, and design systems for handling failure modes in
advance. For safety-critical systems, this is the due diligence required to
avoid product liability nightmares. Historically, system architects have taken
two approaches to this problem: building highly reliable, fail-safe systems
with low probability of failure, or building mostly reliable systems with quick
automated recovery. Both approaches are judged with probability measures.
Reliability is evaluated by mean time to failure, and availability is measured
by service uptime over a year of continuous operation. The RAS (Reliability,
Availability, Serviceability) concept for system design integrates concepts of
design for reliability and for availability along with methods to quickly
service failures that can't be designed for automatic recovery.
Building systems for
very high reliability can be cost prohibitive, so RAS offers an approach to
balance reliability with recovery and servicing features to control cost and
ensure safety and quality of service. This approach is fundamental to systems
where the concern is affordable quality of service, customer retention, and due
diligence for customer safety.
Lessons
from the design of IBM mainframes
You can learn a lot
from the experience built into systems like the IBM® mainframes that have
evolved from a rich heritage of design for reliability, availability, and
serviceability. This example explores elements of IBM mainframe architecture to
assist those developing new architectures by examining the design decisions
made in the big iron mainframes. This article gives background on the evolution
of RAS features developed for IBM mainframes and summarizes significant design
decisions.
To best understand
the evolution of RAS in IBM mainframe architecture, it is useful to step back
in time to 1964 and examine RAS features in the IBM System/360™
(see Resources) and consider how architects have balanced the issues of
cost, reliability, safety, availability, and servicing, and improved upon this
over time. Early systems were most often centralized rather than in the hands
of end users, and may have been less cost-sensitive than today's mainframes,
but the concepts of availability and reliability emerged early and have evolved
over time into the well-proven RAS features now found in the z990.
Often, system and
application requirements will determine if availability is stressed over
reliability or vice versa. For example, the concept of availability has also
been fundamental to telecommunication systems, where most often quality of
service is more of an issue than safety. In contrast, reliability has been
fundamental to systems such as commercial flight control systems where failure
means significant loss of life and assets. The balance of availability and
reliability features should fit the system -- building FAA (Federal Aviation
Administration) levels of reliability into cell phones would make the system
much less affordable and therefore not usable by many customers. Likewise,
safety-critical systems can't simply quote uptime to convince customers that
the systems are not too risky to trust -- fail-safe operation, reliable parts,
triple redundancy, and the extra cost that goes along with these design
features is expected and will be paid for to mitigate risk.
The
difference between availability and reliability
Availability is
simply defined as the percentage of time over a well-defined period that a
system or service is available for users. So, for example, if a system is said
to have 99.999%, or five nines, availability, this system must not be
unavailable more than five minutes over the course of a year. Quick recovery
and restoration of service after a fault greatly increases availability. The
quicker the recovery, the more often the system or service can go down and
still meet the five nines criteria. Five nines is often called high
availability, or HA.
In contrast, high
reliability (HR) is perhaps best described by the old adage that a chain is
only as strong as its weakest link. Building a system from components that have
very low probability of failure leads to maximal system reliability. The
overall expected system reliability simply is the product of all subsystem
reliabilities, and the subsystem reliability is a product of all component
reliabilities. Based upon this mathematical fact, components are required to
have very low probability of failure if the subsystems and system are to also
have reasonably low probability of failure. For example, a system composed of
10 components, each with 99.999% reliability, is (0.99999)10, or 99.99%,
reliable. Any decrease in the reliability of a single component in this type of
single-string design can greatly reduce overall reliability -- for example
adding just one 95% reliable component would drop the overall reliability to
94.99%.
The z990 includes
features that allow it to be serviced and upgraded without service
interruption. The subsystem level of redundancy is the "book," which
is an independently powered, multi-chip module, with cooling, memory cards, and
IO cards. There are also redundant components within a book including CPU
spares (2 per book) and redundant interconnection to level-2 cache.
Furthermore, the z990 provides jumpering to preserve the interconnection of MP
(multi-processor) books while other books are being serviced or replaced in
case of non-recoverable errors.
Service or system
outages can be caused by routine servicing, upgrades, and failures on most
traditional computing systems. Probability of a system or service outage on the
z990 is limited to scenarios where failures are non-recoverable by switching
redundant components or subsystems into operation. In most component or
subsystem failure modes, the z990 is able to isolate the non-recoverable book
or component and continue to operate with some performance degradation.
Redundant books and components allow the z990 to operate without service
interruption until the degraded book is replaced. As you will see later on in
this article, designing interconnection networks for isolation and switching of
modules like the z990 books is complex. While this complexity adds to cost, it
does significantly decrease probability of service interruption. You can find
more detailed information on processor book management in the
"Reliability, availability, and serviceability (RAS) of the IBM eServer
z990" paper (see Resources).
How
high reliability helps
It is theoretically
possible to build a system with low-quality, not-so-reliable components and
subsystems, and still achieve HA. This type of system would have to include
massive redundancy and complex switching logic to isolate frequently failing
components and to bring spares online very quickly in place of those components
that failed to prevent interruption to service. Most often, it is better to
strike a balance and invest in more reliable components to minimize the
interconnection and switching requirements. If you take a very simple example
of a system designed with redundant components that can be isolated or
activated, it becomes clear that the interconnection and switching logic does
not scale well to high levels of redundancy and sparing.
A trade-off can be made
between the complexity of interconnecting components and redundancy management
with the cost of including highly reliable components. The cost of hardware
components with high reliability is fairly well known and can be estimated
based upon component testing, expected failure rates, MTBF (mean time between
failures), operational characteristics, packaging, and the overall physical
features of the component.
System architects
should also consider three simple parameters before investing heavily in HA or
HR for a system component or subsystem:
- Likelihood of unit failure
- Impact of failure on the system
- Cost of recovery versus cost of fail-safe isolation
How
cost and safety factor in
HA design may not
always ensure that a design will be safe. Much depends on how long service
outages will be during recovery scenarios. For safety-critical systems such as
flight control or anti-lock braking, it is possible that even very brief
outages could lead to loss of system stability and total system failure. Thus,
for safety-critical systems, the balance between HA and HR must often favor HR
in order to avoid risky recovery scenarios.
Achieving
HA/HR with redundancy as the primary method
One approach to HA
is to use redundancy and switching to not only increase availability, but so
lower reliability (and most often lower cost) components can be used, which for
some systems yields overall target HA at the lowest overall system cost. The best
example of this approach is RAID (Redundant Array of Inexpensive Disks);
see Resources for examples and links. In fact, numerous RAID
configurations have been designed which make trade-offs between HA/HR and
serviceability by using larger numbers of disk drives of various types
including SCSI, Fiber Channel, Serial-Attached SCSI, and Serial-Attached ATA
drives. RAID also can provide improved storage performance by striping
writes/reads over multiple drives as well as using drives for parity or more
sophisticated error encoding methods. Typical RAID systems with volume
protection allow for a drive failure and replacement with automatic recovery of
the volume and no downtime given a single drive failure. Protection from double
faults or failures while a RAID system is recovering has become of interest
more recently and has led to development of RAID 6. One of the more interesting
aspects of the RAID approach is that it not only relies upon specialized HA
hardware, but on fairly complex software.
RAS
designs should span hardware, firmware, and software layers
Perhaps much harder
to estimate is the cost of highly reliable software. Clearly, reliable hardware
running unreliable software will result in failure modes that are likely to
cause service interruption. It is well accepted that complex software is often
less reliable, and that the best way to increase reliability is with testing.
Testing takes time and ultimately adds to cost and time to market.
Early on, system
architects focused on designing HA and HR hardware with firmware to manage
redundancy and to automate recovery. So, for example, firmware would
reconfigure the components in the example in Table 1 to recover from a
component failure. Traditionally, rigorous testing and verification have
ensured that firmware has no flaws, but history has shown that defects can
still wind up in the field and emerge due to subtle differences in timing or
execution of data-driven algorithms.
High
reliability software often comes at a high cost
Designing firmware
and software for HR can be costly. The FAA requires rigorous documentation and
testing to ensure that flight software on commercial aircraft is highly
reliable. The DO-178B class A standard requires software developers to maintain
copious design, test, and process documentation. Furthermore, testing must
include formal proof that code has been well tested with criteria such as
multiple condition decision coverage (MCDC). This criteria ensures that all
paths and all statements in the code have been exercised and shown to work. It
is very laborious and therefore greatly increases the cost of software
components.
Cost
trade-offs in the hardware layer
Designing for HR
alone can be cost prohibitive, so most often a balance of design for HA and HR
is better. HA at the hardware level is most often achieved through redundancy
(sparing) and switching, which is the case for the z990 and has been
fundamental to IBM mainframe design since the System/360. A trade-off is made
between the cost of duplication and simply engineering higher reliability into
components to reduce the MTBF. Over time, hardware designers have found balance
between HR and HA features to optimize cost and availability. Fundamental to
duplication schemes is the recovery latency. For example, the z990 has a
dynamic CPU sparing (DCS) feature that can cover a failure so that firmware is
unaffected by reconfiguration to isolate the errant CPU and to switch in the
spare.
When considering
component or subsystem duplication for HA, architects must carefully consider
the complexity and latency of the recovery scheme and how this will affect
firmware and software layers. Trade-offs between working to simply increase
reliability instead of increasing availability through sparing should be
analyzed. A simple well-proven methodology that is often employed by systems
engineers is to consider the trade-off of probability of failure, impact of
failure, and cost to mitigate impact or reduce likelihood of failure. This
method is most often referred to as FMEA (Failure Modes and Effects Analysis);
see Resources. Another, less formal process that system engineers often use is
referred to as the "low hanging fruit" process. This process simply
involves ranking system design features under consideration by cost to
implement, reliability improvement, availability improvement, and complexity.
The point of low hanging fruit analysis is to pick features that improve HA/HR
the most for least cost and with least risk. Without existing products and
field testing, the hardest part of FMEA or low hanging fruit analysis is
estimating the probability of failure for components and estimating improvement
to HA/HR for specific features. For hardware, the tried and true method for
estimating reliability is based upon component testing, system testing,
environmental testing, accelerated testing, and field testing. The trade-offs
between engineering reliability and availability into hardware are fairly
obvious, but how does this work with firmware and software?
Cost
trade-offs in the firmware and software layers
Designing and
implementing HR firmware and software can be very costly. The main approach for
ensuring that firmware/software is highly reliable is verification with formal
coverage criteria along with unit tests, integration tests, system tests, and
regression testing. Test coverage criteria include feature points; but for HR
systems, much more rigorous criteria are necessary, including statement, path,
and, in extreme cases, multiple condition decision coverage (MCDC). The IBM
z/OS testing has followed this rigorous approach to ensure HR. (This testing is
described in the article "Testing z/OS; see Resources.) The FEDC
system in z/OS provides support for HA, recognizing that despite rigorous
testing, software/firmware testing cost and time spent must be limited at some
point, and the design should include support for quick operator assisted
recovery. Finally, the FEDC is also very useful for application developers who
most likely would also like to strike a balance between rigorous testing of
their application and provision for recovery.
Component-level
error detection and correction
Ideally, all system,
subsystem, and component errors can be detected and corrected in a hierarchy so
that component errors are detected and corrected without any action required by
the containing subsystem. This hierarchical approach for fault detection and
fault protection/correction can greatly simplify verification of a RAS design.
The z990 ECC memory component provides for single-bit error detection and
automatic correction. The incorporation of ECC memory provides a component
level of RAS, which can increase RAS performance and reduce the complexity of
supporting RAS at higher levels.
Redundancy
management at the subsystem level
The z990 includes a
number of redundancy management features that provide online
replacement/upgrade, automatic recovery with spares, and continuous operation
in degraded modes for double and triple faults that require servicing. The z990
organizes processing, memory, and I/O resources into logical units
called books, which are interconnected so that they can be switched
into or out of operation without interruption to the overall system. This
scheme includes the ability to add and activate processors, memory, and I/O
connections while the system continues to run. The fault-tolerant book
interconnection and cross-book CPU sparing provides excellent automatic
recovery as well for most fault scenarios.
Finally, support
subsystems such as cooling also include redundancy so that the system is not
endangered by thermal control system faults. Redundancy and management of that
redundancy with automated fault detection, isolation, and automatic recovery is
fundamental to the z990 RAS design.
Fail-safe
design
HR systems often
include design elements that ensure that non-recoverable failures result in the
system going out of service, along with safing to reduce risk of losing the
asset, damaging property, or causing loss of life. The z990 may or may not be
used in applications that are considered safety critical -- arguably, even a
database error could result in significant loss of assets (for stock market
applications) or even loss of life (in certain health care applications). The
z990 does incorporate fail-safe modes when recovery is impossible or too costly
to incorporate (for example, double processor faults in a single book) and the
likelihood of a failure is low. In the case of double or triple faults, the
z990 isolates the failed subsystem (a book) and requires operator assistance --
for most users. This is likely a good cost-versus-HA/HR trade-off, given the
built-in support for serviceability in the z990 (on-line replacement).
Serviceability
concepts
The z990 strikes a
nice balance between HA, HR, system safety (permanent loss of data could have
high related risk), and simplicity of operation and servicing. In most cases,
this tracking of errors, data logging and upload to IBM with RETAIN and configuration
tracking of FRUs (field-replaceable units) simplifies service calls, sometimes
(or often) allowing a technician to handle a non-recoverable failure or upgrade
without causing service interruption. The z990 organization of books as
field-replaceable units with support to assist the operator in recovery greatly
increases the z990 serviceability for faults that occur despite the HR/HA
design.
Recovery
concepts
Conceptually,
architects should consider how levels of recovery are handled with varying
degrees of automation, as depicted in Figure 2.
Figure 2. Supporting
multiple levels of recovery autonomy
The z990 approach
includes all levels of recovery automation and management between fully
automatic, operator assisted, and fully manual. Manual recovery requirements
are minimized, and manual recovery still includes isolation features so that
FRUs can be replaced easily to restore full performance without service
interruption and without impact to other processor books.
Putting
it all together
The z990 is an
excellent example of good RAS design because RAS is considered at all levels of
hardware, from components and subsystems to the system level, and because
firmware/software design for RAS is considered in addition to hardware. Perhaps
most important, though, is the fact that the z990 RAS design spans all of these
levels and layers so that overall RAS is a system feature. The well-integrated
RAS features of the z990 no doubt increase the cost of the system considerably.
However, the z990 provides customers with an HA/HR computing platform with low
risk of losses due to downtime. The cost trade-off will vary for each system
design based upon the risk and cost associated with downtime compared to the
cost of decreasing the overall probability of downtime. The system architect
has to find the right balance of cost, complexity, recovery automation,
reliability, and time to market for each, given the system's intended usage.
There is no simple formula for HA/HR analysis, but one place to start is with
careful consideration of what the risk and cost is of being out of service --
if this can be measured in dollars lost per hour or day, potential loss of
life, or loss of customers, this is a good starting point. If my business will
potentially lose thousands of dollars per hour while my enterprise system is
out of service, then I will be more willing to pay much more for HA/HR.
Is
autonomic architecture the future for RAS design?
Strategies for RAS
like the z990 have been refined and improved significantly since the concepts
of continuous availability were introduced in early mainframes like the
System/360. Clearly, RAS requires balance between safety and uninterrupted
service on the one hand and the cost to provide these features on the other.
The cost of additional RAS performance can be high, and this cost must be
balanced with the risk and cost of occasional service failures and safety. How
much is enough? How can the additional cost of better RAS be financed?
One concept is that
systems that require little to no monitoring are not only more cost efficient,
but necessary, if automation is to scale up significantly beyond present day
systems. Enterprise systems include processing, storage, and I/O, with thousands
of interconnections, hundreds of processors, and terabytes of data. Quick
access to widely distributed information for decision support, global
operations, and commerce is required, and the volume and speed of information
flow is scaling beyond the point where traditional monitoring and service
methods can be applied. Autonomic architecture is an alternative to traditional
systems administration that uses RAS as a starting point to further reduce the
human attention required to maintain enterprise systems. A full description of
autonomic architecture is beyond the scope of this review of RAS strategies;
however, you can find more information in the Resources section below.
Resources
This article was
extracted from
Wikipedia:
IBM:
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