Monday, June 24, 2024

Extended Microbenchmarks for Modern Disk Drives

I learnt about this interesting blog post entitled, "Discovering Hard Disk Physical Geometry through Microbenchmarking" by Henry Wong (2019) via @TanelPoder on Twitter. As the title suggests, it is about applying so-called disk drive microbenchmarking to reveal information about the internal mechanical configuration and data layout that is not typically available with other HDD microbenchmark codes; many of which are cited by the author in the References section (something most bloggers are too lazy to do).

While reading it I had some thoughts that turned out to be too long for tweeting a response. So, here I am revisiting my traditional blog, mostly for the formatted space it provides, but also to remind myself that it still lives!

The Article

The article contains a massive amount of very admirable work (~1 year of effort, including benchmark code development) that is exceptionally well written, considering all the details that are covered. I didn't come across any typos or misspellings, either. There are actually TWO articles in one: an extensive summary analysis (not an Executive Summary) and an equally long Appendix containing the detailed measurements obtained from each disk model that was benchmarked.

Quite apart from my subsequent remarks below, the inferences about internal disk structure drawn from the microbenched timing-data are quite remarkable. And the beautiful plots generated from those data (e.g., mapping thousands of sector defects) are something to behold. Understandably, HDD manufacturers are not keen to include such plots in their marketing collateral.

The author states the following as his prime motivations for developing the extended microbenchmark algorithms.

"I had initially set out to detect the number of platters in a disk without opening up a disk, but in modern disks this simple-sounding task requires measuring several other properties before inferring the count of recording surfaces."

"Many of the prior works had practical applications in mind (as opposed to pure curiosity), and were willing to trade measurement speed for a small loss in accuracy. Although [my microbenchmark] algorithms are not fundamentally different, prior work likely avoided exploring these because these measurements can take many hours and there is little practical benefit for applications such as performance modelling or improving data layouts in filesystems."

Although a truly laudible effort, that is largely driven by scientific "curiosity", I can't help but wonder if the author is straining at a gnat.

With a bit of perseverance, I could find several corresponding drive specs online. And, although manufactuers never lie (smirk) the search terms can vary widely.

Platters = disks = discs...
Surfaces = heads = actuators...
Applying the author's terminology:
  1. The number of discrete platters has to be an integer.
  2. The number of surfaces is an even integer (each platter having two sides).
But, that's not what you see in the Summary table of the 17 HDD models that he tested. Let's look at a few examples.

Example Geometries

The following disk models were compared against their respective manufacturer specs that I was able to find online.
  • Tosh X300 5GB has 5 platters and therefore 10 surfaces (1 Gig per platter).
  • Seagate ST1 5GB has 1 platter and therefore 2 surfaces.
  • WD S25, however, has 2 platters but only 3 surfaces. How can that be?
Western Digital built this disk with only 3 actuator arms, not the expected 4. The fourth surface isn't used. Only WD knows why. Cost cutting is one plausible explanation. There is no way to anticpate such quirks. You either have to measure it or read about it.

The author infers the same number of WD S25 used surfaces (3) based on his benching data which, in itself, is quite remarkable. On the other hand, this same information is generally included in the disk manufacturer specs (or similar docs). Worst case, you could actually contact the manufacturer and they would very likely tell you; especially if they thought they could sell you some disks.

Bitter Pill

However, even when you determine all these interesting extended disk metrics (measured, calculated or specified), you are no better off when it comes to using that information for better storage performance — as the author admits (see quote above) — or predicting future HDD internal geometries. Just like with modern multicore microprocessors,
  1. control (especially that related to performance) has progressively been taken away from you and placed inside a silicon module. You can't blue-wire that. Similarly for HDD packaging.
  2. manufacturers tend to implement the cheapest solutions not the best performing solutions, which you can't correct. See WD S25 above. Similarly for HDD controller logic.
The HDD microbenching measurements described in the 2019 article would have to be repeated on each new model release. Any mechanical tweaks or other changes inside the new disk would have to be discovered and then incorporated into updated microbenchmark code ... if possible.

Computer manufacturers of all stripes are in business to make a profit and to avoid being eaten by the competition. They will do whatever it takes (good, bad or ugly), whenever it's deemed necessary, to maintain or improve their market position. An unfortunate casuality of this ongoing commercial warfare is that any painstakingly acquired scientific analysis can become an historical footnote overnight.

Performance Modeling

On the topic of performance modeling of disks (to which the author also alludes), his article is a good reminder of the mind-boggling complexity of data layout in modern disks. And that's essentially just the static picture. Along with that goes the complex dynamics of local request-chain caching and optimization.

In that vein, it's a fool's errand to try and model individual modern disks. In principle, you would need to construct a simulation model to handle all the potential transient behaviors. But that, in turn, requires deciphering the embedded controller algorithms, which are not only highly complex but proprietary. You'll never figure them out and no manufacturer will ever reveal them. So, a performance simulation is actually a non-starter.

On a more positive note, there is a surprisingly counterintuitive technique for constructing accurate performance models of collective disks, like SANs, using the PDQ (Pretty Damn Quick) performance analyzer. That's something I demonstrate in my Guerrilla Capacity and Performance (GCAP) classes.

Related Posts

Tuesday, April 20, 2021

PDQ Online Workshop, May 17-21, 2021

PDQ (Pretty Damn Quick) is a free, open source, performance analyzer available from the Performance Dynamics web site.

All modern computer systems, no matter how complex, can be thought of as a directed graph of individual buffers that hold requests until to be serviced at a shared computational resource, e.g., a CPU or disk. Since a buffer is just a queue, any computer infrastructure, from your laptop up to Facebook.com, can be represented as a directed graph of queues.

The directed arcs or arrows in such a graph correspond to workflows between different queues. In the parlance of queueing theory, a directed graph of queues is called a queueing network model. PDQ is a tool for predicting performance metrics such as, waiting time, throughput, optimal user-load.

Two major benefits of using PDQ are:

  1. confirmation that monitored performance metrics have their expected values
  2. predict performance for circumstances that lie beyond current measurements
Find out more about the workshop and register today.

Thursday, November 26, 2020

PDQ 7.0 is Not a Turkey

Giving Thanks for the release of PDQ 7.0, after a 5-year drought, and just in time for the PDQW workshop next week.

New Featues

  1. The introduction of the STREAMING solution method for OPEN queueing networks. (cf. CANON, which can still be used).
  2. The CreateMultiNode() function is now defined for CLOSED queueing networks and distinguished via the MSC device type (cf. MSO for OPEN networks).
  3. The format of Report() has been modified to make the various types of queueing network parameters clearer.
  4. See the R Help pages in RStudio for details.
  5. Run the demo(package="pdq") command in the R console to review a variety of PDQ 7 models.

Maintenance Changes

The migration of Python from 2 to 3 has introduced maintenance complications for PDQ. Python 3 may eventually be accommodated in future PDQ releases. Perl maintenance has ended with PDQ release 6.2, which to remain compatible with the Perl::PDQ book (2011).

Sunday, November 8, 2020

PDQ Online Workshop, Nov 30-Dec 5, 2020

PDQ (Pretty Damn Quick) is a queueing graph performance analyzer that comes as:
  1. free open source software package
  2. with an online user manual

As shown in the above diagram, any modern computer system can be thought of as a directed graph of individual buffers where requests wait to be serviced at some kind of shared computational resource, e.g., a CPU or disk. Since a buffer is just a queue, any computer infrastructure, from a laptop to Facebook, can be represented as a directed graph of queues. The directed arcs or arrows correspond to workflows between the different queues. In the parlance of queueing theory, a directed graph of queues is called a queueing network model. PDQ is a tool for calculating the performance metrics, e.g., waiting time, throughput, optimal load, of such network models.

Some example PQD applications include models of:

  • Cloud applications
  • Packet networks
  • HTTP + VM + DB apps
  • PAXOS-type distributed apps

Two major benefits of using PDQ are:
  1. confirmation that monitored performance metrics have their expected values
  2. predict performance for circumstances that lie beyond current load-testing
This particular PDQ workshop has been requested for the Central European Time zone.
  • All sessions are INTERACTIVE using Skype (not canned videos)
  • Online sessions are usually 4 hours in duration
  • A typical timespan is 2pm to 6pm CET each business day
  • A nominal 5-10 minute bio break at 4pm CET
  • Attendees are encouraged to bring there own PDQ projects or data to start one
  1. Find out more about the workshop.
  2. Here is the REGISTRATION page.
Hope to see you at PDQW!

Tuesday, February 11, 2020

Converting Between Human Time and Absolute Time in the Shell

This is really more of a note-to-self but it may also be useful for other readers.

Converting between various time zones, including UTC time, is both a pain and error-prone. A better solution is to use absolute time. Thankfully, Unix provides such a time: the so-called Epoch time, which is the integer count of the number of seconds since January 1, 1970.

Timestamps are both very important and often overlooked: the moreso in the context of performance analysis, where time is the zeroth metric. In fact, my preferred title would have been, Death to Time Zones but that choice would probably have made it harder to find the main point here, later on, viz., how to use the Unix epoch time.

Although there are web pages that facilitate such time conversions, there are also shell commands that are often more convenient to use, but not so well known. With that in mind, here are some examples (for future reference) of how to convert between human-time and Unix time.

Examples

The following are the bash shell commands for both MacOS and Linux.

MacOS and BSD

Optionally see the human-readable date-time:

[njg]~% date
Tue Feb 11 10:04:32 PST 2020
Get the Unix integer time for the current date:

[njg]~% date +%s
1581444272
Resolve a Unix epoch integer to date-time:

[njg]~% date -r 1581444272
Tue Feb 11 10:04:32 PST 2020

Linux

Optionally see the human-readable date-time:

[njg]~% date
Tue Feb 11 10:04:32 PST 2020
Get the Unix integer time for the current date:

[njg]~% date +%s
1581444272
Resolve a Unix epoch integer to date-time:

[njg]~% date -d @1581444272
Tue Feb 11 18:04:32 UTC 2020

Monday, June 3, 2019

Book Review: How to Build a Performance Testing Stack From Scratch

Writing a technical book is a difficult undertaking. As a technical author myself, I know that writing well is both arduous and tedious. There are no shortcuts. Over the last 40 years or so, computer-based tools have been developed to help authors write printed textbooks, monographs and technical articles. LaTeX (pronounced lar-tek) reigns supreme in that realm because it's not just word processing software but, a full-blown digital typesetting application that enables authors to produce a single camera-ready PDF file (bits) that is fit for direct printing. Not only does LaTeX correctly typeset characters and mathematical symbols but it can generate the Table of Contents and the corresponding Index of terms, together with correctly cross-referenced callouts for numbered chapters, sections, figures, tables and equations.

In the meantime, over the past 20 years, the nature of the book itself has become progressively more digital. The ability to render the book-block on digital devices as an "e-book" has made printed hardcopies optional. Although purely digital e-books make reading more ubiquitous, e-book file formats and display quality varies across devices, i.e., laptops, phones tablets, and various e-readers. Any reduction in display quality is offset by virtue of e-books being able to include user interaction and even animation: features entirely beyond the printed book. In that sense, there really has been a revolution in the publishing industry: books are no longer about books, they're now about media.

Recently, I became aware of an undergraduate calculus textbook that makes powerful use of animation and audio. The author is a academic mathematician and he published it on YouTube! Is that a book or a movie? Somehow, it's a hybrid of both and, indeed, I wish I'd been able to learn from technical "books" like that. Good visuals and animations can make difficult technical concepts much easier to comprehend. Progressive as all that is, when it comes to technical e-books, I'm not aware of any single authoring tool that can match the quality of LaTeX, let alone incorporate user-interaction and animation. If such a thing did exist, I would be all over it. And Markdown doesn't cut it. But, digital authoring tools are continually evolving.

Matt Fleming (@fleming_matt on Twitter), the author of How to Build a Performance Testing Stack From Scratch, opted to use a static e-book format—not because it produces the most readable result, but because it is the best way to reach a wider audience at lower cost than a more expensive print publisher. The e-publisher in this case is (the relatively unknown) Ministry of Testing Ltd in Brighton, UK and is available on Amazon for the Kindle reader. I was also able to read it using iBooks on Mac OS X.

The range of topics covered is very extensive. I've included the Table of Contents here because it is not viewable on Amazon:

Part 1
  • Step 1: Identify Stakeholders
  • Step 2: Identify What to Measure
  • Step 3: Test Design
  • Step 4: Measuring Test Success and Failure
  • Step 5: Sharing Results
Part 2
  • Understanding Statistics
  • Latency
  • Throughput
  • Statistical Significance
Part 3 The Benchmark Hierarchy Picking Tests Validating Tests Part 4
  • Use a Performance Framework
  • Ensure the Test Duration is Consistent
  • Order Tests by Duration
  • Keep Reproduction Cases Small
  • Setup The Environment Before Each Test
  • Make Updating Tests Easy
  • Errors Should Be Fatal
Part 5
  • Format
  • Use Individual Sample Data
  • Detecting Results in the Noise
  • Outliers
  • Result Precision
  • If All Else Fails Use Test Duration
  • Delivering Results
The overall e-book presentation of "Performance Testing Stack" seems underdeveloped. Most topics could have been greatly expanded. But, as Matt informed me, this is probably due to the content amounting to a concatenation of previously written blog posts. As I said earlier, there are no shortcuts to writing well. The paucity of detail, however, is offset by the shear enthusiasm the Matt brings to his writing: an aspect that deserves separate acknowledgement because performance testing is a very complex subject which can otherwise appear dry and mind-boggling to the uninitiated. And it's the uninitiated that Matt wants to reach. He has written this book in order to encourage the uninitiated reader to seriously consider entering the field.

Some points that could have been developed further, include:

  • Plots and tables can be expanded for legibility by double clicking on them
  • The term "benchmark" needs better explaination
  • Section 2.1 discusses the Harmonic mean but there's no discussion of the Geometric mean.
  • Section 3.3 on Distributions does not clearly distinguish between analytic (parametric) distributions and sample distributions (which usually have no analytic form).
  • Section 5 (p.85) on Result Precision needs to discuss the difference between accuracy, precision, and error.

Conversely, Matt's enthusiasm may have gone a bit overboard in his choice of title. The book promises:

This book will walk you through designing and building a performance testing stack from scratch; step by step from planning and running performance tests through to understanding and analysing the results. If you’re new to performance testing or looking to expand your understanding of this topic then this book is for you!
Unfortunately, this book doesn't provide enough details to actually build a test stack—which would've been very cool. Rather, it presents a comprehensive overview of all the major concepts that one needs to absorb in order to develop a running performance testing stack. But even with the more limited scope, this book is still important because, off hand, I don't know of any other source where one can be introduced to performance testing without drowning in a sea of terminology, procedures and architectures.

Ultimately, this e-book is a great starting point for newbies, as well was being a good reminder for seasoned testers about what should be done in good performance tests.

Wednesday, January 2, 2019