Understanding SSD Advertised Performance and Its Purchase Implications – An SSD Primer

UNDERSTANDING OUR COMPUTER USE

In the end, it confirms something we always thought but just didn’t really understand.  Large sequential read and write access (high sequential and 512kb) are utilized by the average user less than 1% of the time yet the most used method of access is smaller random write access as shown by the 4k write at over 50%.

Manufacturers showcase high sequential disk transfer speeds in selling their SSDs solely for their lightning fast appearance yet, these high sequential speeds are actually the disk transfer method that is used the least at less than 1% in  total.  As much as we would like to lay blame solely on the SSD companies, the majority of blame actually goes right back to the new SSD consumer who knows of no other way to differentiate between the myriad of SSDs available other than through high sequential performance.

Since the small 4k random write score is accessed at over 50%, lets examine that for a bit. Take a look at the two above benchmarks and tell me which is the better. Do you see a pattern at all? Look closely at the small random 4kb access speed results and how they differ.  Would you believe that the result on the left is that of a hard drive while the SSD is to the right?

The truth is that the low 4k random write performance of the SSD on the right is 81 times faster than that of the hard drive on the left and this disk transfer method is the most used at well over 50% of the time.  Next to disk access speed, low 4k data movement is primarily responsible for the visible upgrade we see when moving from a hard drive to an SSD.

LOW 4K WRITE PERFORMANCE EXPLAINED

When you start your computer and/or any software applications, your system relies on several dynamic link libraries (DLLs) to get the OS and applications up and running properly.  DLLs are simply hundreds of smaller programs that are called upon by the main program when needed and do not reside in your RAM.  These DLLs are very small in size and are loaded through 4-8kb random disk access which means that the faster they load, the faster the OS loads as well as its software.  It also means that the faster these DLLs can be loaded while a program is in use, the quicker the program.

In other words, the 4kb random write access is the single most crucial access that results in better visible SSD performance. If you are considering buying an SSD and want the absolute best visible upgrade from your present system, you simply find the SSD with the best transfer results at the 4-8 kb random access level.

UNDERSTANDING DISK ACCESS SPEED

Some may read this and think that we should have opened with an explanation of ‘disk access’, however, the truth is our selection of an SSD will have very little to do with disk access speeds whatsoever.  While it is true that the largest visible performance upgrade we see when moving from a hard drive to an SSD is definitely a result of disk access speeds, all SSDs have very close disk access speeds which are around 0.01-0.02ms, as compared to a hard drive which is typically 9ms or more.  Lets take a look at a benchmark of the Intel Series 520 240GB SSD where the disk access speed is shown as ‘Resp. Time’:

Once again, when we compare the disk access at low 4k random write to that of the typical hard drive, we are looking at a disk access decrease of 90 times which accounts for the stealth of the SSD in use.

FINAL THOUGHTS

It’s a given that the most visible upgrade we see when moving from a hard drive to a SSD is accomplished through the lightning fast information retrieval accomplished by the the access speed of the SSD first and foremost.  This is a ‘nice to know’, however, holds little merit when deciding what SSD is best simply because the access speeds of all solid state drives are so close.

Low 4k random write disk performance now becomes paramount because it is used well over 50% of the time in typical computer use and software relies on this access to start as fast as it can.  Today many SSDs are similar in their low 4k random write performance, however, the difference in 4k random transfer speeds is more important to the typical user than the difference of the high sequential performance that is used primarily to enhance SSD sales.

Why would I even care if my SSD is capable of 550MB/s read and 520MB/s write performance if I will never reach these speeds in my typical use?  Low 4K random transfer speeds, however, I always use.  That is what should interest me first!

In case this article has reached you through one of the many search engines, we should mention that this is the third in our series of  SSD Primer articles meant to help us all along in our understand of solid state drives.

  1. BENEFITS OF A SOLID STATE DRIVE – AN SSD PRIMER
  2. SSD COMPONENTS AND MAKE UP – AN SSD PRIMER
  3. SSD TYPES AND FORM FACTORS – AN SSD PRIMER
  4. SSD ADVERTISED PERFORMANCE – AN SSD PRIMER
  5. SSD MIGRATION OR FRESH INSTALL – AN SSD PRIMER
  6. GC AND TRIM IN SSDS EXPLAINED – AN SSD PRIMER

 

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Frank
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Frank

This is a very interesting test. It does indeed shed light on a fallacy of typical SSD advertising. One other thing it sheds light on is even more surprising. How wasteful the OS is of write cycles. Look at the numbers again. 56.53% of all accesses in his test were 8K writes. 8K reads were nuber 2 at only 7.6%. When is the computer reading all of this data that it is writing? If you do some math on the full results does it show something closer to balance in total K read versus total K written? Given that JEDEC… Read more »

Nick
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Nick

I certainly agree with you on principle, but I wonder how much of that data could be writes to the same block. Consider a program with a while loop and a counter. If I only care about the value of the counter after the while loop’s exit condition is met, I may potentially be writing a value there many times before a single read is needed. Surely other similar situations also exist. (Granted, in this trivial case, that information need not be stored to disk.)

Molgor
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Molgor

Also keep in mind that PCs are running more and more .Net applications, not to mention Windows, Office, and other Microsoft Software. At least some of the code, if not half or more, is MSIL, not machine language. As the CLR interprets the MSIL, it is constantly upgrading the code to Native Mode. That could account for some of the 8K writes as well.

David
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David

I would love to hear your analysis and recommendations on using a RAM disk in conjunction with an SSD. I’m doing that now and have installed my two most write-intensive applications (stock charting and anti-virus) into it as well as locating my web browser caches (IE and Firefox) and user temp file locations into it. I’m using DataRam’s free RAMDisk driver.
I bring this up as I’ve read that Windows 7 caches much of what’s in RAM on the disk anyway – thus negating my “protecting” my SSD from the writes of applications writing to the RAMDisk.

Q
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Q

Not sure on Win7, but I assume it shouldn’t be much different… In WinXP, for something like 4 years or so, I disabled the paging file altogether. What this means are 2 things: 1 – (the bad) if you do not have enough RAM windows will simply tell you so and if you use more, one of the programs will get closed. Nowadays 4Gb should take care of most (95%) of situations, 8Gb if 4Gb doesn’t cut it will solve your problems; 2 – (the great) Never again will windows copy something out of the RAM into the disk in… Read more »

Sam
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Q, I had my paging file disabled for a long time, too. In the end, I did some measurements, and was quite surprised: while I felt good having the paging file disabled, it actually did nothing for my computers performance. In the end I did not bother disabling it after the last reinstall, since it does not help measured performance in any way, and instead can pose problems if you run out of memory. SITE RESPONSE: I have been the biggest advocate of ‘no pagefile’ for years and have never said that it alone will increase performance. I have also… Read more »

Trane Francks
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Q: It’s important to note that a 32-bit Windows system cannot use more than 4 GB of RAM, even with Physical Address Extension enabled. As such, your 8 GB suggestion wouldn’t improve things at all unless the user also upgraded to a 64-bit OS. Even then, the Windows 7 Starter version is limited to 2 GB RAM.

Gonzo_V
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Gonzo_V

WOW… now I am noways close to even writing code much less really knowing exactly where and the inter reaction too all the terms you used to explain your view of the subject. However I did understand what you were talking about in terms of it’s actual event/product to the operating system……….thanks guys really insightful.

Gonzo

SITE RESPONSE: Thank you for the favorable return.

wx4sno
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wx4sno

The 4k read chart above, was that conducted on 34nm or 25nm NAND? I’ve heard recently that the OCZ Vertex 2 has been shipping with 25nm and everyone is reporting much slower response times…your thoughts??? I just bought a Vertex 2 but I don’t know if it will arrive as 34nm or 25nm….