The SSD Anthology: Understanding SSDs and New Drives from OCZ
by Anand Lal Shimpi on March 18, 2009 12:00 AM EST- Posted in
- Storage
Latency vs. Bandwidth: What to Look for in a SSD
It took me months to get my head wrapped around it, but I think I finally get it. We often talk about the concepts of bandwidth and latency but rarely are they as tangible as they are here today.
When I speak of latency I’m talking about how long it takes to complete a request, or fetch a block of data. When I mention bandwidth, I’m talking about how much you can read/write at once. Think of latency as the speed limit and bandwidth as the number of lanes on a high way.
If you’re the only car on the highway, you’re going to notice the impact of latency more than bandwidth. A speed limit of 70 mph instead of 35 is going to impact you much more than if you added more lanes to the road.
If you’re a city planner however and your only concern is getting as many people to work and back, you’re going to notice the impact of bandwidth more than latency. It doesn’t matter how fast a single car can move, what matters is how many cars you can move during rush hour traffic.
I’d argue that if you’re a desktop user and you’re using an SSD as a boot/application drive, what will matter most is latency. After you’ve got your machine setup the way you want it, the majority of accesses are going to be sequential reads and random reads/writes of very small file sizes. Things like updating file tables, scanning individual files for viruses, writing your web browser cache. What influences these tasks is latency, not bandwidth.
If you were constantly moving large multi-gigabyte files to and from your disk then total bandwidth would be more important. SSDs are still fairly limited in size and I don’t think you’ll be backing up many Blu-ray discs to them given their high cost per GB. It’s latency that matters here.
Obviously I’ll be testing both latency and bandwidth, but I wanted to spend a moment talking about the synthetic latency tests.
Iometer is a tool that can simulate any combination of disk accesses you can think of. If you know how an application or OS hits the disk, iometer can simulate it. While random disk accesses are the reason that desktop/notebook hard drives feel so slow, the accesses are generally confined to particular areas of the disk. For example, when you’re writing a file the OS needs to update a table mapping the file you’re writing to the LBAs it allocated for the file. The table that contains all of the LBA mapping is most likely located far away from the file you’re writing, thus the process of writing files to the same area can look like random writes to two different groups of LBAs. But the accesses aren’t spread out across the entire drive.
In my original X25-M article I ran a 4KB random write test over the entire span of the drive. That’s a bit more ridiculous than even the toughest user will be on his/her desktop. For this article I’m limiting the random write test to an 8GB space of the drive; it makes the benchmark a little more realistic for a desktop/notebook workload.
The other thing I’ve done is increased the number of outstanding IOs from 1 to 3. I’ve found that in a multitasking user environment Vista will generally have a maximum of 3 or 4 outstanding IOs (read/write requests).
The combination of the two results in a 100% random file write of 4KB files with 3 outstanding IOs to an 8GB portion of the drive for 3 minutes. That should be enough time to get a general idea of how well these drives will perform when it comes to random file write latency in a worst case, but realistic usage scenario.
Facebook
Twitter
RSS
250 Comments
View All Comments
GlItCh017 - Wednesday, March 18, 2009 - link
I just wanted to comment that the backstory portion to this article is simply the most interesting part to an article (or almost even an article inside the main article). On top of that, it is easily the most interesting article I have ever read simply because of that section. Really really must say that I enjoyed reading it!radguy - Wednesday, March 18, 2009 - link
I have been waiting for this one for a while and it was very informative. Thank you very much for it. I did pick up one of the patriot warp drives for my netbook. I was really happy until I installed avg free. So not running an antivirus on it anymore but I have drive image backup incase it goes bad. Overall pretty happy as it was only 80 bucks if I get my mir.I think I'm going to wait until windows 7 till I upgrade my primary desktop. 2 of those vertexs in raid 0 would be sweet though.
sleepeeg3 - Wednesday, March 18, 2009 - link
They were one of the first SSDs you reviewed and they use their own controller. How does their random write performance compare to everything else out now?These reviews made me totally reassess the purchase of the two Samsungs I bought. I had no idea the random writes on the Samsung drives were so bad. Other reviews show the Samsung drives doing better or at least near the X25-M in write tests: http://www.techreport.com/articles.x/15433/6">http://www.techreport.com/articles.x/15433/6 However, those tests probably would have been somewhat sequential.
nubie - Wednesday, March 18, 2009 - link
Grammatically awkward sentence on Page 21:"so if you own one of these drives - you owned a fixed version."
The tense is incorrect (own/owned). I think "own a fixed version" is still awkward, perhaps "you have the fixed version", also the "so" may be superfluous. You can replace the ", so if" with a "; if". Here is how I might re-write the sentence:
"The old firmware never shipped thanks to OCZ's quick acting; if you own one of these drives - you have a fixed version."
(I am not an expert, so feel free to correct me if I am wrong.)
Awesome article btw, thanks for setting me straight on SSD, I have been steering clear of them. I hope soon you can review SSD's and most are good to excellent. :)
Flyboy27 - Wednesday, March 18, 2009 - link
This article has answered every question I've had regarding SSDs recently. Thanks Anand!Flyboy27 - Wednesday, March 18, 2009 - link
If a 120gb Vertex was around $250 I would get one yesterday. I suppose I can wait though.7Enigma - Thursday, March 19, 2009 - link
For me, 2 60's or 2 80's for around that price and I'm sold. Want the Raid0.kgwagner - Wednesday, March 18, 2009 - link
I almost didn't read this article, as everybody and their brother seems to want to explain SSDs these days and most of the articles aren't much more than glorified press releases. But, this one truly took the drives to task and presented some valid information and explanations about the state of the art and where it needs to go. Kudos, Anand. Awesome show. Good job.Mr Perfect - Wednesday, March 18, 2009 - link
"Needless to say, there was some definite fallout from that review. I’m used to negative manufacturer response after a GPU review, but I’m always a bit surprised when it happens in any other segment."Obviously you can't make a business out of irritating manufacturers, but when there really are issues, the readers want to know about them. After all, that's why we come here!
gwolfman - Wednesday, March 18, 2009 - link
You own Anand. Keep up the good work. I've seen you cited from many sites about the work you've done, in particular with SSDs. Best article I've read in months!