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.

The Verdict The Return of the JMicron based SSD
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  • ryedizzel - Thursday, March 19, 2009 - link

    Anand,

    I just wanted to thank you for an amazing article. I am a very picky buyer and technology critic, so I always come to your site for the ‘real’ story on things. In fact for the amount of time, research, and (useful) testing methodologies you invest, I almost feel guilty receiving this information for free. Especially since your findings benefit the industry as a whole since it causes manufacturer’s to fix/improve their products (well at least the smart ones do). The i7 motherboard roundup was another great example of this. Seriously, if you have a place for donations I would send you $50 in a heartbeat. I know it’s not much but if others did the same it would add up to a decent token of appreciation.

    Oh and please don’t take people’s grammar or nitpicky corrections the wrong way. Yes it can be annoying, but in the end it does help the article become closer to perfect since others catch little things overlooked by human error. In the end we are all grateful for these articles, otherwise we wouldn’t be here reading them!
    Reply
  • bsoft16384 - Thursday, March 19, 2009 - link

    You know, the JMicron SSDs really aren't so bad. Yes, you'd have to be crazy to spend $400 on one when there are better options like the X25-M (or the new Vertex series for that matter).

    But I paid $65 for my 30GiB OCZ "Core" SSD. Yeah, random writes are piss slow. I knew this. The drive replaced the 5400.5 that came with my EEE PC.

    I'm not going to be doing any 'extensive multitasking' on my EEE. I'm not running a file server, I certainly don't have antivirus in the background, and I don't spend all day installing new apps.

    I'm running 7. Compared with the 5400.5, the system boots faster, Chrome loads faster, but Windows updates take longer. That's a trade-off I'm willing to deal with, considering that I get less noise and more battery life in return.

    I can tell you this - the JMicron SSDs beat the pants off of the PCIe MLC SSDs that ship with many netbooks.

    So, yeah, I guess this is a product that's "unfit for market". But it's perfect for some of us. If I wanted a boot drive for a Linux media/backup server (along with HDDs for storage), I could see choosing a $50 SSD over a $50 HDD.

    It's all about your needs. No one is pretending that JMicron SSDs are fast, at least not at writing. But if your usage is mostly read-centric (or "nothing-centric", as is frequently the case on netbooks), and your chief criteria are low power and low price, the JMicron SSDs do quite well.
    Reply
  • petersterncan - Thursday, March 19, 2009 - link

    Anand,

    That was an awesome article... and good for you to give OCZ to do the right thing... and they did!

    Kudos also to OCZ for actually listening to feedback and doing something about it.

    Reply
  • Adul - Thursday, March 19, 2009 - link

    Good work Anand :) Reply
  • SSDMaster - Thursday, March 19, 2009 - link

    The stuttering problem with SSD's can be fixed with diskpart. Go do some research before you post an article this massive and convoluted. Reply
  • goinginstyle - Thursday, March 19, 2009 - link

    No it does not, it will help if the partition is properly aligned but it is not a cure, neither is the rest of the voodoo magic being spouted on the OCZ forums. They help, but do not cure the stuttering problem with the JMicron based drives. I just love the fact that OCZ wants to sell you a drive (Core series) that does not work properly, have you purchase third party software, and then hack the OS in order to improve the performance of the product. Glad to see it fixed with the Vertex and Summit drives though, but it will take a long time before I even think about using an OCZ product again. I was hoping to see the new ADATA and SuperTalent drives in this article, maybe those are coming in the next segment he mentioned. Reply
  • SSDMaster - Friday, March 20, 2009 - link

    Yes, it does.
    I have a Core series 60GB OCZ drive. I bought it right before Newegg increased the price on the drives. The stuttering was horrible, and worthless even as a secondary drive if it was formatted with XP. Also, after using Diskpart and aligning the drive I could not install XP on the drive and have it bootable.. Which sucked.

    But there's ways around that, and guess what, I have a stutter free flash drive for cheap that gets very good performance numbers, and boots Server 2008 in under 10 seconds.
    Reply
  • bsoft16384 - Thursday, March 19, 2009 - link

    OCZ's product works fine. You may not like the performance, but it's certainly not unusable.

    The $65 I paid for my 30GiB OCZ "Solid" SSD is about what you'd pay for a USB flash drive. The disk I have has a USB interface, which is very convenient, plus it's plenty fast enough for my EEE PC.

    I'm glad that Anand has done these reviews. People need to understand what they're getting into when they by a JMicron SSD. If you don't expect much, you won't be disappointed.

    Arguably for a normal notebook/desktop you should buy a normal hard drive if your budget is under $100. But the JMicron SSDs do a good job in netbooks (which, again, aren't too fast to begin with) at a very low price.

    I have aligned my partitions and disabled swap on my Windows 7 install (on my EEE). I also have 2GB of memory in my EEE. I haven't done any fancy tweaking on the OS.

    Compared with the EEE 900A that I had briefly (PCIe SSD), my EEE 900HA is dramatically faster. You can't run XP or Vista on PCIe SSDs unless you have a lot of patience. You *can* run it on a JMicron SSD.

    I honestly don't notice any stuttering. I don't run antivirus and I don't multitask much on this machine. If I demanded from my EEE what I demand from my desktop (Q9300 + 8GB + WD6400AAKS), I know that the SSD would choke up. But I'm not going to do that on a 1.6GHz single-core Atom anyway.

    So, yeah, OK. I guess I think that trashing JMicron SSDs is a little like trashing USB flash drives for being slow. Paying $300 for a UFD would be stupid, as is paying $300 for a JMicron SSD. But in the sub-$100 category, you don't expect much.

    When better SSDs drop below $100, maybe I'll upgrade. Until then, I'm enjoying 5 hours of battery life on my 900HA.
    Reply
  • tomoyo - Thursday, March 19, 2009 - link

    Well I think it's unusable for my needs if I'm running it as an operating system drive. I place a giant important on the latency of the drive at that point and it certainly includes random writes. Which is why I would never ever buy an ssd that's majorly below the Intel write performance. It's not worth the price premium or loss of storage size compared to the standard hard drive. Reply
  • Bikerskummm - Thursday, March 19, 2009 - link

    Fantastic article Anand!

    Just a few thoughts:

    I have had some trouble replicating some of the Samsung SLC results...

    Despite filling the drive up and emulating a well used drive as described in the article I cannot get my Sammy drive's performance to degrade as much as you managed to especially regarding random write performance...

    Now my system used for testing is a socket 775 (qx9650) and I was testing on ICH9R and WINXP (SP3) but still I would expect to see similar figures ...

    I do not have an X58 system to test on at the moment but I would be very interested if The Sammy SLC drives were shown to degrade faster / perform worse with a X58 / ICH10R / Vista x64 (SP1?) setup...
    Reply

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