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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  • mikepers - Tuesday, March 24, 2009 - link

    Anand,

    I wanted to compliment you on what I think was an excellent article. This is the type of thing I really have always liked from Anandtech. The detailed background, the technical reasons for the issues and then a thorough review of the current state of things.

    I just finished upgrading my desktop. The only remaining item I wanted to replace was the hard disk. I had been thinking about getting a Velociraptor but instead I just ordered a 60GB Vertex from Newegg.

    Thanks again for all the work.

    Mike P.
    Reply
  • ameatypie - Monday, March 23, 2009 - link

    That sure was a lot to take in! Fantastic article though, it has really opened my eyes to the possibilities that Solid State Drives provide. Probably wont be buying one in the immediate future given the so-called depression and such things, but i will certainly keep up with SSD progress.
    Thanks again for your fantastic articles - im sure im not the only one who really appreciates them :)
    Reply
  • coopchennick - Monday, March 23, 2009 - link

    Hey Anand, I just finished reading through this whole article and I'm very impressed with the thoroughness and how informative it was.

    You just acquired a new regular reader.
    Reply
  • zdzichu - Sunday, March 22, 2009 - link

    Very nice and thorough article. I only lack more current status of TRIM command support in current operating systems. For example, Linux supports it since last year:

    http://kernelnewbies.org/Linux_2_6_28#head-a1a9591...">http://kernelnewbies.org/Linux_2_6_28#h...a9591f48...
    Reply
  • Sinned - Sunday, March 22, 2009 - link

    Outstanding article that really helped me understand SSD drives. I wonder how much of an impact the new SATA III standard will have on SSD drives? I believe we are still at the beginning stage for SSD drives and your article shows that much more work needs to be done. My respect for OCZ and how they responded in a positive and productive way should be a model for the rest of the SSD makers. Thank you again for such a concise article.
    Respectfully,
    Sinned
    Reply
  • 529th - Sunday, March 22, 2009 - link

    The first thing I thought of was Democracy. Don't know why. Maybe it was because a company listened to our common goal of performance. Thank you OCZ for listening, I'm sure it will pay off!!! Reply
  • araczynski - Saturday, March 21, 2009 - link

    very nice read. the 4/512 issue seems a rather stupid design decision, or perhaps more likely a stupid problem to find this 4/512 solution as 'acceptable'.

    although a great marketing choice, built in automatic life expectancy reduction.

    sounds like the manufacturers want the hard drives to become a disposable medium like styrofoam cups.

    perhaps when they narrow the disparity down to 4/16, i might consider buying an ssd. that, or when they beat the 'old school' physical platters in price.

    until then, get back to the drawing board and stop crapping out these half arsed 'should be good enough' solutions.
    Reply
  • IntelUser2000 - Sunday, March 22, 2009 - link

    araczynski: The 4/512 isn't done by accident. It's done to lower prices. The flash technology used in SSDs are meant to replace platter HDDs in the future. There's no way of doing that without cost reductions like these. Even with that the SSDs still cost several times more per storage space. Reply
  • araczynski - Tuesday, March 24, 2009 - link

    i understand that, but i don't remember original hard drives being released and being slower than the floppy drives they were replacing.

    this is part of the 'release beta' products mentality and make the consumer pay for further development.

    the 5.25" floppy was better than the huge floppy in all respects when it was released. the 3.5" floppy was better than the 5.25" floppy when it was released. the usb flash drives were better than the 3.5" floppies when they were released.

    i just hate the way this is being played out at the consumer's expense.
    Reply
  • hellcats - Saturday, March 21, 2009 - link

    Anand,

    What a great article. I usually have to skip forwards when things bog down, but they never did with this long, but very informative article. Your focus on what matters to users is why I always check anandtech first thing every morning.
    Reply

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