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
POST A COMMENT

250 Comments

View All Comments

  • Natfly - Wednesday, March 18, 2009 - link

    Reply
  • DangerMouse4269 - Tuesday, April 13, 2010 - link

    Nicely written. Even a very out of practice Comp Eng understood that. Reply
  • geekforhire - Monday, June 14, 2010 - link

    I have just replaced the hard drive in this 3 year old Dell Inspiron 9400 notebook computer with a new and very quick OCZ SSD, manually configured the partition with a 1024 offset, freshly installed the OS, freshly downloaded all of the latest and greatest drivers from Dell, and applied all currently available OS updates from Msft.

    The problem is that when the machine resumes from Standby, it will /reliably/ (4 out of 4 attempts) produce a BSOD 0xF4 after the power button is pressed to resume the machine from standby.

    Here's the sequence to recreate the problem:

    0) Machine is booted normally into Windows, and log in to an account which has administrative privs.
    1) Click on Start -> Shut Down -> Standby.
    2) See display turn black, disk I/O light flashes then stops, then the power indicator light begins to flash on and off slowly.
    3) Wait until the power light has made 2 slow flashes.
    4) Press the power button.
    5) See the Dell Bios splash screen, then disappear
    6) Boom: See the BSOD 0xF4

    The values reported after the STOP are:
    (0x00000003, 0x865b3020, 0x865b3194, 0x805d2954)

    Note that I've been in contact with OCZ before about this SSD+computer, because the previous BSOD that was produced was 0x77. Their recommendation was to create the partition with an offset with a 64 interval, and to reflash the SSD with their modern firmware. This was done, the OS was reinstalled as described, and now I'm getting a different BSOD code. Another mention was a question whether the notebook computer uses a SATA2 controller (definitely compatible) or SATA1 (which may have troubles).

    I've run Spinrite on the SSD, and there are lots of ECC errors being reported. I've been in contact with Spinrite, and they chalk this up to the SSD being chatty (which they like), but since SSD's are new and magnetic disks are common, they want to stay focussed on magnetic disks.

    When the machine boots back up, the OS reports that a serious error has occurred, and asks that a problem report be submitted, which I do. Then an attractive but somewhat generic page is displayed with common causes (Aging or failing hard disks, large file transfers from secondary media to local hd, loss of power to a hard drive, hard disk intensive processes (eg: antivirus scanners), recently installed hardware that might have compatibility and performance problems)

    Has anyone else encountered this kind of problem, and do you have any suggestions?
    Reply
  • angavar - Thursday, September 9, 2010 - link

    As a medical student I can appreciate a well researched and analytical article when I see it. This is by far the best computer hardware review I have ever read! Thank-you for your time and effort in producing what is clearly a thoroughly researched and detailed analysis. Reply
  • mac021 - Wednesday, October 17, 2012 - link

    Thank you for the lesson and helping me understand SSD drives. May I just ask for your advice...

    For everyday use designing and generating prototypes for websites and running typical office s/w like word and excel for long documentations while listening to music or just having some video play in the background, then the occasional gaming of, say Star Craft 2 and Dead Space 3, and lets assume I do this on a 5 hours a day average for 365 days in a year, how long before I need to replace an OCZ Vertex/Summit SSD? And does format/reinstall help in prolonging the life of an SSD just as it does for my old hard drives (from a computer that's 6 years old and counting)? Or there's no stopping the SSD's death after reaching 10,000 times of being erased and rewritten on? I'm not one who keeps upgrading or buying new computer systems for every new thing that comes out, i'm more of a keeper and maintainer for as long as the system servers my needs... but when I make a purchase, I make sure it will be enough to last me another 6-12 years IF possible! Which is why I'm still considering SATA for my next purchase late this year or early next year (and I'm only buying a new PC just because I made a mistake buying a foxconn motherboard that can't support anything higher than XP, not even Vista... weird, anyway I found that out too late).

    Also, would you know of a motherboard that supports SSD, Windows 8, Nvidea, third gen i5/i7, and up to 64GB ram?

    Thanks so much!
    Reply
  • windows10 - Wednesday, November 30, 2016 - link

    This article is meaningfull, interesting. thank you for sharing Reply
  • susanbones - Tuesday, September 12, 2017 - link

    I was wondered to these many responses here. Reply

Log in

Don't have an account? Sign up now