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.
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sbuckler - Wednesday, March 18, 2009 - link
Depends on how smart the controller is? Shuffling around the data now and again in the background would make sense.Frallan - Wednesday, March 18, 2009 - link
@AT
This is why i come to AT to read up on the developments.
@OCZ
Well played :0)
The ruler of the roost are the Intels however I will be able to afford one of those when there are cows enjoying themselfs by dancing on the moon. My next upgrade will be a Vertex - not only bc its Valu for money but equally much bc. OCZ obviously takes care of thier customers and listens to reason.
pmonti80 - Wednesday, March 18, 2009 - link
This is the kind of article that makes me come back here.nowayout99 - Wednesday, March 18, 2009 - link
OCZ, you should listen to Uncle Anand. ;) Hopefully Mr. Petersen understands that it's tough love.And the final product seems perfectly cool -- great performance at a better price than Intel. It's the first SSD I'd be able to reasonably consider.
SOLIDNecro - Wednesday, March 18, 2009 - link
Thx for this article Anand, I have been in a hotly contested debate over OCZ vs Samsung with my "Asperger Enhanced" nemisis/close friend...(In all fairness, I should mention I use the BiPolar SSE instruction set myself)
He was only looking at Samsung, I said he should look into what OCZ has now.
His reply was "I don't know them, and don't want to be disapointed"
(Long story behind that...He's from the Server/Workstain/HPC crowd, I am from the hardcore OC/Gamer/Desktop group, so he is not familiar with OCZ)
Looks like the Samsung (And alot of others) has "Issues" with performance degrading over time that are somewhat solved by Intel and OCZ (Plus maybe a few other companies that use the Rev B JMicron controller on there low cost SSD's)
I agree the OCZ Vertex offers the best bang for low buck SSD today, and I am tempted to grab one. But a year from now, anyone that bought a current gen MLC SSD will be saying "I coulda had a V-8" if that TRIM technology does what it promises!!!
James5mith - Wednesday, March 18, 2009 - link
As people continue to try and push the envelope of storage performance in a variety of ways, and as 6gbps SATA becomes available, the performance of SSD's will only go up.As always, I wanted to say thanks for the great article and keep them coming. It's the only way the rest of us can keep pace with what's happening out there in the world of performance storage.
vailr - Wednesday, March 18, 2009 - link
Is there any benefit in using 2 SSD's in a Raid 0 configuration?And: any differences between motherboard Intel Raid vs. a Raid controller card from Areca, for example. Also: can the "Trim" command work while in Raid mode? Probably not, I'm guessing...
7Enigma - Thursday, March 19, 2009 - link
Raid0 is really the holy grail for SSD's. The low risk of failure of SSD's which normally makes Raid0 with typical mechanical HD's more dangerous is very appealing. My personal storage-size goal is ~120-160gigs. Once they reach that size for under $300 I think I'm going to jump in. But I'm more likely to grab 2 60's or 2 80's and Raid0 them than get a single large SSD. The added performance will outweigh the higher power draw of 2 drives, and should make them extremely competitive with Intel's offerings (or whatever holds the crown at the time).I figure it will be about a year or so until the prices are in that range, as 2 60gig Vertex drives will currently run you about $400 after rebate.
I can't wait to jump on that upgrade and will then put my current 250gig mechanical drive as the storage drive (I don't use a ton of space in general as I have a 320gig external backup).
Rasterman - Thursday, March 19, 2009 - link
The problem with doing that is if you want to move your drives to another system they won't work, so upgrading is a pain. You could image them I guess, but plugging one drive in is much simpler. I had an older XP install that made it through 3-4 different systems.I would also question real world results, if you're going at 250MB/s or 500MB/s its not even going to be noticeable unless you are doing some massive video editing or some other huge file operations, and as Anand says, SSDs don't fill this role right now as they are super expensive per GB. So if you really are editing video a lot, you are going to need a hell of a lot more space than SSDs can offer you.
Gasaraki88 - Friday, March 20, 2009 - link
RAID is a universal standard so if you take two RAID0 drives out and move them to another computer with a RAID controller, it SHOULD just work if the original RAID was doing it correctly.