Take the 2-minute tour ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

From what I understand, USB flash drives and solid-state drives (SSDs) are based on similar technologies, NAND flash memory.

But, USB flash drives are usually quite slow, with a read and write speed of 10-25 MB per second, while SSDs are usually very fast, about 200-600 MB per second.

Why are SSDs so much faster than USB flash drives? And why aren't USB flash drives faster than 10-25 MB per second?

Is it simply that SSDs use parallel access to the NAND flash memory or are there other reasons?

share|improve this question
1  
I benchmark my USB flash drives at usbflashspeed.com, and as you can see, they are faster than you state. –  paradroid Mar 5 '11 at 15:22
3  
Begs the question, what's the speed of a SSD when you put it into a USB enclosure? –  Mark Ransom Mar 9 '11 at 3:09
add comment

5 Answers

up vote 51 down vote accepted

Parallelism, and how the controller makes use of it is the main factor. There usually isn't room for the 8-16 discrete NAND chips you'd find in an SSD. The controllers in USB stick usually aren't nearly as complex either, to efficiently make use of available parallelism.

Other factors that matter are the quality of the flash. Many USB sticks use cheaper flash which you have to read and write slower in order to avoid errors. USB drives don't have lots of over-provisioning to help keep clean blocks around. And USB 2.0 is limited to around 35 MB/s. And many SSDs come with a big RAM cache.

The main reason for these differences is price. There are some SSDs in a USB stick form factor, like the LaCie FastKey.

share|improve this answer
add comment

USB sticks also cost quite a bit less than SSDs. Which is because they use cheaper slow Flash instead of the expensive fast Flash used in SSDs.

share|improve this answer
    
And was is the main difference between cheaper slow Flash compared to expensive fast Flash except the price? –  Jonas Mar 5 '11 at 15:08
1  
Fast nonvolatile RAM is quite a bit more expensive to fabricate; the cost has come down quite a bit, but nowhere near enough for it to be used in cheap USB keys. –  geekosaur Mar 5 '11 at 15:13
    
Yes, but you still only mention the price. Are they using the same technology? and how do they become faster? –  Jonas Mar 5 '11 at 15:30
2  
I on;y used to support chip fabricators; I don't know their business. But I'd imagine it involves the same basic technology but possibly with more exotic materials; as an example which may or may not actually be relevant in this case, a GaAs substrate with specific doping that must be a specific density with very high tolerance. You can look up details, but this generally means vacuum chambers and precisely controlled temperatures, and even so yields can be quite low (that is, up to 70% of the resulting material might not have the required tolerance). –  geekosaur Mar 5 '11 at 15:41
add comment

The USB interface is a huge bottleneck. I happen to own a really cool Kanguru eFlash drive and it makes a huge difference.

This device has a USB and a eSata connector. It transfers at 45 MB/s on the USB side and 90 MB/s on the eSata side. From the same device! This truly points to that USB is the limiting factor. As others have said though, some drives a built too cheaply to even be limited by USB.

share|improve this answer
add comment

Some factors I've seen:

  • Parallelism: SSD's use a lot of Flash devices and access them heavily in parallel, pipelining and interleaving them.

  • SLC vc MLC: Multi Level Cells store two bits in each cell, but the access time is longer and Single Level Cells, which store one bit per cell. Besides, SLCs sustain a lot more write/erase cycles than MLCs. All USB thumb flashdrives and SD cards are MLC because it's cheaper. Note that some 'consumer' SSD's, like Intel's X25-M, also use MLC, reserving SLC for 'enterprise' drives, like the X25-E.

  • Complex allocation algorithms. Not only for wear-leveling (but that's very important too), they also detach the erasing from the writing, so when you have to write, the drive already has several pre-erased cells ready to be written.

  • Interfaces: SATA2 and now SATA3 are a lot faster than USB. Not only on raw bitrate, but also have a much higher efficiency. You never reach 100% of USB's theoretical speed, but on SATA, SAS, and IEE1394 you get it consistently.

  • Some extra hardware tricks; like a small battery-backed writeback cache. You write to this small RAM, and the drive itself writes to Flash after acknowledging the write.

share|improve this answer
add comment

Primary difference is the use of technology: SLC (Single Cell), MLC (Multi Cell), TLC (Three-level Cell). While SLC is the reliable and the fastest, it is also limited to maximum capacity in GB sizes on each Flash Chip. Slower Flash Drives that you are mentioning use MLC and therefore are slower, whereas majority of SSD are still using SLC.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.