USB drives faster?

[dbarton]

I have a laptop. The drive spec says 100mbps transfer rate, 2m cache, 5400rpm. 12ms seek time.

Im looking at a USB drive.This drive has an 8mb cache, 5400rpm. 9ms seek time.

USB is up to 480mbps, yes?

Even though this is USB, isnt it going to be faster all around?

 

[Zepper]

Typical thruput on USB 2.0 with a 7200 rpm desktop drive is around 20MB/sec. A 5400 rpm notebook drive drive won't come close to that. FireWire-400 is a bit better in the real world with HDDs even though its rating is only 400mb/sec (~40MB/sec) - 20% less than USB 2.0. If you must stay with a notebook size drive, there are 7200 rpm units that would be considerably faster but will probably require its own power supply like the desktop drives do.

.bh.

 

[Lord Evermore]

For teaching purposes: 8 megabits = 1 megabyte. 8 bits per byte. 480Mb/8=50MB.

100MBps is the transfer rate of the interface (ATA100) for an ATA hard drive. A USB hard drive is nothing but an ATA hard drive in a case, with an adapter chip to let it talk over USB. Most hard drives can't get anywhere near the speed of ATA100 except when bursting data to or from the cache. However 7200RPM drives do easily exceed USB's 50MBps maximum, at least when transferring on the fastest parts of the disk.

A 5400RPM drive probably comes in well under that maximum, however 50MBps is only the theoretical maximum. Actual throughput over USB tends to be significantly less, and a 5400RPM drive is most likely capable of reaching that real maximum, or close to it. A range of 20MBps for writing to 40MBps for reading.

A 7200RPM drive on USB will give better performance, but generally you're not using an external drive to DO much, not running applications off of it. You're usually transferring large files. A 7200RPM drive will have lower access latency than a 5400RPM drive, so it's more "responsive". But if you're transferring a large file, that's almost unnoticeable, since the drive is not having to do a lot of seeking, it's just continuously reading/writing, a sequential transfer. A faster drive will also ensure that the transfer speed stays closer to the maximum that USB can handle. A 7200RPM drive's maximum throughput at the outer edges of the disk might be 50MBps, but as it moves towards the inner parts of the disk, throughput reduces, down to 35MBps for example. 35MBps is still enough to flood USB's maximum real-world throughput. But a 5400RPM drive may drop down to 20MBps on the inner tracks, which might not be able to fully utilize the USB bandwidth.

So with a fast drive, USB does become a limitation, but depending on your needs, the unused bandwidth with a slower drive might not matter at all. I would not suggest using a USB drive for running applications (a FireWire drive might work).

Laptop drives in general are slower than desktop drives of course. However a laptop 5400RPM drive should still be a better performer than a USB 5400RPM drive, regardless of whether it's a laptop or desktop drive. USB also adds a lot of latency to the transfers. The drive itself may only have 9ms access latency, but that's not what you'd actually get when using it.

Also laptop drives actually have lower average latency than a desktop drive, because the platters are smaller, so the heads have to move less to cover the whole drive. That's one reason servers are now starting to use smaller drives.

Changing the hard drive in a laptop is normally not a terribly difficult thing to do. You might save money getting a faster laptop drive. There are 7200RPM models that only use about the same amount of power as a 5400RPM drive, and with more cache.