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True transfer rate?

Posted on 2013-01-13
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Last Modified: 2013-01-13
I have a Macbook Pro with an SSD and an external Firewire 800 7200RPM drive.  I cloned 210GB of data from the MBP to the ext drive.  It took 1 hour and 10 minutes.  

Firewire 800 transfers at 780Mbs, which translates to 5820MB per hour or 582GB per hour.  I got less than 1/2 of that in an hour.  The 7200RPM drive and SSD factor in.  How do I get some idea on how fast the actual transfer rate was?
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Question by:brettr
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by:Dave Baldwin
Dave Baldwin earned 150 total points
ID: 38772418
With variable size files, it is difficult to get a better estimate.  The extra time is taken up by the disk, file, and directory overhead and some network overhead.  Before each file or directory can be transferred it has to be created on the destination.  For that reason, transferring one large file will seem to go quicker than transferring the same amount of data in a lot of small files.
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Frosty555 earned 200 total points
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Your real-world transfer speed worked out to be:

210GB * 1024 / 70 minutes / 60 seconds = 51.2 Megabytes per second.

Firewire-800 has a theoretical maximum bandwidth of 800MBPS. Theoretically the fastest transfer speed possible on this interface is:

800MBPS / 8 = 100 Megabytes per second.

However, this is just the raw speed of the wire, you have to take a lot of other things into account:

     - Communication protocol overhead
     - Network overhead
     - Application level overhead (reading the data on the SSD, processing it etc)
     - Capabilities of the receiving device (it's a 7200rpm disk. There is latency, overhead and limitations when writing to that physical device)

My guess is the bottleneck was the disk speed of your external firewire hard drive, and 51.2 Megabytes per second is probably quite reasonable.
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by:garycase
garycase earned 150 total points
ID: 38772618
"...  How do I get some idea on how fast the actual transfer rate was? "  ==>   You just did exactly that.    You transferred a known amount of data in a known amount of time -- so you just do the math (as frosty did above) ==> in your case the rate is 51.2 MB/s.

Note that a modern 7200 rpm drive easily exceeds that rate, even on the slower inner cylinders (and will double it on the outer cylinders), so it's not likely your drive caused any slowdown UNLESS you were transferring a lot of relatively small files.

If you want to be sure the drive performance isn't a notable factor, create a single very large file (100MB or more);  then measure the time to copy that.
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by:brettr
ID: 38772629
There are small files involved with the transfer and many folders must be created.  I can't say how many but assume lots.  That is probably the source of the slow down.  Thanks.
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