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Hey guys say I have 2 drives and I am swaping the files in them...

Would it be faster to copy files a-d on drive A to drive B then copy files e-h on drive B to drive A or....would it be faster to do them at the same time?

so.... copy files a-d on drive A to drive B AND copy files e-h on drive B to drive A.

Hope that makes sense...thanks...(Mechanical drives)

Basically better to have both copies going on at once or 1 at a time?

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2 Answers

up vote 3 down vote accepted

In almost all cases, it will be faster to copy sequentially (one at a time). The reason is that if you launch it simultaneously, the disk drive heads will be constantly switching from the position of the file being read to the position of the file being written, and this head movement is relatively slow (about 10-20ms).

If you do copy sequentially, one drive will be mostly reading big chuncks of sequential data (about 25-50 Mb/sec), and the other one will be mostly writing it.

By the way, make sure you have enough space left on both devices.

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How about when you are using RAID-0? That will be an interesting case :) –  Juha Untinen Nov 21 '12 at 14:04
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Short answer:
It is faster is a mechanical disk only is executing a single copy at the same time because that most likely results in sequential access to that drive.

Longer answer:

Copying from disk 1 to disk 2 means that the heads on disk 1 are over a specific place of the disk, reading the data to be copied. The heads of drive 2 are over a empty place of drive 2, sequentially writing data. (assuming no fragmentation of free space on drive 2).

The copy should proceed at the lowest common dominator of drive 1's read speed and drive 2's write speed.

The same is true with the reverse copy (just swap drive 1 and drive 2)


Now when you try to do both ate the same time then the head on both drives will have to move between the place where they can read the data to be copied, and the empty place on disk where they can write the data. While they are moving no data can be read or written. Thus disk performance is lower.


Note: this holds explicitly for two drives. Two volumes behave differently.

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